BROKEN PROMISES OF PRIVACY: RESPONDING
TO THE SURPRISING FAILURE OF ANONYMIZATION*Paul Ohm

Computer scientists have recently undermined our faith in the privacyprotecting power of anonymization, the name for techniques that protect theprivacy of individuals in large databases by deleting information like names andsocial security numbers. These scientists have demonstrated that they can often“reidentify” or “deanonymize” individuals hidden in anonymized data withastonishing ease. By understanding this research, we realize we have made a
mistake, labored beneath a fundamental misunderstanding, which has assured usmuch less privacy than we have assumed. This mistake pervades nearly every
information privacy law, regulation, and debate, yet regulators and legal scholars
have paid it scant attention. We must respond to the surprising failure ofanonymization, and this Article provides the tools to do so.

INTRODUCTION..................................................................................................................1703
I. ANONYMIZATION AND REIDENTIFICATION ..............................................................1706
A. The Past: Robust Anonymization .....................................................................1706
1. Ubiquitous Anonymization .......................................................................1707
a. The Anonymization/Reidentification Model ...................................1707*
Associate Professor, University of Colorado Law School. This Article was presented at thePrivacy Law Scholars Conference and at conferences and faculty workshops at Harvard’s Center for
Research and Computer Science and Berkman Center, Princeton’s Center for Information Technology
Policy, Fordham University Center for Law and Information Policy, University of Washington Schoolof Law, University of Washington’s Computer Science & Engineering Department, NYU Information
Law Institute, DePaul Center for IP Law and Information Technology, International Association ofPrivacy Professionals Global Privacy Summit, and the University of Colorado Law School. I thankall participants for their comments.
Thanks in particular to Caspar Bowden, Ramon Caceres, Ryan Calo, Deborah Cantrell, Danielle
Citron, Nestor Davidson, Pierre de Vries, Vasant Dhar, Cynthia Dwork, Jed Ela, Ed Felten, Victor
Fleischer, Susan Freiwald, Brett Frischmann, Michael Froomkin, Simson Garfinkel, Lauren Gelman, Eric
Goldman, James Grimmelmann, Mike Hintze, Chris Hoofnagle, Clare Huntington, Jeff Jonas, Jerry
Kang, Nancy Kim, Jon Kleinberg, Sarah Krakoff, Tim Lee, William McGeveran, Deven McGraw, Viva
Moffat, Tyler Moore, Arvind Narayanan, Helen Nissenbaum, Scott Peppett, Jules Polonetsky, Foster
Provost, Joel Reidenberg, Ira Rubinstein, Andrew Schwartz, Ari Schwartz, Vitaly Shmatikov, Chris
Soghoian, Dan Solove, Latanya Sweeney, Peter Swire, Salil Vadhan, Michael Waggoner, Phil
Weiser, Rebecca Wright, Felix Wu, and Michael Zimmer for their comments. This research was
supported by a pre-tenure research leave grant by the University of Colorado Law School, and for this I
thank Dean David Getches and Associate Dean Dayna Matthew. Finally, I thank my research assistant,
Jerry Green.

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57 UCLA LAW REVIEW 1701 (2010)b. The Reasons to Anonymize...............................................................1708
c. Faith in Anonymization ....................................................................1710
2. Anonymization Techniques: The Release-and-Forget Model .................1711
B. The Present and Future: Easy Reidentification ................................................1716
1. How Three Anonymized Databases Were Undone .................................1717
a. The AOL Data Release .....................................................................1717
b. ZIP, Sex, and Birth Date....................................................................1719
c. The Netflix Prize Data Study ............................................................1720
2. Reidentification Techniques .....................................................................1723
a. The Adversary....................................................................................1723
b. Outside Information ..........................................................................1724
c. The Basic Principle: Of Crossed Hands and Inner Joins..................1725
3. Responding to Objections .........................................................................1727
a. No Harm, No Foul .............................................................................1728
b. Examples of Bad Anonymization ......................................................1728
c. The Problem of Public Release..........................................................1729
d. The Myth of the Superuser................................................................1730
4. The Intuition Gap .....................................................................................1731
II. HOW THE FAILURE OF ANONYMIZATION DISRUPTS PRIVACY LAW .........................1731
A. The Evolution of Privacy Law...........................................................................1732
1. The Privacy Torts: Compensation for Harm ............................................1732
2. Shift to Broad Statutory Privacy: From Harm to Prevention
and PII ........................................................................................................1733
3. How Legislatures Have Used Anonymization to Balance Interests.........1735
a. How HIPAA Used Anonymization to Balance Health Privacy......1736
b. How the EU Data Protection Directive Used Anonymization
to Balance Internet Privacy ...............................................................1738
B. How the Failure of Anonymization Disrupts Privacy Law...............................1740
C. The End of PII ...................................................................................................1742
1. Quitting the PII Whack-a-Mole Game ....................................................1742
2. Abandoning “Anonymize” and “Deidentify” ...........................................1744
III. HALF MEASURES AND FALSE STARTS .......................................................................1745
A. Strictly Punish Those Who Harm ....................................................................1746
1. The Accretion Problem.............................................................................1746
2. The Database of Ruin ................................................................................1748
3. Entropy: Measuring Inchoate Harm..........................................................1749
4. The Need to Regulate Before Completed Harm ......................................1750
B. Wait for Technology to Save Us.......................................................................1751
1. Why Not to Expect a Major Breakthrough ..............................................1752
a. Utility and Privacy: Two Concepts at War ......................................1752
b. The Inverse and Imbalanced Relationship .......................................1753
2. The Prospect of Something Better Than Release-and-Forget .................1755
3. The Limitations of the Improved Techniques..........................................1756
C. Ban Reidentification..........................................................................................1758
IV. RESTORING BALANCE TO PRIVACY LAW AFTER THE FAILURE
OF ANONYMIZATION..................................................................................................1759

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1703

A. Which Database Owners Should We Regulate Anew? ...................................1759
B. Regulatory Principles.........................................................................................1761
1. From Math to Sociology ............................................................................1761
2. Support for Both Comprehensive and Contextual Regulation ...............1762
C. The Test.............................................................................................................1764
1. Five Factors for Assessing the Risk of Privacy Harm................................1765
2. Applying the Test ......................................................................................1768
D. Two Case Studies...............................................................................................1769
1. Health Information....................................................................................1769
2. IP Addresses and Internet Usage Information..........................................1771
a. Are IP Addresses Personal?................................................................1772
b. Should the Data Protection Directive Cover Search Queries?........1774
CONCLUSION .....................................................................................................................1776

INTRODUCTION
Imagine a database packed with sensitive information about many people.
Perhaps this database helps a hospital track its patients, a school its students, or a bank its customers. Now imagine that the office that maintains
this database needs to place it in long-term storage or disclose it to a third
party without compromising the privacy of the people tracked. To eliminate
the privacy risk, the office will anonymize the data, consistent with contemporary, ubiquitous data-handling practices.
First, it will delete personal identifiers like names and social security
numbers. Second, it will modify other categories of information that act like
identifiers in the particular context—the hospital will delete the names ofnext of kin, the school will excise student ID numbers, and the bank will
obscure account numbers.
What will remain is a best-of-both-worlds compromise: Analysts will stillfind the data useful, but unscrupulous marketers and malevolent identity
thieves will find it impossible to identify the people tracked. Anonymization
will calm regulators and keep critics at bay. Society will be able to turn its collective attention to other problems because technology will have solved this one.
Anonymization ensures privacy.
Unfortunately, this rosy conclusion vastly overstates the power of anonymization. Clever adversaries can often reidentify or deanonymize the people
hidden in an anonymized database. This Article is the first to comprehensively
incorporate an important new subspecialty of computer science, reidentification

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science, into legal scholarship.1 This research unearths a tension that shakes
a foundational belief about data privacy: Data can be either useful or perfectly
anonymous but never both.
Reidentification science disrupts the privacy policy landscape by undermining the faith we have placed in anonymization. This is no small faith, fortechnologists rely on it to justify sharing data indiscriminately and storing data
perpetually, while promising users (and the world) that they are protecting
privacy. Advances in reidentification expose these promises as too often illusory.
These advances should trigger a sea change in the law because nearly
every information privacy law or regulation grants a get-out-of-jail-free card
to those who anonymize their data. In the United States, federal privacy statutes
carve out exceptions for those who anonymize.2 In the European Union, thefamously privacy-protective Data Protection Directive extends a similar safe
harbor through the way it defines “personal data.”3 Yet reidentification science exposes the underlying promise made by these laws—that anonymization
protects privacy—as an empty one, as broken as the technologists’ promises.
At the very least, lawmakers must reexamine every privacy law, asking whether
the power of reidentification and fragility of anonymization have thwarted
their original designs.
The power of reidentification also transforms the public policy debate over
information privacy. Today, this debate centers almost entirely on squabbles
over magical phrases like “personally identifiable information” (PII) or “personal
data.” Advances in reidentification expose how thoroughly these phrases missthe point. Although it is true that a malicious adversary can use PII such as aname or social security number to link data to identity, as it turns out, theadversary can do the same thing using information that nobody would classify
as personally identifiable.

1.
A few legal scholars have considered the related field of statistical database privacy. E.g.
Douglas J. Sylvester & Sharon Lohr, The Security of Our Secrets: A History of Privacy and Confidentiality
in Law and Statistical Practice, 83 DENV. U. L. REV. 147 (2005); Douglas J. Sylvester & Sharon Lohr,Counting on Confidentiality: Legal and Statistical Approaches to Federal Privacy Law After the USA
PATRIOT Act, 2005 WIS. L. REV. 1033. In addition, a few law students have discussed some of thereidentification studies discussed in this Article, but without connecting these studies to larger questions
about information privacy. See, e.g., Benjamin Charkow, Note, The Control Over the De-Identificationof Data, 21 CARDOZO ARTS & ENT. L.J. 195 (2003); Christine Porter, Note, De-Identified Data andThird Party Data Mining: The Risk of Re-Identification of Personal Information, 5 SHIDLER J.L. COM. &TECH. 3 (2008) (discussing the AOL and Netflix stories).
2.
See infra Part II.B.
3.
Council Directive 95/46 on the Protection of Individuals with Regard to the Processing of
Personal Data and on the Free Movement of Such Data, 1995 O.J. (L281) 31 [hereinafter EU Data
Protection Directive].

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How many other people in the United States share your specificcombination of ZIP code, birth date (including year), and sex? According to alandmark study, for 87 percent of the American population, the answer is zero;these three pieces of information uniquely identify each of them.4 How
many users of the Netflix movie rental service can be uniquely identified bywhen and how they rated any three of the movies they have rented?
According to another important study, a person with this knowledge can iden5
tify more than 80 percent of Netflix users. Prior to these studies, nobody
would have classified ZIP code, birth date, sex, or movie ratings as PII. As aresult, even after these studies, companies have disclosed this kind of information connected to sensitive data in supposedly anonymized databases, withabsolute impunity.
These studies and others like them sound the death knell for the idea that
we protect privacy when we remove PII from our databases. This idea, which
has been the central focus of information privacy law for almost forty years,
must now yield to something else. But to what?
In search of privacy law’s new organizing principle, we can derive fromreidentification science two conclusions of great importance:
First, the power of reidentification will create and amplify privacy harms.
Reidentification combines datasets that were meant to be kept apart, and indoing so, gains power through accretion: Every successful reidentification,even one that reveals seemingly nonsensitive data like movie ratings, abets
future reidentification. Accretive reidentification makes all of our secrets fundamentally easier to discover and reveal. Our enemies will find it easier to connect
us to facts that they can use to blackmail, harass, defame, frame, or discriminate
against us. Powerful reidentification will draw every one of us closer to what I
call our personal “databases of ruin.”6
Second, regulators can protect privacy in the face of easy reidentification only at great cost. Because the utility and privacy of data are intrinsicallyconnected, no regulation can increase data privacy without also decreasing data4.
Latanya Sweeney, Uniqueness of Simple Demographics in the U.S. Population (Laboratory for
Int’l Data Privacy, Working Paper LIDAP-WP4, 2000). For more on this study, see infra Part I.B.1.b.
More recently, Philippe Golle revisited Dr. Sweeney’s study, and recalculated the statistics based on
year 2000 census data. Dr. Golle could not replicate the earlier 87 percent statistic, but he did calculatethat 61 percent of the population in 1990 and 63 percent in 2000 were uniquely identified by ZIP, birthdate, and sex. Philippe Golle, Revisiting the Uniqueness of Simple Demographics in the US Population, 5
ACM WORKSHOP ON PRIVACY IN THE ELEC. SOC’Y 77, 78 (2006).
5.
Arvind Narayanan & Vitaly Shmatikov, Robust De-Anonymization of Large Sparse Datasets,in PROC. OF THE 2008 IEEE SYMP. ON SECURITY AND PRIVACY 111, 121 [hereinafter Netflix Prize
Study]. For more on this study, see infra Part I.B.1.c.
6.
See infra Part III.A.

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utility. No useful database can ever be perfectly anonymous, and as the utilityof data increases, the privacy decreases.
Thus, easy, cheap, powerful reidentification will cause significant harmthat is difficult to avoid. Faced with these daunting new challenges, regulators must find new ways to measure the risk to privacy in different contexts.
They can no longer model privacy risks as a wholly scientific, mathematical
exercise, but instead must embrace new models that take messier humanfactors like motive and trust into account. Sometimes, they may need toresign themselves to a world with less privacy than they would like. But more
often, regulators should prevent privacy harm by squeezing and reducing theflow of information in society, even though in doing so they may need tosacrifice, at least a little, important counter values like innovation, free speech,
and security.
The Article proceeds in four Parts. Part I describes the dominant role
anonymization plays in contemporary data privacy practices and debates. It
surveys the recent, startling advances in reidentification science, tellingstories of how sophisticated data handlers—America Online, the state of
Massachusetts, and Netflix—suffered spectacular, surprising, and embarrassing
failures of anonymization. It then looks closely at the science of reidentification,
borrowing heavily from a computer science literature heretofore untapped bylegal scholars. Part II reveals how these powerful advances in reidentification
thwart the aims of nearly every privacy law and regulation. Part III considers
three simple and appealing responses to these imbalances, but ultimately
rejects them as insufficient and incomplete. Finally, Part IV offers a way forward,
proposing a test for deciding when to impose new privacy restrictions on
information flow and demonstrating the test with examples from health andinternet privacy.

I.
A.

ANONYMIZATION AND REIDENTIFICATIONThe Past: Robust Anonymization

Something important has changed. For decades, technologists havebelieved that they could robustly protect people’s privacy by making smallchanges to their data, using techniques surveyed below. I call this the robust
anonymization assumption. Embracing this assumption, regulators and technologists have promised privacy to users, and in turn, privacy is what users
have come to expect. Today, anonymization is ubiquitous.
But in the past fifteen years, computer scientists have established what I
call the easy reidentification result, which proves that the robust anonymization

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assumption is deeply flawed—not fundamentally incorrect, but deeply flawed.
By undermining the robust anonymization assumption, easy reidentification
will topple the edifices of promise and expectation we have built upon anonymization. The easy reidentification result will also wreak havoc on our legal
systems because our faith in robust anonymization has thoroughly infiltrated
our privacy laws and regulations, as Part II explores. But before we deploy the
wrecking balls, this Part reviews the story of how we built these grand structures,to explain what we are about to lose.
1.

Ubiquitous Anonymization

Anonymization plays a central role in modern data handling, forming
the core of standard procedures for storing or disclosing personal information.
What is anonymization, why do people do it, and how widespread is it?
a. The Anonymization/Reidentification Model
Let us begin with terminology. A person or entity, the data administrator,
possesses information about individuals, known as data subjects. The data
administrator most often stores the information in an electronic database, but
it may also maintain information in other formats, such as traditional paper
records.
Data administrators try to protect the privacy of data subjects by anonymizing data. Although I will later argue against using this term,7 I am notquite ready to let it go, so for now, anonymization is a process by which information in a database is manipulated to make it difficult to identify data subjects.
Database experts have developed scores of different anonymization
techniques, which vary in their cost, complexity, ease of use, and robustness.
For starters, consider a very common technique: suppression.8 A data administrator suppresses data by deleting or omitting it entirely. For example, ahospital data administrator tracking prescriptions will suppress the names ofpatients before sharing data in order to anonymize it.
The reverse of anonymization is reidentification or deanonymization.9
10
A person, known in the scientific literature as an adversary, reidentifies

7.
See infra Part II.C.2.
8.
See Latanya Sweeney, Achieving k-Anonymity Privacy Protection Using Generalization andSuppression, 10 INT’L J. ON UNCERTAINTY, FUZZINESS & KNOWLEDGE-BASED SYS. 571, 572 (2002).
9.
E.g., Netflix Prize Study, supra note 5, at 111–12.
10.
Id.

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anonymized data by linking anonymized records to outside information, hoping
to discover the true identity of the data subjects.
b. The Reasons to Anonymize
Data administrators anonymize to protect the privacy of data subjects
when storing or disclosing data. They disclose data to three groups. First, they
release data to third parties: For example, health researchers share patient
data with other health researchers,11 websites sell transaction data to adver12
tisers, and phone companies can be compelled to disclose call logs to law
enforcement officials.13 Second, administrators sometimes release anonymized
14
data to the public. Increasingly, administrators do this to engage in what
is called crowdsourcing—attempting to harness large groups of volunteer
users who can analyze data more efficiently and thoroughly than smallergroups of paid employees.15 Third, administrators disclose anonymized data to16others within their organization. Particularly within large organizations,data collectors may want to protect data subjects’ privacy even from others in
the organization.17 For example, large banks may want to share some datawith their marketing departments, but only after anonymizing it to protect
customer privacy.
Lawrence Lessig’s four regulators of behavior—norms and ethics, themarket, architecture, and law—each compel administrators to anonymize.18
Anonymization norms and ethics often operate through best practice
documents that recommend anonymization as a technique for protecting
privacy. For example, biomedical guidelines often recommend coding genetic
11.
National Institutes of Health, HIPAA Privacy Rules for Researchers, http://privacyruleandresearch.nih.gov/faq.asp (last visited June 12, 2010).
12.
E.g., Posting of Susan Wojcicki, Vice President, Product Management to The Official Google
Blog, Making Ads More Interesting, http://googleblog.blogspot.com/2009/03/making-ads-moreinteresting.html (Mar. 11, 2009, 2:01 EST) (announcing a new Google initiative to tailor ads to “thetypes of sites you visit and the pages you view”).
13.
E.g., In re Application of United States for an Order for Disclosure of Telecommunications
Records and Authorizing the Use of a Pen Register and Trap and Trace, 405 F. Supp. 2d 435 (S.D.N.Y.
2005) (granting the government the authority to compel a provider to provide information suggesting
the location of a customer’s cell phone).
14.
See infra Part I.B.1 (describing three public releases of databases).
15.
See CLAY SHIRKY, HERE COMES EVERYBODY: THE POWER OF ORGANIZING WITHOUTORGANIZATIONS (2008); JAMES SUROWIECKI, THE WISDOM OF CROWDS (2004).
16.
See Posting of Philip Lenssen to Google Blogoscoped, Google-Internal Data Restrictions,
http://blogoscoped.com/archive/2007-06-27-n27.html (June 27, 2007) (detailing how Google andMicrosoft limit internal access to sensitive data).
17.
See id.
18.
See LAWRENCE LESSIG, CODE: VERSION 2.0, at 123 (2006) (listing four regulators of online
behavior: markets, norms, laws, and architecture).

Broken Promises of Privacy

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data—associating stored genes with nonidentifying numbers—to protect
privacy.19 Other guidelines recommend anonymization in contexts such as
20
21
22electronic commerce, internet service provision, data mining, and national
23
security data sharing. Academic researchers rely heavily on anonymization
to protect human research subjects, and their research guidelines recommend
anonymization generally,24 and specifically in education,25 computer network
26
27
monitoring, and health studies. Professional statisticians are duty-bound to28
anonymize data as a matter of professional ethics.
Market pressures sometimes compel businesses to anonymize data. For
example, companies like mint.com and wesabe.com provide web-based
personal finance tracking and planning.29 One way these companies add
value is by aggregating and republishing data to help their customers
compare their spending with that of similarly situated people.30 To makecustomers comfortable with this type of data sharing, both mint.com andwesabe.com promise to anonymize data before sharing it.31
32
Architecture, defined in Lessig’s sense as technological constraints, often
forces anonymization, or at least makes anonymization the default choice. As
one example, whenever you visit a website, the distant computer with which
you communicate—also known as the web server—records some information
19.
Roberto Andorno, Population Genetic Databases: A New Challenge to Human Rights, in
ETHICS AND LAW OF INTELLECTUAL PROPERTY 39 (Christian Lenk, Nils Hoppe & Roberto Andornoeds., 2007).
20.
ALEX BERSON & LARRY DUBOV, MASTER DATA MANAGEMENT AND CUSTOMER DATAINTEGRATION FOR A GLOBAL ENTERPRISE 338–39 (2007).
21.
See infra Part II.A.3.b.
22.
G.K. GUPTA, INTRODUCTION TO DATA MINING WITH CASE STUDIES 432 (2006).
23.
MARKLE FOUND. TASK FORCE, CREATING A TRUSTED NETWORK FOR HOMELAND
SECURITY 144 (2003), available at http://www.markle.org/downloadable_assets/nstf_report2_full_report.pdf.
24.
See THE SAGE ENCYCLOPEDIA OF QUALITATIVE RESEARCH METHODS 196 (Lisa M.
Given ed., 2008) (entry for “Data Security”).
25.
LOUIS COHEN ET AL., RESEARCH METHODS IN EDUCATION 189 (2003).
26.
See Ruoming Pang et al., The Devil and Packet Trace Anonymization, 36 COMP. COMM.
REV. 29 (2006).
27.
INST. OF MED., PROTECTING DATA PRIVACY IN HEALTH SERVICES RESEARCH 178 (2000).
28.
European Union Article 29 Data Protection Working Party, Opinion 4/2007 on theConcept of Personal Data, 01248/07/EN WP 136, at 21 (June 20, 2007) [hereinafter 2007 Working
Party Opinion], available at http://ec.europa.eu/justice_home/fsj/privacy/docs/wpdocs/2007/wp136_en.pdf.
29.
See Eric Benderoff, Spend and Save the Social Way—Personal Technology, SEATTLE TIMES,
Nov. 8, 2008, at A9.
30.
See Carolyn Y. Johnson, Online Social Networking Meets Personal Finance, N.Y. TIMES, Aug.
7, 2007, available at http://www.nytimes.com/2007/08/07/technology/07iht-debt.1.7013213.html.
31.
See, e.g., Wesabe, Security and Privacy, http://www.wesabe.com/page/security (last visitedJune 12, 2010); Mint.com, How Mint Personal Finance Management Protects Your Financial Safety,
http://www.mint.com/privacy (last visited June 12, 2010).
32.
LESSIG, supra note 18, at 4.

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about your visit into what is called a log file.33 The vast majority of web servers
collect much less than the maximum amount of information available about
your visit, not due to the principled privacy convictions of their owners, but
because the software saves only a limited amount of information by default.34
c. Faith in Anonymization
Many defend the privacy-protecting power of anonymization and hold it
out as a best practice despite evidence to the contrary. In one best practices
guide, the authors, after cursorily acknowledging concerns about the power ofanonymization, conclude that, “[w]hile we recognize that [reidentification] is aremote possibility in some situations, in most cases genetic research data anonymization will help to ensure confidentiality.”35 Similarly, Google has said, “[i]tis difficult to guarantee complete anonymization, but we believe [Google’s log
file anonymization techniques] will make it very unlikely users could be
identified.”36
Government officials and policymakers embrace anonymization as well.
Two influential data mining task forces have endorsed anonymization. In
2004, the Technology and Privacy Advisory Committee (TAPAC), a Defense
Department−led group established in the wake of controversy over the government’s Total Information Awareness program, produced an influential reportabout government data mining.37 The report recommends anonymization“whenever practicable” and thus restricts all of its other recommendations
only to databases that are not “known or reasonably likely to include personallyidentifiable information.”38
Likewise, the Markle Foundation task force, which included among its
39
members now–Attorney General Eric Holder, produced a similar report.
Like TAPAC, the Markle Foundation group concluded that “anonymizing
technologies could be employed to allow analysts to perform link analysis
among data sets without disclosing personally identifiable information . . . [so]33.
STEPHEN SPAINHOUR & ROBERT ECKSTEIN, WEBMASTER IN A NUTSHELL 458–59 (2002).
34.
Apache, Apache HTTP Server Version 1.3 Log Files, http://httpd.apache.org/docs/1.3/logs.
html (last visited June 12, 2010) (describing the default “common log format” which logs less information
than the alternative “combined log format”).
35.
ADIL E. SHAMOO & DAVID B. RESNICK, RESPONSIBLE CONDUCT OF RESEARCH 302 (2009).
36.
Chris Soghoian, Debunking Google’s Log Anonymization Propaganda, Surveillance State,
CNET NEWS, Sept. 11, 2008, http://news.cnet.com/8301-13739_3-10038963-46.html.
37.
TECHNOLOGY & PRIVACY ADVISORY COMM., REPORT: SAFEGUARDING PRIVACY IN
THE FIGHT AGAINST TERRORISM 35–36 (2004), available at http://www.cdt.org/security/usapatriot/
20040300tapac.pdf.
38.
Id. at 50 (Recommendation 2.2).
39.
See MARKLE FOUND. TASK FORCE, supra note 23, at 34.

Broken Promises of Privacy

1711

analysts can perform their jobs and search for suspicious patterns without theneed to gain access to personal data until they make the requisite showing
for disclosure.”40
Many legal scholars share this faith in anonymization.41 Ira Rubinstein,
Ronald Lee, and Paul Schwartz state a “consensus view” that “[w]ith the goalof minimizing the amount of personal information revealed in the course of
running pattern-based searches, the anonymization of data (such as names,
42
addresses, and social security numbers) is essential.” Barbara Evans, a prominent medical privacy scholar, speaks about “anonymized” data “that have hadpatient identifiers completely and irrevocably removed before disclosure, suchthat future reidentification would be impossible.”43 Many other legal scholars
44
have made similar claims premised on deep faith in robust anonymization.
The point is not to criticize or blame these people for trusting anonymization;
as we will see, even computer scientists have been surprised by the success ofrecent attacks on anonymization.
2.

Anonymization Techniques: The Release-and-Forget Model

How do people anonymize data? From among the scores of different anonymization techniques, I will focus on an important and large subset that I
call release-and-forget anonymization.45 As the name suggests, when a data
administrator practices these techniques, she releases records—either publicly,40.
Id. at 34.
41.
Regulators do too. See infra Part II.A (listing laws and regulations that assume robust anonymization).
42.
Ira S. Rubinstein et al., Data Mining and Internet Profiling: Emerging Regulatory andTechnological Approaches, 75 U. CHI. L. REV. 261, 266, 268 (2008).
43.
Barbara J. Evans, Congress’ New Infrastructural Model of Medical Privacy, 84 NOTRE DAME
L. REV. 585, 619–20 (2009). Professor Evans has clarified that the quote did not reflect her personal
opinions about the feasibility of definitive anonymization but rather reflected how the term ‘anonymization’ has commonly been understood by regulators and others in bioethics. Email From Barbara Evans,
Assoc. Professor, Univ. of Houston Law Ctr., to Paul Ohm, Assoc. Professor, Univ. of Colorado Law Sch.
(July 21, 2010) (on file with author).
44.
See, e.g., Fred H. Cate, Government Data Mining: The Need for a Legal Framework, 43 HARV.
C.R.-C.L. L. REV. 435, 487 (2008); Matthew P. Gordon, A Legal Duty to Disclose Individual Research
Findings to Research Subjects?, 64 FOOD & DRUG L.J. 225, 258–59 (2009); Bartha Maria Knoppers
et al., Ethical Issues in Secondary Uses of Human Biological Material From Mass Disasters, 34 J.L. MED.
& ETHICS 352, 353 (2006); Susan M. Wolf et al., Managing Incidental Findings in Human Subjects
Research: Analysis and Recommendations, 36 J.L. MED. & ETHICS 219, 226–27 (2008); Irfan Tukdi,
Comment, Transatlantic Turbulence: The Passenger Name Record Conflict, 45 HOUS. L. REV. 587, 618–
19 (2008).
45.
Other means of making data more anonymous include releasing only aggregated statistics;
interactive techniques, in which administrators answer directed questions on behalf of researchers, insteadof releasing data in its entirety; and “differential privacy” techniques, which protect privacy by adding
carefully calibrated noise to the data. See discussion infra Part III.B.2.

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57 UCLA LAW REVIEW 1701 (2010)privately to a third party, or internally within her own organization—andthen she forgets, meaning she makes no attempt to track what happens to
the records after release. Rather than blithely put her data subjects at risk, before
she releases, she modifies some of the information.
I focus on release-and-forget anonymization for two reasons. First, thesetechniques are widespread.46 Because they promise privacy while allowing
the broad dissemination of data, they give data administrators everything they47
want without any compromises, and data administrators have embraced them.
Second, these techniques are often flawed. Many of the recent advances in thescience of reidentification target release-and-forget anonymization in particular.48
Consider some common release-and-forget techniques.49 First, we needa sample database to anonymize, a simplified and hypothetical model of a
hospital’s database for tracking visits and complaints:50
TABLE 1: Original (Nonanonymized) Data
Name

Race

Birth Date

Sex

Sean
DanielKate
MarionHelen
Reese
ForestHilaryPhilip
Jamie
Sean
Adrien

Black
Black
Black
Black
Black
Black
White
White
White
White
White
White

9/20/1965
2/14/1965
10/23/1965
8/24/1965
11/7/1964
12/1/1964
10/23/1964
3/15/1965
8/13/1964
5/5/1964
2/13/1967
3/21/1967

Male
Male
FemaleFemaleFemaleFemaleMale
Female
Male
Male
Male
Male

ZIP
Code
02141
02141
02138
02138
02138
02138
02138
02139
02139
02139
02138
02138

Complaint
Short of breath
Chest pain
Painful eye
Wheezing
Aching joints
Chest pain
Short of breath
Hypertension
Aching joints
Fever
Vomiting
Back pain46.
See Laks V.S. Lakshmanan & Raymond T. Ng, On Disclosure Risk Analysis of Anonymized
Itemsets in the Presence of Prior Knowledge, 2 ACM TRANSACTIONS ON KNOWLEDGE DISCOVERY FROMDATA 13, 13:2 (2008) (“Among the well-known transformation techniques, anonymization is arguably
the most common.”).
47.
Id. (“Compared with other transformation techniques, anonymization is simple to carry out,
as mapping objects back and forth is easy.”).
48.
See Justin Brickell & Vitaly Shmatikov, The Cost of Privacy: Destruction of Data-Mining
Utility in Anonymized Data Publishing, in 2008 KNOWLEDGE DISCOVERY & DATA MINING CONF. 70, 70.
49.
The following discussion is only a survey; it will make an expert of no one.
50.
All of the hypothetical data in this table aside from the “Name” column comes from a paperby Latanya Sweeney. Sweeney, supra note 8, at 567 fig.4. Where the first names come from is left as
an exercise for the reader.

Broken Promises of Privacy

1713

Using standard terminology, we call this collection of data a table; eachrow is a row or record; each column is a column, field, or attribute, identified
by a label (in bold) called a field name or attribute name; each record has aparticular value for a given attribute.51
To protect the privacy of the people in this table, the hospital databaseadministrator will take the following steps before releasing this data:
Singling Out Identifying Information: First, the administrator will single
out any fields she thinks one can use to identify individuals. Often, she will
single out not only well-known identifiers like name and social security number,but combinations of fields that when considered together might link a recordin the table to a patient’s identity.52 Sometimes an administrator will select thepotentially identifying fields herself, either intuitively (by isolating types ofdata that seem identifying) or analytically (by looking for uniqueness in theparticular data). For example, no two people in our database share a birth date,so the administrator must treat birth date as an identifier.53 If she did not,then anyone who knew Forest’s birth date (and who knew Forest had been
54
admitted to the hospital) would be able to find Forest in the anonymized data.
In other cases, an administrator will look to another source—such as astatistical study, company policy, or government regulation—to decide whether
or not to treat a particular field as identifying. In this case, assume the administrator decides, based on one of these sources, to treat the following four fields
as potential identifiers: name, birth date, sex, and ZIP code.55
Suppression: Next, the administrator will modify the identifying fields. She
might suppress them, removing the fields from the table altogether.56 In our
example, the administrator might delete all four potential identifiers, producing
this table:

51.
GAVIN POWELL, BEGINNING DATABASE DESIGN 38–41 (2005).
52.
Claudio Bettini et al., The Role of Quasi-Identifiers in k-Anonymity Revisited (DICo Univ.
Milan Tech. Rep. RT-11-06, July 2006).
53.
See id. Because these sorts of identifiers do not link directly to identity, researchers sometimes
refer to them as quasi-identifiers.
54.
That large numbers of people could know Forest’s birth date is far from an idle worry. Today,
more than ever, people are sharing this kind of information widely. For example, “at least 10 million
U.S. residents make publicly available or inferable their birthday information on their [social networking]
online profiles.” Alessandro Acquisti & Ralph Gross, SSN Study-FAQ, http://www.heinz.cmu.edu/~acquisti/ssnstudy (last visited June 12, 2010).
55.
See infra Part I.B.1.b (discussing research about using the combination of ZIP code, birth date,and sex as an identifier).
56.
Sweeney, supra note 8, at 3.

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57 UCLA LAW REVIEW 1701 (2010)

TABLE 2: Suppressing Four Identifier Fields
Race
Black
Black
Black
Black
Black
Black
White
White
White
White
White
White

Complaint
Short of breath
Chest pain
Painful eye
Wheezing
Aching joints
Chest pain
Short of breath
Hypertension
Aching joints
Fever
Vomiting
Back pain

Here we first encounter a fundamental tension. On the one hand, withthis version of the data, we should worry little about privacy; even if one knows
Forest’s birth date, sex, ZIP code, and race, one still cannot learn Forest’s complaint. On the other hand, aggressive suppression has rendered this data almost
useless for research.57 Although a researcher can use the remaining data to
track the incidence of diseases by race, because age, sex, and residence havebeen removed, the researcher will not be able to draw many other interesting
and useful conclusions.
Generalization: To better strike the balance between utility and privacy,
the anonymizer might generalize rather than suppress identifiers.58 This means
she will alter rather than delete identifier values to increase privacy while
preserving utility. For example, the anonymizer may choose to suppress thename field, generalize the birth date to only the year of birth, and generalize ZIP
codes by retaining only the first three digits.59 The resulting data would look
like this:57.
See infra Part III.B.1 (discussing the relationship between utility and privacy).
58.
Sweeney, supra note 8, at 3.
59.
Under the HIPAA Privacy Rule, these three changes would qualify the resulting table asdeidentified health information. See U.S. Health & Human Services, Standards for Privacy ofIndividually Identifiable Health Information, 45 C.F.R. §§ 160, 164 (2009). For more on HIPAA and thePrivacy Rule, see infra Part II.A.3.a.

Broken Promises of Privacy

1715

TABLE 3: Generalized
Race

Birth
Year

Sex

ZIP
Code*

Complaint

Black
Black
Black
Black
Black
Black
White
White
White
White
White
White

1965
1965
1965
1965
1964
1964
1964
1965
1964
1964
1967
1967

Male
Male
FemaleFemaleFemaleFemaleMale
Female
Male
Male
Male
Male

021*
021*
021*
021*
021*
021*
021*
021*
021*
021*
021*
021*

Short of breath
Chest pain
Painful eye
Wheezing
Aching joints
Chest pain
Short of breath
Hypertension
Aching joints
Fever
Vomiting
Back pain

Now, even someone who knows Forest’s birth date, ZIP code, sex, andrace will have trouble plucking out Forest’s specific complaint. The records
in this generalized data (Table 3) are more difficult to reidentify than they
were in the original data (Table 1), but researchers will find this data much
more useful than the suppressed data (Table 2).
Aggregation: Finally, to better understand what qualifies as release-andforget anonymization, consider a commonly used technique that does notobey release-and-forget. Quite often, an analyst needs only summary statistics,
not raw data. For decades, statisticians have investigated how to release aggregate statistics while protecting data subjects from reidentification.60 Thus, if
researchers only need to know how many men complained of shortness ofbreath, data administrators could release this:TABLE 4: Aggregate Statistic
Men Short of Breath260.
E.g., Nabil R. Adam & John C. Wortmann, Security-Control Methods for Statistical Databases:
A Comparative Study, 21 ACM COMPUTING SURVEYS 515 (1989); Tore Dalenius, Towards a
Methodology for Statistical Disclosure Control, 15 STATISTISK TIDSKRIFT 429 (1977) (Swed.); I.P. Fellegi,
On the Question of Statistical Confidentiality, 67 J. AM. STAT. ASS’N 7 (1972).

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57 UCLA LAW REVIEW 1701 (2010)

As it happens, Forest is one of the two men described by this statistic—hecomplained about shortness of breath—but without a lot of additional information, one would never know. His privacy is secure.61
Privacy lawyers tend to refer to release-and-forget anonymization techniques using two other names: deidentification62 and the removal of
personally identifiable information (PII).63 Deidentification has taken on
special importance in the health privacy context. Regulations implementing
the privacy provisions of the Health Insurance Portability and Accountability
Act (HIPAA) expressly use the term, exempting health providers andresearchers who deidentify data before releasing it from all of HIPAA’s many
onerous privacy requirements.64
B.

The Present and Future: Easy Reidentification

Until a decade ago, the robust anonymization assumption worked wellfor everybody involved. Data administrators could protect privacy when sharing
data with third parties; data subjects could rest assured that their secrets
would remain private; legislators could balance privacy and other interests(such as the advancement of knowledge) by deregulating the trade in anonymized records;65 and regulators could easily divide data handlers into twogroups: the responsible (those who anonymized) and the irresponsible (those
who did not).
About fifteen years ago, researchers started to chip away at the robust
anonymization assumption, the foundation upon which this state of affairs
has been built. Recently, however, they have done more than chip away;
they have essentially blown it up, casting serious doubt on the power ofanonymization, proving its theoretical limits and establishing what I call theeasy reidentification result. This is not to say that all anonymization techniques
fail to protect privacy—some techniques are very difficult to reverse—butresearchers have learned more than enough already for us to reject anonymization as a privacy-providing panacea.

61.
For additional discussion of privacy techniques other than release-and-forget, see infra Part
III.B.2.
62.
National Institutes of Health, De-identifying Protected Health Information Under the Privacy
Rule, http://privacyruleandresearch.nih.gov/pr_08.asp (last visited June 12, 2010).
63.
ERIKA MCCALLISTER ET AL., NAT’L INST. OF STANDARDS & TECH., SPECIAL PUB. NO.
800-122, GUIDE TO PROTECTING THE CONFIDENTIALITY OF PERSONALLY IDENTIFIABLE INFORMATION(PII) (2010), available at http://csrc.nist.gov/publications/nistpubs/800-122/sp800-122.pdf.
64.
45 C.F.R. §§ 164.502(d)(2), 164.514(a)–(b) (2009). See infra Part II.A.3.a.
65.
See infra II.A.

Broken Promises of Privacy
1.

1717

How Three Anonymized Databases Were Undone

Consider three recent, spectacular failures of anonymization. In eachcase, a sophisticated entity placed unjustified faith in weak, release-and-forgetanonymization. These stories, which I will use as examples throughout this
Article, provide two important lessons: They demonstrate the pervasivenessof release-and-forget anonymization even among supposedly sophisticateddata administrators, and they demonstrate the peril of this kind of anonymization in light of recent advances in reidentification.
a. The AOL Data Release
On August 3, 2006, America Online (AOL) announced a new initiative
called “AOL Research.”66 To “embrac[e] the vision of an open research community,” AOL Research publicly posted to a website twenty million search queries
for 650,000 users of AOL’s search engine, summarizing three months ofactivity.67 Researchers of internet behavior rejoiced to receive this treasure
trove of information, the kind of information that is usually treated by searchengines as a closely guarded secret.68 The euphoria was short-lived, however,as AOL and the rest of the world soon learned that search engine queries arewindows to the soul.
Before releasing the data to the public, AOL had tried to anonymize it
to protect privacy. It suppressed any obviously identifying information suchas AOL username and IP address in the released data.69 In order to preserve
the usefulness of the data for research, however, it replaced these identifierswith unique identification numbers that allowed researchers to correlatedifferent searches to individual users.70
In the days following the release, bloggers pored through the data
spotlighting repeatedly the nature and extent of the privacy breach. These
bloggers chased two different prizes, either attempting to identify users or66.
Posting of Abdur Chowdhury, cabdur@aol.com, to SIGIR-IRList, irlist-editor@acm.org,
http://sifaka.cs.uiuc.edu/xshen/aol/20060803_SIG-IRListEmail.txt (last visited July 19, 2010).
67.
Id. Others have reported that the data contained thirty-six million entries. Paul Boutin,
You Are What You Search, SLATE, Aug. 11, 2006, http://www.slate.com/id/2147590.
68.
See Katie Hafner, Researchers Yearn to Use AOL Logs, but They Hesitate, N.Y. TIMES, Aug 23,
2006, at C1 (describing the difficulty that academic researchers experience accessing raw search data).
69.
See Michael Barbaro & Tom Zeller, Jr., A Face Is Exposed for AOL Searcher No. 4417749,
N.Y. TIMES, Aug. 9, 2006, at A1. IP addresses, discussed infra in Part II.A.3.b, are numbers that identify
computers on the internet and can be used to track internet activity.
70.
Barbaro & Zeller, Jr., supra note 69.

1718

57 UCLA LAW REVIEW 1701 (2010)“hunt[ing] for particularly entertaining or shocking search histories.”71 Thanks
to this blogging and subsequent news reporting, certain user identification
numbers have become sad little badges of infamy, associated with pitiful orchilling stories. User “No. 3505202 ask[ed] about ‘depression and medical
leave.’ No. 7268042 type[d] ‘fear that spouse contemplating cheating.’”72
User 17556639 searched for “how to kill your wife” followed by a string ofsearches for things like “pictures of dead people” and “car crash photo.”73
74
While most of the blogosphere quickly and roundly condemned AOL,a few bloggers argued that the released data, while titillating, did not violate
privacy because nobody had linked actual individuals with their anonymized
queries.75 This argument was quickly silenced by New York Times reporters
Michael Barbaro and Tom Zeller, who recognized clues to User 4417749’s
identity in queries such as “‘landscapers in Lilburn, Ga,’ several people with thelast name Arnold and ‘homes sold in shadow lake subdivision gwinnett county
georgia.’”76 They quickly tracked down Thelma Arnold, a sixty-two-year-old
widow from Lilburn, Georgia who acknowledged that she had authored thesearches, including some mildly embarrassing queries such as “numb fingers,”“60 single men,” and “dog that urinates on everything.”77
The fallout was swift and crushing. AOL fired the researcher who released
the data and also his supervisor.78 Chief Technology Officer Maureen Govern
79
resigned. The fledgling AOL Research division has been silenced, and ayear after the incident, the group still had no working website.80
71.
Id. These twin goals demonstrate an important information dichotomy revisited later: Whensomeone talks about the sensitivity of data, they may mean that the information can cause harm if
disclosed, or they may mean that the information can be used to link anonymized information to identity.
As we will see, regulators often misunderstand the difference between these two classes of information.
See infra Part II.A.
72.
See Barbaro & Zeller, Jr., supra note 69.
73.
Markus Frind, AOL Search Data Shows Users Planning to Commit Murder, Paradigm Shift Blog
(Aug. 7, 2006), http://plentyoffish.wordpress.com/2006/08/07/aol-search-data-shows-users-planningto-commit-murder.
74.
See, e.g., Posting of Michael Arrington to TechCrunch, AOL Proudly Releases Massive
Amounts of Private Data (Aug. 6, 2006), http://www.techcrunch.com/2006/08/06/aol-proudly-releasesmassive-amounts-of-user-search-data (“The utter stupidity of this is staggering.”).
75.
Greg Linden, for example, complained that “no one actually has come up with an example
where someone could be identified. Just the theoretical possibility is enough to create a privacy firestorm
in some people’s minds.” Greg Linden, A Chance to Play With Big Data: Geeking With Greg, http://glinden.
blogspot.com/2006/08/chance-to-play-with-big-data.html (Aug. 4, 2006, 19:53 PST).
76.
Barbaro & Zeller, Jr., supra note 69.
77.
Id.
78.
Tom Zeller, Jr., AOL Executive Quits After Posting of Search Data, N.Y. TIMES, Aug. 22, 2006,
http://www.nytimes.com/2006/08/22/technology/22iht-aol.2558731.html.
79.
Id.
80.
Chris Soghoian, AOL, Netflix and the End of Open Access to Research Data, Surveillance State,
CNET NEWS, Nov. 30, 2007, http://news.cnet.com/8301-13739_3-9826608-46.html.

Broken Promises of Privacy

1719

b. ZIP, Sex, and Birth Date
Recall from the Introduction the study by Latanya Sweeney, professorof computer science, who crunched 1990 census data and discovered that87.1 percent of people in the United States were uniquely identified bytheir combined five-digit ZIP code, birth date (including year), and sex.81
According to her study, even less-specific information can often reveal identity,
as 53 percent of American citizens are uniquely identified by their city, birthdate, and sex, and 18 percent by their county, birth date, and sex.82
Like the reporters who discovered Thelma Arnold, Dr. Sweeney offereda hyper-salient example to drive home the power (and the threat) of reidentification techniques. In Massachusetts, a government agency called theGroup Insurance Commission (GIC) purchased health insurance for state
employees.83 At some point in the mid-1990s, GIC decided to release records
summarizing every state employee’s hospital visits at no cost to any researcher
who requested them.84 By removing fields containing name, address, social
security number, and other “explicit identifiers,” GIC assumed it had protectedpatient privacy, despite the fact that “nearly one hundred attributes per” patientand hospital visit were still included, including the critical trio of ZIP code,85
birth date, and sex.
At the time that GIC released the data, William Weld, then−Governorof Massachusetts, assured the public that GIC had protected patient privacy
86
by deleting identifiers. In response, then−graduate student Sweeney started
hunting for the Governor’s hospital records in the GIC data.87 She knew that
Governor Weld resided in Cambridge, Massachusetts, a city of fifty-four
thousand residents and seven ZIP codes. For twenty dollars, she purchased thecomplete voter rolls from the city of Cambridge—a database containing,among other things, the name, address, ZIP code, birth date, and sex of every
voter. By combining this data with the GIC records, Sweeney found Governor
81.
Sweeney, supra note 4. A subsequent study placed the number at 61 percent (for 1990
census data) and 63 percent (for 2000 census data). Golle, supra note 4, at 1.
82.
Sweeney, supra note 4.
83.
Massachusetts Executive Office for Administration and Finance, Who is the GIC?,
http://mass.gov/gic (follow “Who is the GIC?” hyperlink) (last visited June 15, 2010).
84.
Recommendations to Identify and Combat Privacy Problems in the Commonwealth: Hearingon H.R. 351 Before the H. Select Comm. on Information Security, 189th Sess. (Pa. 2005) (statementof Latanya Sweeney, Associate Professor, Carnegie Mellon University), available at http://dataprivacylab.
org/dataprivacy/talks/Flick-05-10.html.
85.
Id.
86.
Henry T. Greely, The Uneasy Ethical and Legal Underpinnings of Large-Scale Genomic Biobanks,
8 ANN. REV. GENOMICS & HUM. GENETICS 343, 352 (2007).
87.
Id.

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57 UCLA LAW REVIEW 1701 (2010)

Weld with ease. Only six people in Cambridge shared his birth date; only threewere men, and of the three, only he lived in his ZIP code.88 In a theatricalflourish, Dr. Sweeney sent the governor’s health records (including diagnoses
and prescriptions) to his office.89
c. The Netflix Prize Data StudyOn October 2, 2006, about two months after the AOL debacle, Netflix,
the “world’s largest online movie rental service,” publicly released one
hundred million records revealing how nearly a half-million of its users had
rated movies from December 1999 to December 2005.90 In each record, Netflix
disclosed the movie rated, the rating assigned (from one to five stars), and thedate of the rating.91 Like AOL and GIC, Netflix first anonymized the records,
removing identifying information like usernames, but assigning a unique user
identifier to preserve rating-to-rating continuity.92 Thus, researchers could tellthat user 1337 had rated Gattaca a 4 on March 3, 2003, and Minority Reporta 5 on November 10, 2003.
Unlike AOL, Netflix had a specific profit motive for releasing these93
records. Netflix thrives by being able to make accurate movie recommendations; if Netflix knows, for example, that people who liked Gattaca will also
like The Lives of Others, it can make recommendations that keep its customers
coming back to the website.
To improve its recommendations, Netflix released the hundred million
records to launch what it called the “Netflix Prize,” a prize that took almost
three years to claim.94 The first team that used the data to significantly improveon Netflix’s recommendation algorithm would win one million dollars.95 Aswith the AOL release, researchers have hailed the Netflix Prize data releaseas a great boon for research, and many have used the competition to refine ordevelop important statistical theories.96
88.
Sweeney, supra note 4.
89.
Greely, supra note 86.
90.
The Netflix Prize Rules, http://www.netflixprize.com/rules (last visited June 12, 2010).
91.
Id.
92.
Netflix Prize: FAQ, http://www.netflixprize.com/faq (last visited June 12, 2010) (answering
the question, “Is there any customer information in the dataset that should be kept private?”).
93.
See Clive Thompson, If You Liked This, You’re Sure to Love That, N.Y. TIMES MAG., Nov.
23, 2008, at 74, available at http://www.nytimes.com/2008/11/23/magazine/23Netflix-t.html.
94.
Posting of Steve Lohr, Netflix Challenge Ends, but Winner is in Doubt, N.Y. TIMES BITS BLOG,
http://bits.blogs.nytimes.com/2009/07/27/netflix-challenge-ends-but-winner-is-in-doubt (July 27, 2009,
16:59 EST).
95.
See The Netflix Prize Rules, supra note 90.
96.
See Thompson, supra note 93.

Broken Promises of Privacy

1721

Two weeks after the data release, researchers from the University ofTexas, Arvind Narayanan and Professor Vitaly Shmatikov, announced that“an attacker who knows only a little bit about an individual subscriber caneasily identify this subscriber’s record if it is present in the [Netflix Prize]
dataset, or, at the very least, identify a small set of records which include thesubscriber’s record.”97 In other words, it is surprisingly easy to reidentify people
in the database and thus discover all of the movies they have rated with only
a little outside knowledge about their movie-watching preferences.
The resulting research paper is brimming with startling examples of theease with which someone could reidentify people in the database, and has
been celebrated and cited as surprising and novel to computer scientists.98 If
99
an adversary—the term used by computer scientists —knows the precise
ratings a person in the database has assigned to six obscure movies,100 and101
nothing else, he will be able to identify that person 84 percent of the time.
If he knows approximately when (give or take two weeks) a person in the
database has rated six movies, whether or not they are obscure, he canidentify the person 99 percent of the time.102 In fact, knowing when ratings
were assigned turns out to be so powerful that knowing only two movies arating user has viewed (with the precise ratings and the rating dates give ortake three days), an adversary can reidentify 68 percent of the users.103
To summarize, the next time your dinner party host asks you to list yoursix favorite obscure movies, unless you want everybody at the table to know
every movie you have ever rated on Netflix, say nothing at all.
To turn these abstract results into concrete examples, Narayanan andShmatikov compared the Netflix rating data to similar data from the Internet
97.
Arvind Narayanan & Vitaly Shmatikov, How to Break the Anonymity of the Netflix Prize
Dataset, ARVIX, Oct. 16, 2006, at 1, http://arxiv.org/abs/cs/0610105v1 (v.1) [hereinafter Netflix Prize v1].
Narayanan and Shmatikov eventually published the results in 2008. Netflix Prize Study, supra note 5.
98.
In 2008, the paper was awarded the “Award for Outstanding Research in Privacy Enhancing
Technologies” or PET Award, given jointly by Microsoft and the Privacy Commissioner of Ontario,
Canada. Press Release, EMEA Press Ctr., Microsoft, Privacy to the Test—Exploring the Limits of Online
Anonymity and Accountability (July 23, 2008), http://www.microsoft.com/emea/presscentre/pressreleases/
23072008_PETSFS.mspx. E.g., Cynthia Dwork, An Ad Omnia Approach to Defining and Achieving
Private Data Analysis, in PRIVACY, SECURITY, AND TRUST IN KDD 1, 2 (2008), available at http://www.
springerlink.com/content/85g8155l38612w06/fulltext.pdf.
99.
See infra Part I.B.2.a.
100.
By obscure movie, I mean a movie outside the top five hundred movies rated in the
database, ranked by number of ratings given. See generally Netflix Prize Study, supra note 5.
101.
Id. at 121, 122 fig.8. The authors emphasize that this result would apply to most of the rating
users, as 90 percent of them rated five or more obscure movies and 80 percent rated ten or more obscure
movies. Id. at 121 tbl.
102.
Id. at 121, 120 fig.4.
103.
Id.

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57 UCLA LAW REVIEW 1701 (2010)

Movie Database (IMDb),104 a movie-related website that also gives users the
chance to rate movies. Unlike Netflix, IMDb posts these ratings publicly on
its website, as Amazon does with user-submitted book ratings.
Narayanan and Shmatikov obtained ratings for fifty IMDb users.105 From
106
this tiny sample, they found two users who were identifiable, to a statisticalnear-certainty, in the Netflix database.107 Because neither database compriseda perfect subset of the other, one could learn things from Netflix unknowable108
only from IMDb, and vice versa, including some things these users probably
did not want revealed. For example, the authors listed movies viewed by one
user that suggested facts about his or her politics (“Fahrenheit 9/11”), religious
views (“Jesus of Nazareth”), and attitudes toward gay people (“Queer as Folk”).109
Soon after it awarded the first Netflix Prize, the company announced
that it would launch a second contest, one involving “demographic andbehavioral data . . . includ[ing] information about renters’ ages, gender, ZIP
codes, genre ratings, and previously chosen movies.”110 In late 2009, a few
Netflix customers brought a class action lawsuit against the company for
privacy violations stemming from the release of their information through the
Netflix Prize.111 The suit alleged violations of various state and federal privacy
112
laws. A few months later, after the FTC became involved, Netflix announced
that it had settled the suit and shelved plans for the second contest.113

104.
Internet Movie Database, http://www.imdb.com (last visited June 12, 2010).
105.
Ideally, the authors would have imported the entire IMDb ratings database to see how many
people they could identify in the Netflix data. The authors were afraid, however, that the IMDb terms ofservice prohibited this. Netflix Prize Study, supra note 5, at 122. As of Feb. 11, 2009, the IMDb termsof service prohibited, among other things, “data mining, robots, screen scraping, or similar data
gathering and extraction tools.” Internet Movie Database, IMDb Copyright and Conditions of Use,
http://www.imdb.com/help/show_article?conditions (last visited June 12, 2010).
106.
IMDb reports that 57 million users visit its site each month. Internet Movie Database, IMDb
History, http://www.imdb.com/help/show_leaf?history (last visited June 12, 2010).
107.
Netflix Prize Study, supra note 5, at 123.
108.
Id.
109.
Id.
110.
Posting of Steve Lohr, Netflix Awards $1 Million Prize and Starts a New Contest, N.Y. TIMESBITS BLOG, http://bits.blogs.nytimes.com/2009/09/21/netflix-awards-1-million-prize-and-starts-a-newcontest (Sep. 21, 2009, 10:15 EST).
111.
Posting of Ryan Singel, Netflix Spilled Your Brokeback Mountain Secret, Lawsuit Claims, WIREDTHREAT LEVEL BLOG, http://www.wired.com/threatlevel/2009/12/netflix-privacy-lawsuit (Dec. 17,
2009, 16:29 EST).
112.
Id.
113.
Posting of Steve Lohr, Netflix Cancels Contest Plans and Settles Suit, N.Y. TIMES BITS BLOG,
http://bits.blogs.nytimes.com/2010/03/12/netflix-cancels-contest-plans-and-settles-suit (Mar. 12, 2010,
2:46 PM EST).

Broken Promises of Privacy
2.

1723

Reidentification Techniques

How did Sweeney discover William Weld’s diagnoses? How did Barbaro
and Zeller find Thelma Arnold? How did Narayanan and Shmatikov reidentify
the people in the Netflix Prize dataset? Each researcher combined two setsof data—each of which provided partial answers to the question “who does thisdata describe?”—and discovered that the combined data answered (or nearlyanswered) the question.
Even though administrators had removed any data fields they thought
might uniquely identify individuals, researchers in each of the three cases
unlocked identity by discovering pockets of surprising uniqueness remaining
in the data. Just as human fingerprints left at a crime scene can uniquely
identify a single person and link that person with “anonymous” information,
so too do data subjects generate “data fingerprints”—combinations of valuesof data shared by nobody else in their table.114
Of course, researchers have long understood the basic intuition behinda data fingerprint; this intuition lay at the heart of endless debates about
personally identifiable information (PII). What has startled observers about the
new results, however, is that researchers have found data fingerprints in nonPII data, with much greater ease than most would have predicted. It is thiselement of surprise that has so disrupted the status quo. Sweeney realized thesurprising uniqueness of ZIP codes, birth dates, and sex in the U.S. population;
Barbaro and Zeller relied upon the uniqueness of a person’s search queries;
and Narayanan and Shmatikov unearthed the surprising uniqueness of theset of movies a person had seen and rated. These results suggest that maybe
everything is PII to one who has access to the right outside information.
Although many of the details and formal proofs of this work are beyond thescope of this Article, consider a few aspects of the science that are relevant tolaw and policy.
a. The AdversaryComputer scientists model anonymization and reidentification as an adversarial game, with anonymization simply an opening move.115 They call the114.
See BBN Tech., Anonymization & Deidentification, http://www.bbn.com/technology/hci/
security/anon (last visited June 12, 2010) (referring to services to remove ‘“fingerprints’ in the data”).
115.
See Irit Dinur & Kobbi Nissim, Revealing Information While Preserving Privacy, in PROC. 22ND
ACM SYMP. ON PRINCIPLES DATABASE SYS. 202, 203 (2003), available at http://portal.acm.org/citation.
cfm?id=773173.

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57 UCLA LAW REVIEW 1701 (2010)

person trying to reidentify the data the “adversary.”116 They seem not to moralize
the adversary, making no assumptions about whether he or she wants toreidentify for good or ill. The defining feature of the adversary seems to be thathe or she is, no surprise, adversarial—motivated to do something the data
administrator wishes not to happen.
Who are these potential adversaries who might have a motive to reidentify? Narayanan and Shmatikov suggest “stalkers, investigators, nosy colleagues,
employers, or neighbors.”117 To this list we can add the police, national security
analysts, advertisers, and anyone else interested in associating individuals
with data.
b. Outside Information
Once an adversary finds a unique data fingerprint, he can link that data
to outside information, sometimes called auxiliary information.118 Many anonymization techniques would be perfect, if only the adversary knew nothingelse about people in the world. In reality, of course, the world is awash in data
about people, with new databases created every day. Adversaries combine
anonymized data with outside information to pry out obscured identities.
Computer scientists make one appropriately conservative assumption aboutoutside information that regulators should adopt: We cannot predict the typeand amount of outside information the adversary can access.119 It is naïve toassume that the adversary will be unable to find the particular piece of dataneeded to unlock anonymized data.120 In computer security, this discredited121
attitude is called “security through obscurity.” Not only do reidentification
scientists spurn security through obscurity, but they often assume that theadversary possesses the exact piece of data—if it exists—needed to unlock
anonymized identities, in order to design responses that protect identity evenin this worst case.122
116.
Id.
117.
Arvind Narayanan & Vitaly Shmatikov, De-Anonymizing Social Networks, in PROC. 200930TH IEEE SYMP. ON SECURITY & PRIVACY 173, 203 [hereinafter De-Anonymizing Social Networks](for a draft version of this article that includes unpublished appendices, see Narayanan & Shmatikov,
infra note 169).
118.
See Netflix Prize Study, supra note 5, at 112.
119.
Id.
120.
Id.
121.
SIMSON GARFINKEL ET AL., PRACTICAL UNIX AND INTERNET SECURITY 61 (2003)(describing “[t]he problem with security through obscurity”).
122.
Cf. Cynthia Dwork, Differential Privacy, in AUTOMATA, LANGUAGES AND PROGRAMMING,
33RD INT’L COLLOQUIUM PROC. PART II 1, 2 (2006), available at http://www.springerlink.com/content/
383p21xk13841688/fulltext.pdf.

Broken Promises of Privacy

1725

It seems wise to adopt this aggressively pessimistic assumption of perfectoutside information given the avalanche of information now available on theinternet123 and, in particular, the rise of blogs and social networks. Neverbefore in human history has it been so easy to peer into the private diaries ofso many people.124 Alessandro Acquisti and Ralph Gross—researchers whodeveloped an efficient algorithm for using public data to guess people’s social
security numbers125—call this the “age of self-revelation.”126
As only one example among many, in early 2009, many Facebook users
began posting lists called “25 random things about me.”127 The implicit pointof the exercise was to bare one’s soul—at least a little—by revealing secrets
about oneself that friends would not already know.128 “25 random things about
129me” acts like a reidentification virus because it elicits a vast amount of secret
information in a concise, digital format. This is but one example of the richoutside information available on social networking websites. It is no surprisethat several researchers have already reidentified people in anonymized social
networking data.130
c. The Basic Principle: Of Crossed Hands and Inner Joins
One computer security expert summarized the entire field of reidentification to me with a simple motion: He folded his hands together, interleaving
his fingers, like a parishioner about to pray. This simple mental image nicely
summarizes the basic reidentification operation. If you imagine that your left
hand is anonymized data, your right hand is outside information, and yourinterleaved fingers are places where information from the left matches theright, this image basically captures how reidentification is achieved.

123.
See Lakshmanan & Ng, supra note 46, at 13:3 (“The assumption that there is no partial[outside] information out there is simply unrealistic in this Internet era.”).
124.
Cf. De-Anonymizing Social Networks, supra note 117, at 173–74 (describing sharing of
information obtained from social networks).
125.
Alesandro Acquisti & Ralph Gross, Predicting Social Security Numbers from Public Data, 106
NAT’L ACAD. SCI. 10975 (2009).
126.
Acquisti & Gross, supra note 54.
127.
Douglas Quenqua, Ah, Yes, More About Me? Here are ‘25 Random Things’, N.Y. TIMES, Feb.
4, 2009, at E6.
128.
See id.
129.
E.g., Michael Kruse, 25 Random Things About Me to Keep You Caring, ST. PETERSBURG TIMES,
Feb. 23, 2009, available at http://www.tampabay.com/features/humaninterest/article978293.ece.
130.
De-Anonymizing Social Networks, supra note 117, at 177; see also Lars Backstrom, Cynthia
Dwork & Jon Kleinberg, Wherefore Art Thou R3579X? Anonymized Social Networks, Hidden Patterns,
and Structural Steganography, in 16TH INT’L WORLD WIDE WEB CONFERENCE PROC. 181 (2007), available
at http://portal.acm.org/citation.cfm?id=1242598.

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Database administrators call the hand-folding operation an “inner join.”131
An inner join is an operation combining two database tables, connecting
rows from one to rows from the other by matching shared information.132
When the rows in the tables represent people, an inner join assumes thatrows in which critical fields match refer to the same person, and can be
combined into one row in the output table.133 For example, if an adversary has
one table that looks like this:TABLE 5: Anonymized Database
Race

Birth Date

Sex

Black
Black
Black
Black
Black
Black
White
White
White
White
White
White

9/20/1965
2/14/1965
10/23/1965
8/24/1965
11/7/1964
12/1/1964
10/23/1964
3/15/1965
8/13/1964
5/5/1964
2/13/1967
3/21/1967

Male
Male
FemaleFemaleFemaleFemaleMale
Female
Male
Male
Male
Male

ZIP
Code
02141
02141
02138
02138
02138
02138
02138
02139
02139
02139
02138
02138

Complaint
Short of breath
Chest pain
Painful eye
Wheezing
Aching joints
Chest pain
Short of breath
Hypertension
Aching joints
Fever
Vomiting
Back pain

131.
Indeed, in common database systems “INNER JOIN” is the command used to perform suchan operation. See, e.g., ALAN BEAULIEU, LEARNING SQL 77 (2005); ANDY OPPEL & ROBERT SHELDON,SQL: A BEGINNER’S GUIDE 264 (2009); ALLEN G. TAYLOR, SQL ALL-IN-ONE DESK REFERENCE FOR
DUMMIES 309 (2007); PAUL WILTON & JOHN COLBY, BEGINNING SQL 90–93 (2005).
132.
See BEAULIEU, supra note 131.
133.
See id. This simple example necessarily masks some complexity. For example, reidentifiersmust contend with noisy data—errors that cause false positives and false negatives in the inner join.
They use probability theory to spot both of these kinds of errors. See Netflix Prize Study, supra note
5, at 120.

Broken Promises of Privacy

1727

and a separate table that looks like this:TABLE 6: Database Including PII
NameBirth Date

Sex

DanielForestHelen
HilaryKate
Marion2/14/1965
10/23/1964
11/7/1964
3/15/1965
10/23/1965
8/24/1965

Male
Male
Feale
FemaleFemaleFemale

ZIP
Code
02141
02138
02138
02139
02138
02138

Smoker?
Yes
Yes
No
No
No
Yes

and she performs an inner join on the birth date, sex, and ZIP code columns, shewould produce this:TABLE 7: Inner Join of Tables 5 and 6 on Birth Date/ZIP/Sex
Name

Race

Birth Date

Sex

DanielKate
MarionHelen
ForestHilary

Black
Black
Black
Black
White
White

2/14/1965
10/23/1965
8/24/1965
11/7/1964
10/23/1964
3/15/1965

Male
FemaleFemaleFemaleMale
Female

ZIP
Code
02141
02138
02138
02138
02138
02139

Complaint

Smoker?

Chest pain
Painful eye
Wheezing
Aching joints
Short of breath
Hypertension

Yes
No
Yes
No
Yes
No

Notice that with the two joined tables, the sum of the information isgreater than the parts. From the first table alone, the adversary did not knowthat the white male complaining of shortness of breath was Forest, nor did heknow that the person was a smoker. From the second table alone, the adversary
knew nothing about Forest’s visit to the hospital. After the inner join, theadversary knows all of this.
3.

Responding to Objections

In the rest of this Article, I draw many lessons from the three stories
presented above and use these lessons to call for aggressive regulatory
responses to the failure of anonymization. I anticipate, and in some cases I
have confronted, several objections to these interpretations and prescriptions
that deserve responses.

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57 UCLA LAW REVIEW 1701 (2010)a. No Harm, No Foul

The three stories above demonstrate well the power of reidentification,
but they do not demonstrate how reidentification can be used to harm people.
The researchers described are professional journalists or academics, and ethicalrules and good moral judgment limited the harm they caused. But do not be
misled if the results of these studies seem benign. In Part III, I show how thetechniques used in these studies can lead to very real harm, by assembling
chains of inferences connecting individuals to harmful facts.134
b. Examples of Bad Anonymization
Several people have expressed the opinion that the three stories I describe
highlight only the peril of bad anonymization.135 These people have argued
that the State of Massachusetts, AOL, and Netflix should have foreseen the vulnerability of their approaches to anonymization.136 I have many responses.
First, and most fundamentally, the phrase “bad anonymization” is redundant. At least for forget-and-release methods, computer scientists havedocumented theoretical limits about the type of privacy that can be achieved,
137
which I describe below. Although some researchers have developed newtechniques that do better than forget-and-release anonymization, these techniques have significant limitations, and I explore both the techniques andlimitations below.138
Second, the fact that such sophisticated data handlers were responsible
for these three data releases belies the idea that these were the mistakes ofamateurs. Indeed, Netflix boasted about how it perturbed the Netflix Prizedata before it released it to protect privacy.139 Likewise, AOL’s data release was
stewarded by PhDs who seemed aware that they were dealing with sensitive140
information and approved by high-ranking officials. With hindsight it is easy
to argue that these breaches were foreseeable—nobody questions anymore
134.
135.

See infra Part III.A (describing “the database of ruin”).
E.g., Khaled El Emam, Has There Been a Failure of Anonymization?, ELECTRONIC HEALTHINFORMATION & PRIVACY, Aug. 19, 2009, http://ehip.blogs.com/ehip/2009/08/has-there-been-afailure-of-anonymization.html (“Ohm has taken examples of poorly de-identified datasets that werere-identified and drew broad conclusions from those.”).
136.
Id.
137.
See infra Part III.B.1.
138.
See infra Part III.B.2 and III.B.3.
139.
Netflix Prize: FAQ, supra note 92 (“Even if, for example, you knew all your own ratings andtheir dates you probably couldn’t identify them reliably in the data because only a small sample wasincluded (less than one-tenth of our complete dataset) and that data was subject to perturbation.”).
140.
Zeller, Jr., supra note 78.

Broken Promises of Privacy

1729

whether search queries can be used to identify users—but the past failure offoresight by sophisticated data handlers should give us pause about present
claims of bad anonymization.
Third, when one considers the mistakes that have been made by sophisticated data handlers, one can begin to imagine the mistakes being made by
the legions of less-sophisticated data handlers, the thousands of IT professionals
with no special training in anonymization who are responsible for anonymizing millions of corporate databases. Even if we can divide anonymization
cases into good and bad piles, it is safe to assume that the bad towers over the
good.
Finally, even if we could teach every data handler in the world how to
avoid the mistakes of the past—a daunting and expensive proposition—our
new, responsible approach to anonymization would still do nothing to protect
all of the data anonymized in the past. Database owners could reanonymize
databases they still controlled, but they would not be able to secure the data
they shared or redistributed in the past.
c. The Problem of Public Release
It would also be a mistake to conclude that the three stories demonstrate
only the peril of public release of anonymized data. Some might argue that
had the State of Massachusetts, AOL and Netflix kept their anonymized data tothemselves, or at least shared the data much less widely, we would not have had
to worry about data privacy.
There is obviously some logic to this objection. In Part IV, I argue that
regulators should treat publicly released data differently than privately used
data.141
On the other hand, we should not be surprised that we learned thelessons of reidentification only after public releases of data. Reidentification
researchers can only reidentify that which they can access. But other people
with access to less-public information might be reidentifying in private, keeping
the results to themselves. Any time data is shared between two private parties,
we should worry about the possibility of reidentification.
Moreover, we must not forget that anonymization is also used by companies as an internal privacy control—to allow Department A to share datawith Department B without breaching customer privacy.142 Just because data is
kept wholly within a company does not put to rest concerns about expectations
141.
142.

Infra Part IV.C.1.
See supra notes 16–17 and accompanying text.

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57 UCLA LAW REVIEW 1701 (2010)of privacy. If a company promises, for example, to share behavioral data withits marketing arm only in anonymized form, we should worry that the powerof easy reidentification gives the company the tools needed to break thatpromise.
d. The Myth of the Superuser
Finally, some might object that the fact that reidentification is possible
does not necessarily make it likely to happen. In particular, if there are no
motivated, skilled adversaries, then there is no threat. I am particularly sensitive to this objection, because I have criticized those who try to influence policy
by exploiting fears of great power, a tactic that relies on what I have called
the “Myth of the Superuser.”143
The power of reidentification, however, is not a Myth of the Superuser
story for three reasons: First, reidentification techniques are not Superuser techniques. The Netflix study reveals that it is startlingly easy to reidentify people
in anonymized data.144 Although the average computer user cannot perform
an inner join, most people who have taken a course in database management
or worked in IT can probably replicate this research using a fast computerand widely available software like Microsoft Excel or Access.145 Second, theAOL release reminds us about the power of a small group of bored bloggers.
And third, there are great financial motivations pushing people to reidentify.146
Moreover, I did not claim that feats of great power never happen online.
Such a conclusion is provably false. Instead, I argued that because it is so easy
to exaggerate power, we should hold those offering stories about online power totry to influence policy to a high standard of proof.147 I concede that my claim ofreidentification power should be held to the high standard of proof, and I
argue that I have met that standard.

143.
See generally Paul Ohm, The Myth of the Superuser: Fear, Risk, and Harm Online, 41 U.C.
DAVIS L. REV. 1327 (2008).
144.
Netflix Prize Study, supra note 5, at 112.
145.
The INNER JOIN command is taught in beginner database texts. See, e.g., OPPEL &SHELDON, supra note 131; TAYLOR, supra note 131, at 309; WILSON & COLBY, supra note 131, at 501.
146.
See Salvador Ochoa et al., Reidentification of Individuals in Chicago’s Homicide Database: A
Technical Legal Study (unpublished student paper) (2001), available at http://web.mit.edu/sem083/www/assignments/reidentification.html (discussing financial motives pressing people to reidentify including
those affecting marketers and blackmailers).
147.
See Ohm, supra note 143, at 1402.

Broken Promises of Privacy
4.

1731

The Intuition Gap

What each of the foregoing objections highlights is the gap in intuition
that persists among privacy experts. These privacy experts, primarily lawyers andbusiness executives charged with protecting their companies’ users, clients,
and customers, cling to the idea that although anonymization may be weaker
than we assumed, it has not failed. They may concede the need to change
privacy policies or invest a bit more heavily in technology and expertise in
response to the studies cited above, but they hope they need only small tweaks
like these and not overhauls.
In the meantime, I predict that computer scientists and talented amateurs
will continue to release new examples of powerful reidentification, witheach announcement shaking those who still cling to false faiths. As have thepast announcements, these future announcements will surprise experts by
how cheaply, quickly, and easily supposedly robust anonymization will fall. I
make these predictions confidently, because the power of reidentification traces
two curves, both moving upward incessantly: the power of computer hardware
and the richness of outside information.
The future of anonymization and reidentification thus promises years of
awkward transition, as the privacy experts on the wrong side of the intuition
gap weaken and then finally abandon their faith in anonymization. It may
take years—maybe five, maybe more—before most privacy experts accept thatthey should abandon faith in anonymization, and these will be years filledwith dashed hopes and recalibrated expectations. The gap will probably takelonger to close than it fairly should, as companies and other interests vested
in the cheap, easy promises of anonymization will try to convince others topersist in their faith despite the evidence.
The rest of this Article will mostly skip past the coming, painful years oftransition while the intuition gap closes. Instead, it will plan for what happens
next, after the intuition gap closes, once we realize that anonymization has
failed. What does the failure of anonymization mean for privacy law?

II.

HOW THE FAILURE OF ANONYMIZATION DISRUPTSPRIVACY LAW

Policymakers cannot simply ignore easy reidentification, because for decades they enacted laws and regulations while laboring under the robust
anonymization assumption. They must now reexamine every privacy law
and regulation to see if the easy reidentification result has thwarted their
original designs.

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57 UCLA LAW REVIEW 1701 (2010)

Modern privacy laws tend to act preventatively, squeezing down the flowof particular kinds of information in order to reduce predictable risks of harm.
In order to squeeze but not cut off valuable transfers of information, legislators
have long relied on robust anonymization to deliver the best of both worlds:
the benefits of information flow and strong assurances of privacy. The failureof anonymization has exposed this reliance as misguided, throwing carefullybalanced statutes out of equilibrium.
At the very least, legislators must abandon the idea that we protect
privacy when we do nothing more than identify and remove PII. The idea that
we can single out fields of information that are more linkable to identity thanothers has lost its scientific basis and must be abandoned.
A.

The Evolution of Privacy Law

In the past century, the regulation of privacy in the United States andEurope has evolved from scholarly discussion, to limited common law torts, tobroad statutory schemes. Before deciding how to respond to the rise of easy
reidentification, we must recognize three themes from this history of privacy
law. First, while privacy torts focus solely on compensating injured victims ofprivacy harms, more recent privacy statutes shift the focus from post hoc
redress to problem prevention. Second, this shift has led to the hunt for PII
through quasi-scientific exercises in information categorization. Third, legislatures have tried to inject balance into privacy statutes, often by relying on
robust anonymization.
1.

The Privacy Torts: Compensation for Harm

Most legal scholars point to a celebrated nineteenth-century law review
article by Samuel Warren and Louis Brandeis, The Right to Privacy,148 as thewellspring of information privacy law. In the article, Warren and Brandeis,
alarmed by the rise of tabloid journalism, advocated a new right of privacy,
urging courts to allow plaintiffs to bring new privacy torts.149 The conceptof harm—intangible, incorporeal harm to mere feelings, but harm all thesame—loomed large in the article. For example, Warren and Brandeis
describe victims of privacy deprivations as experiencing “mental suffering,”150“mental pain and distress, far greater than could be inflicted by mere bodily
148.
Samuel D. Warren & Louis D. Brandeis, The Right to Privacy, 4 HARV. L. REV. 193 (1890).
149.
Irwin R. Kramer, The Birth of Privacy Law: A Century Since Warren and Brandeis, 39 CATH.
U. L. REV. 703, 709 (1990).
150.
Warren & Brandeis, supra note 148, at 213.

Broken Promises of Privacy

1733

injury,”151 and “injury to the feelings.”152 That the authors focused on harm isunsurprising because the entire article is a call for “[a]n action of tort for damages
in all cases.”153
Seventy years later, William Prosser synthesized the case law inspired byWarren and Brandeis into the four privacy torts commonly recognized in U.S.
jurisdictions today: (1) intrusion upon the plaintiff’s seclusion or solitude, orinto his private affairs, (2) public disclosure of embarrassing private facts
about the plaintiff, (3) publicity that places the plaintiff in a false light in the
public eye, and (4) appropriation, for the defendant’s advantage, of the plaintiff’sname or likeness.154 All four require actual injury, as do all torts.155
2.

Shift to Broad Statutory Privacy: From Harm to Prevention and PII

Courts took the lead during the evolution of the privacy torts,156 while
legislatures stayed mostly in the background, doing little more than occasionally codifying privacy torts.157 Then, about forty years ago, legislatures began
to move to the forefront of privacy regulation, enacting sweeping new statutory
privacy protections. The fear of computerization motivated this shift.
In the 1960s, the U.S. government began computerizing records about
its citizens, combining this data into massive databases. These actions sparked
great privacy concerns.158 Throughout the decade, commentators describedthreats to privacy from computerization and helped defeat several government
proposals.159 Spurred by this, in 1973 an advisory committee created by thesecretary of health, education, and welfare issued a report that proposed a
new framework called “Fair Information Principles” (FIPS).160 The FIPS have151.
Id. at 196.
152.
Id. at 197.
153.
Id. at 219.
154.
William L. Prosser, Privacy, 48 CAL. L. REV. 383 (1960). Prosser was also the reporter forthe second Restatement of Torts, in which he also promulgated his four privacy torts. RESTATEMENT(SECOND) OF TORTS § 652B (1977).
155.
W. PAGE KEETON ET AL., PROSSER & KEETON ON TORTS 5 (5th ed. 1984) (defining torts
as “a body of law which is directed toward the compensation of individuals . . . for losses which they
have suffered”).
156.
Prosser, supra note 154, at 386–89.
157.
E.g., N.Y. CIV. RIGHTS LAW §§ 50–51 (McKinney 2007).
158.
PRISCILLA M. REGAN, LEGISLATING PRIVACY: TECHNOLOGY, SOCIAL VALUES, AND
PUBLIC POLICY 82 (1995).
159.
Daniel J. Solove, A Taxonomy of Privacy, 154 U. PA. L. REV. 477, 506–07 & nn.138–45(2006).
160.
U.S. DEP’T OF HEALTH, EDUC., & WELFARE, RECORDS, COMPUTERS, AND THE RIGHTS
OF CITIZENS (1973).

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57 UCLA LAW REVIEW 1701 (2010)

been enormously influential, inspiring statutes,161 law review articles,162 andmultiple refinements.163
FIPS require a data protection scheme that provides, among other things,
notice and consent, access, data integrity, enforcement, and remedies,164 but
for the present discussion, what the FIPS say is less important than what the
FIPS wrought: a very different approach to privacy law, one that embraces
rights of privacy that do more than solely redress past harm. Influenced by
the FIPS, legislatures have enacted statutes designed to avoid “privacy problems”
that have nothing to do with the “injury to feelings” at the heart of theprivacy torts. As Dan Solove puts it, “These problems are more structural innature. . . . They involve less the overt insult or reputational harm to a person
and more the creation of the risk that a person might be harmed in the future.”165
Thus, beginning in the 1970s, Congress began to enact statutes designed
to reduce the risk of harm. Congress’s approach for crafting these laws is best
described as Linnaean. After first identifying a problem—“a risk that aperson might be harmed in the future”166—lawmakers try to enumerate andcategorize types of information that contribute to the risk. They categorize
on a macro level (distinguishing between health information, education information, and financial information) and on a micro level (distinguishing between
names, account numbers, and other specific data fields). Through this process,they have filled many pages of the U.S. Code with taxonomies of information
types that deserve special treatment because of their unusual tendency tocause harm.167
Congress has thus embraced a wholly data-centric approach, the PII
approach, to protecting privacy. This approach assumes that lawmakers canevaluate the inherent riskiness of data categories, assessing with mathematicalprecision whether or not a particular data field contributes to the problem
enough to be regulated. In doing so, it tends to ignore messier, human factors
161.
E.g., The Privacy Act of 1974 “requires agencies to follow the Fair Information Practices
when gathering and handling personal data.” Daniel J. Solove & Chris Jay Hoofnagle, A Model
Regime of Privacy Protection, 2006 U. ILL. L. REV. 357, 361 (citing 5 U.S.C. § 552a(e) (2000)).
162.
E.g., Marc Rotenberg, Fair Information Practices and the Architecture of Privacy (What Larry
Doesn’t Get), 2001 STAN. TECH. L. REV. 1; Paul M. Schwartz, Preemption and Privacy, 118 YALE L.J.
902, 906–22 pt. I (2009).
163.
ORGANISATION FOR ECONOMIC COOPERATION & DEV., OECD GUIDELINES ON THE
PROTECTION OF PRIVACY AND TRANSBORDER FLOWS OF PERSONAL DATA (2001), available at
http://www.uhoh.org/oecd-privacy-personal-data.PDF; Federal Trade Commission, Fair Information
Practice Principles, http://www.ftc.gov/reports/privacy3/fairinfo.shtm (last visited June 12, 2010).
164.
Federal Trade Commission, supra note 163.
165.
Solove, supra note 159, at 487–88.
166.
Id.
167.
See infra notes 203–207 (giving examples of statutes that list categories of information).

Broken Promises of Privacy

1735

that should also factor into a risk assessment, such as the likelihood thatsomeone will be motivated enough to care about a particular dataset.168
It is necessary, however, to distinguish between two very different
legislative motivations for singling out categories of information. The easy reidentification result calls into question only the second of these motivations.
First, some statutes restrict sensitive information, the kind of information thatcauses fully-realized harm when disclosed.169 For example, the Driver’s Privacy
Protection Act (DPPA) singles out “highly restricted personal information,”including sensitive categories like “photograph” and “medical or disability
information.”170 Easy reidentification has not disrupted the logic of provisions
like this one. Even though robust anonymization has failed, it still makes
sense to treat specially those kinds of information that can be used directly tocause harm.
In contrast, lawmakers often single out categories of data for special
treatment under the mistaken belief that these categories (and only these)
increase the linkability of anonymized data. For instance, the DPPA singles
out a second category of personal information, including linkable data fields
like social security number and driver identification number, for special, butless restrictive, treatment.171 The law implicitly assumes that this list includes
every data field that can link database records to identity—but easy reidentification proves otherwise. When legislators focus on linkability andidentifiability in this way, they enshrine release-and-forget, deidentification,
PII-removal approaches to anonymization into law. This approach to legislation
makes little sense in light of the advances in easy reidentification.
3.

How Legislatures Have Used Anonymization to Balance Interests

Writing about the privacy torts, William Prosser said that “[i]n determining
where to draw the line the courts have been invited to exercise nothing lessthan a power of censorship over what the public may be permitted to read.”172
So too is every privacy statute an “exercise [in] the power of censorship.”173
These laws restrict the free flow of information. This should give lawmakers
168.
See infra Part IV.B (discussing motive).
169.
Arvind Narayanan & Vitaly Shmatikov, De-Anonymizing Social Networks, http://userweb.cs.
utexas.edu/~shmat/shmat_oak09.pdf, app. B (last visited June 12, 2010) (noting that some laws single
out information that “itself is sensitive,” while others seek to prevent “deductive disclosure”). This
paper was later published without appendices. See De-Anonymizing Social Networks, supra note 117.
170.
18 U.S.C. § 2725(3)-(4) (2006).
171.
Id.
172.
Prosser, supra note 154, at 413.
173.
Id.

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great pause. The free flow of information fuels the modern economy, nourishes
our hunger for knowledge, shines a light on the inner workings of powerful
institutions and organizations, and represents an exercise of liberty.174 Before
enacting any privacy law, lawmakers should weigh the benefits of unfetteredinformation flow against its costs and must calibrate new laws to impose
burdens only when they outweigh the harms the laws help avoid.
But for the past forty years, legislators have deployed a perfect, silver
bullet solution—anonymization—that has absolved them of the need toengage in overt balancing. Anonymization liberated lawmakers by letting
them gloss over the measuring and weighing of countervailing values likesecurity, innovation, and the free flow of information. Regardless of whether
those countervailing values weighed heavily, moderately, or barely at all, theywould always outweigh the minimized risk to privacy of sharing anonymized
data, which lawmakers believed to be almost nil thanks to anonymization.
The demise of robust anonymization will throw the statutes legislatures havewritten out of balance, and lawmakers will need to find a new way to regainbalance lost.
Consider how legislatures in two jurisdictions have relied upon
anonymization to bring supposed balance to privacy law: the U.S.’s Health
Insurance Portability and Accountability Act (HIPAA) and the EU’s Data
Protection Directive.
a. How HIPAA Used Anonymization to Balance Health Privacy
In 1996, the U.S. Congress enacted the Health Insurance Portability
and Accountability Act (HIPAA), hoping to improve healthcare and healthinsurance in this country.175 Among the other things it accomplishes, HIPAA
is a significant privacy law. Title II of the Act mandates compliance withhealth privacy regulations, which have been promulgated by the Department

174.
See Kent Walker, Where Everybody Knows Your Name: A Pragmatic Look at the Costs ofPrivacy and the Benefits of Information Exchange, 2000 STAN. TECH. L. REV. 2, 7–21 (enumerating thebenefits of shared information).
175.
Pub. L. No. 104-191, 110 Stat. 1936 (1996). According to the preamble to the Act, thepurpose of HIPAA is:
To amend the Internal Revenue Code of 1986 to improve portability and continuity of healthinsurance coverage in the group and individual markets, to combat waste, fraud, and abusein health insurance and health care delivery, to promote the use of medical savings accounts,
to improve access to long-term care services and coverage, to simplify the administration ofhealth insurance, and for other purposes.
Id.

Broken Promises of Privacy

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of Health and Human Services (HHS) and are now known as the HIPAA
Privacy Rule.176
In many ways, the HIPAA Privacy Rule represents the high-water markfor use of PII to balance privacy risks against valuable uses of information.177
HIPAA demonstrates Congress’s early sensitivity to the power of reidentification, through its treatment of what it calls the “de-identification of healthinformation” (DHI).178 HIPAA itself exempts data protected by DHI from179
any regulation whatsoever, but defines DHI so as to allow for further regulatoryinterpretation—and HHS has used this statutory mandate to define DHI as
information that “does not identify an individual” nor provide “a reasonablebasis to believe that the information can be used to identify an individual.”180
HHS’s Privacy Rule elaborates this vague reasonability standard further
in two alternate ways. First, under the so-called “statistical standard,” data is
DHI if a statistician or other “person with appropriate knowledge . . . andexperience” formally determines that the data is not individually identifiable.181
Second, data is DHI under the so-called “safe harbor standard” if the covered
182
entity suppresses or generalizes eighteen enumerated identifiers. The Privacy
Rule’s list is seemingly exhaustive—perhaps the longest such list in any privacy regulation in the world. Owing to the release of Dr. Sweeney’s study
around the same time, the Privacy Rule requires the researcher to generalize
birth dates to years183 and ZIP codes to their initial three digits.184
Congress and HHS concluded simply that by making data unidentifiable,
health professionals could trade sensitive information without impinging on
patient privacy. Moreover, they froze these conclusions in amber, enumerating
a single, static list, one they concluded would protect privacy in all healthprivacy contexts.185 In promulgating the Privacy Rule, regulators relied on their176.
Id. § 264 (directing the secretary of Health and Human Services to submit standards forprotecting privacy); HIPAA Privacy Rule, 45 C.F.R. §§ 160, 164 (2009).
177.
Jay Cline, Privacy Matters: When Is Personal Data Truly De-Identified?, COMPUTERWORLD,
July 24, 2009, http://www.computerworld.com/s/article/9135898/Privacy_matters_When_is_personal_
data_truly_de_identified (“No other country has developed a more rigorous or detailed guidance for
how to convert personal data covered by privacy regulations into non-personal data.”). HIPAA is not
the most recent information privacy law enacted in the U.S. See, e.g., Gramm-Leach-Bliley Act of
1999, Pub. L. No. 106-102, (codified as 15 U.S.C. §§ 6801–6809 (2006)); Children’s Online Privacy
Protection Act of 1998, Pub. L. No. 106-170, (codified as 15 U.S.C. §§ 6501–6506 (2006)).
178.
See 45 C.F.R. §§ 164.502(d)(2), 164.514(a), (b) (2009).
179.
Id.
180.
Id. § 164.514(a).
181.
Id. § 164.514(b)(1).
182.
Id. § 164.514(b)(2).
183.
Id. § 164.514(b)(2)(C).
184.
Id. § 164.514(b)(2)(B) (allowing only two digits for ZIP codes with 20,000 or fewer residents).
185.
Since promulgating the safe harbor list almost a decade ago, HHS has never amended it.

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faith in the power of anonymization as a stand-in for a meaningful costbenefit balancing. This is an opportunity lost, because it is hard to imagine
another privacy problem with such starkly presented benefits and costs. On
one hand, free exchange of information among medical researchers can help
them develop treatments to ease human suffering and save lives. On the other
hand, medical secrets are among the most sensitive we hold. It would havebeen quite instructive to see regulators explicitly weigh such stark choices.
By enumerating eighteen identifiers, the Privacy Rule assumes that anyother information that might be contained in a health record cannot be used
to reidentify. We now understand the flaw in this reasoning, and we should
consider revising the Privacy Rule as a result.186
b. How the EU Data Protection Directive Used Anonymization
to Balance Internet Privacy
EU lawmakers have also relied upon the power of anonymization to avoid
difficult balancing questions. Unlike the American approach with HIPAA,
however, the EU enacted a broad, industry-spanning law,187 the Data Protection
Directive, which purports to cover any “personal data” held by any data
administrator.188 Data is personal data if it can be used to identify someone“directly or indirectly, in particular by reference to an identification number
or to one or more factors specific to his physical, physiological, mental, economic, cultural or social identity.”189
The EU never intended the Directive to apply to all data. Instead, it
meant for “personal data” to exclude at least some data—data that was not“directly or indirectly” identifiable, such as anonymized data—from regulation.
Like their U.S. counterparts, EU lawmakers imagined they could strike abalance through the power of technology. If anonymization worked, data
administrators could freely share information so long as data subjects were nolonger “directly or indirectly” identifiable. With this provision, EU lawmakers
sought to preserve space in society for the storage and transfer of anonymized
data, thereby providing room for unencumbered innovation and free expression.

186.
See infra Part IV.D.1.
187.
The Directive obligates EU countries to transpose its rules into domestic laws within a
set time frame. Eur. Comm’n Justice & Home Affairs, Transposition of the Data Protection Directive,
http://ec.europa.eu/justice_home/fsj/privacy/lawreport/index_en.htm (last visited June 12, 2010).
188.
EU Data Protection Directive, supra note 3, art. 2(a).
189.
Id.

Broken Promises of Privacy

1739

Whether and to what extent the Directive retains such a preserve has been
debated in the internet privacy context.190 For several years, the EU has clashedwith companies like Google, Yahoo, and Microsoft over what they must do toprotect databases that track what their users do online.191 Much of this debate
has turned on what companies must do with stored IP addresses. An IP address
is a numeric identifier assigned to every computer on the internet.192 Just as asocial security number identifies a person, an IP address identifies a computer,
193
Every
so an IP address can tie online conduct to location and identity.
194
computer reveals its IP address to every other computer it contacts, so every
195
time I visit Google, my computer reveals its IP address to a Google computer.
Following longstanding industry practice, Google records my IP address alongwith details about what I am doing when using Google’s services.196
Google has argued to the EU that it protects the privacy of its users using
anonymization, by throwing away part, not all, of every IP address.197 Specifically, an IP address is composed of four equal pieces called octets,198 and Google
stores the first three octets and deletes the last, claiming that this practice199
protects user privacy sufficiently. Google’s competitors, Microsoft and Yahoo,
are much more thorough, throwing away entire IP addresses.200
At its core, this too is a debate about balance—between the wonderful
innovations Google promises it can deliver by studying our behavior,201 and the190.
See, e.g., Frederick Lah, Note, Are IP Addresses “Personally Identifiable Information?”, 4 I/S:
J.L. & POL’Y FOR INFO. SOC’Y 681 (2008).
191.
E.g., Posting of Saul Hansell, Europe: Your IP Address Is Personal, N.Y. TIMES BITS BLOG,
http://bits.blogs.nytimes.com/2008/01/22/europe-your-ip-address-is-personal (Jan. 22, 2008).
192.
DOUGLAS COMER, 1 INTERNETWORKING WITH TCP/IP 42 (5th ed. 2006).
193.
Id. at 43–44.
194.
Id. at 35–36.
195.
Id.
196.
SIMSON GARFINKEL & GENE SPAFFORD, WEB SECURITY, PRIVACY AND COMMERCE 211(2002).
197.
Letter From Google to Congressman Joe Barton 14–15 (Dec. 21, 2007), available at
http://searchengineland.com/pdfs/071222-barton.pdf.
198.
COMER, supra note 192, at 53.
199.
Letter From Google to Congressman Joe Barton, supra note 197, at 14–15.
200.
Behavioral Advertising: Industry Practice and Consumers’ Expectations, Hearings Before the H.
Comm. on Energy and Commerce, Subcomm. on Communications, Technology and the Internet and Subcomm.
on Commerce, Trade and Consumer Protection, 111th Cong. 1 (2009) (statement of Anne Toth, Headof Privacy, Yahoo! Inc.); Posting of Peter Cullen, Chief Privacy Strategist at Microsoft, Microsoft Privacy& Safety, Microsoft Supports Strong Industry Search Data Anonymization Standards, MICROSOFT PRIVACY
AND SAFETY BLOG, http://blogs.technet.com/privacyimperative/archive/2008/12/08/microsoft-supportsstrong-industry-search-data-anonymization-standards.aspx (Dec. 8, 2008).
201.
In 2008, to try to placate those worried about privacy, Google authored a series of blog
posts “about how [they] harness the data [they] collect to improve [their] products and services for [their]users.” E.g., Posting of Matt Cutts, Software Engineer, Using Data to Fight Webspam, THE OFFICIAL

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possible harm to users whose IP addresses are known or revealed. Again, claims
that we should trust robust anonymization stand in for nuanced, careful costbenefit balancing arguments. Google promises we can have our cake while iteats it too—by placing our trust in data anonymization.
B.

How the Failure of Anonymization Disrupts Privacy Law

In addition to HIPAA and the EU Data Protection Directive, almost
every single privacy statute and regulation202 ever written in the U.S. andthe EU embraces—implicitly or explicitly, pervasively or only incidentally—theassumption that anonymization protects privacy, most often by extending safe
harbors from penalty to those who anonymize their data. At the very least,
regulators must reexamine every single privacy law and regulation. The lossof robust anonymization reveals the lurking imbalance in these privacy laws,
sometimes shifting in favor of protecting privacy too much and sometimes
favoring the flow of information too much.
Easy reidentification makes PII-focused laws like HIPAA underprotectiveby exposing the arbitrariness of their intricate categorization and line drawing.
Although HIPAA treats eighteen categories of information as especially
identifying,203 it excludes from this list data about patient visits—like hospital
name, diagnosis, year of visit, patient’s age, and the first three digits of ZIP
code—that an adversary with rich outside information can use to defeat
anonymity.
Many other laws follow the same categorization-and-line-drawing
approach. The Driver’s Privacy Protection Act requires special handling for“personal information” including, among other things, “social security number, driver identification number, name, address . . . , [and] telephone
number,”204 while requiring much less protection of “the 5-digit zip code” and“information on vehicular accidents, driving violations, and driver’s status.”205
Similarly, the Federal Education Rights and Privacy Act (FERPA) singles out
for protection “directory information,” including, among other things, “name,GOOGLE BLOG, http://googleblog.blogspot.com/2008/06/using-data-to-fight-webspam.html (June 27,
2008, 4:51 EST) (linking to earlier posts in the series).
202.
In this Article, I focus on statutes and regulations for several reasons. First, these rules provide
a concrete set of texts about which I can make correspondingly concrete observations. Second, Americanand European approaches to privacy legislation differ somewhat, providing a comparative study. Third,
when it comes to dictating how information is collected, analyzed, and disclosed in modern life, noother source of law has the influence of privacy statutes and regulations.
203.
45 C.F.R. §§ 164.502(d)(2), 164.514(a), (b) (2009).
204.
18 U.S.C. § 2725(3) (2006).
205.
Id.

Broken Promises of Privacy

1741

address, telephone listing, date and place of birth, [and] major field of study.”206
Federal Drug Administration regulations permit the disclosure of “records
about an individual” associated with clinical trials “[w]here the names andother identifying information are first deleted.”207 These are only a few of manylaws that draw lines and make distinctions based on the linkability of information. When viewed in light of the easy reidentification result, these provisions,like HIPAA, seem arbitrary and underprotective.
In contrast, easy reidentification makes laws like the EU Data Protection
Directive overbroad—in fact, essentially boundless. Because the Directive
turns on whether information is “directly or indirectly” linked to a person,208
each successful reidentification of a supposedly anonymized database extends
the regulation to cover that database. As reidentification science advances, it
expands the EU Directive like an ideal gas to fit the shape of its container. A
law that was meant to have limits is rendered limitless, disrupting the careful
legislative balance between privacy and information and extending datahandling requirements to all data in all situations.
Notice that the way the easy reidentification result disrupts the Directiveis the mirror image of the way it impacts HIPAA. Easy reidentification makes
the protections of HIPAA illusory and underinclusive because it deregulates thehandling of types of data that can still be used to reidentify and harm. On
the other hand, easy reidentification makes laws like the EU Data Protection
Directive boundless and overbroad. We should tolerate neither result because
both fail to achieve the balance that was originally at the heart of both typesof laws.
Most privacy laws match one of these two forms. Even the few that do
not fit neatly into one category or the other often contain terms that are madeindeterminate and unpredictable by easy reidentification. As one example,
the Stored Communications Act in the U.S. applies to “record[s] or otherinformation pertaining to a subscriber . . . or customer,” without specifying
what degree of identifiability makes a record “pertain.”209 As reidentification
science advances, courts will struggle to decide whether anonymized records
fall within this definition. The vagueness of provisions like this will invite costly
litigation and may result in irrational distinctions between jurisdictions andbetween laws.

206.
207.
208.
209.

20 U.S.C. § 1232g(a)(5)(A) (2006).
21 C.F.R. § 21.70(a)(3)(i) (2009).
EU Data Protection Directive, supra note 3, art. 2(a).
18 U.S.C. § 2702(c) (2006).

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57 UCLA LAW REVIEW 1701 (2010)C.

The End of PII

1.

Quitting the PII Whack-a-Mole GameAt the very least, we must abandon the pervasively held idea that wecan protect privacy by simply removing personally identifiable information(PII). This is now a discredited approach. Even if we continue to follow it inmarginal, special cases, we must chart a new course in general.
The trouble is that PII is an ever-expanding category. Ten years ago,almost nobody would have categorized movie ratings and search queries as
PII, and as a result, no law or regulation did either.210 Today, four years after
computer scientists exposed the power of these categories of data to identify,
no law or regulation yet treats them as PII.
Maybe four years has not been enough time to give regulators the
chance to react. After all, HIPAA’s Privacy Rule, which took effect in 2003,
does incorporate Dr. Sweeney’s research, conducted in the mid-1990s.211 It
expressly recognizes the identifying power of ZIP code, birth date, and sex,
and carves out special treatment for those who delete or modify them, alongwith fifteen other categories of information.212 Should this be the model offuture privacy law reform—whenever reidentification science finds fields of datawith identifying power, should we update our regulations to encompass the
new fields? No. This would miss the point entirely.
HIPAA’s approach to privacy is like the carnival whack-a-mole game:
As soon as you whack one mole, another will pop right up. No matter how
effectively regulators follow the latest reidentification research, folding newly
identified data fields into new laws and regulations, researchers will always findmore data field types they have not yet covered.213 The list of potential PII
214
will never stop growing until it includes everything.
Consider another reidentification study by Narayanan and Shmatikov.215
The researchers have reidentified anonymized users of an online social network
based almost solely on the stripped-down graph of connections between
210.
The Video Privacy Protection Act, enacted in 1988, protects lists of movies watched not
because they are PII, but because they are sensitive. 18 U.S.C. § 2710 (2006). For more on the distinction, see supra Part II.A.2.
211.
See supra Part I.B.1.b (describing Sweeney’s research).
212.
45 C.F.R. §§ 164.502(d)(2), 164.514(a)-(b) (2009).
213.
See Narayanan & Shmatikov, supra note 169 (“While some data elements may be uniquely
identifying on their own, any element can be identifying in combination with others.”).
214.
Cf. id.; Dinur & Nissim, supra note 115, at 202 (“[T]here usually exist other means ofidentifying patients, via indirectly identifying attributes stored in the database.”).
215.
See Narayanan & Shmatikov, supra note 169.

Broken Promises of Privacy

1743

people.216 By comparing the structure of this graph to the nonanonymized
graph of a different social network, they could reidentify many people evenignoring almost all usernames, activity information, photos, and every othersingle piece of identifying information.217
To prove the power of the method, the researchers obtained and anonymized the entire Twitter social graph, reducing it to nameless, identity-freenodes representing people connected to other nodes representing Twitter’s“follow” relationships. Next, they compared this mostly deidentified husk
of a graph218 to public data harvested from the Flickr photo-sharing socialnetwork site. As it happens, tens of thousands of Twitter users are also
Flickr users, and the researchers used similarities in the structures of Flickr’s“contact” graph and Twitter’s “follow” graph to reidentify many of theanonymized Twitter user identities. With this technique, they could reidentify
the usernames or full names of one-third of the people who subscribed to both
Twitter and Flickr.219 Given this result, should we add deidentified husks ofsocial networking graphs—a category of information that is almost certainly220
unregulated under U.S. law, yet shared quite often —to the HIPAA Privacy
Rule list and to the lists in other PII-focused laws and regulations? Of course not.
Instead, lawmakers and regulators should reevaluate any law or regulationthat draws distinctions based solely on whether particular data types can belinked to identity, and should avoid drafting new laws or rules grounded insuch a distinction. This is an admittedly disruptive prescription. PII has longserved as the center of mass around which the data privacy debate hasorbited.221 But although disruptive, this proposal is also necessary. Too often,
the only thing that gives us comfort about current data practices is that an
administrator has gone through the motions of identifying and deleting PII—
and in such cases, we deserve no comfort at all. Rather, from now on weneed a new organizing principle, one that refuses to play the PII whack-amole game. Anonymization has become “privacy theater”;222 it should nolonger be considered to provide meaningful guarantees of privacy.
216.
See De-Anonymizing Social Networks, supra note 117, at 182–85.
217.
Id. at 184.
218.
Id. To make their study work, the researchers first had to “seed” their data by identifying
150 people who were users of both Twitter and Flickr. They argue that it would not be very difficult
for an adversary to find this much information, and they explain how they can use “opportunistic seeding”
to reduce the amount of seed data needed. Id. at 181–85.
219.
Id.
220.
Id. at 174–75 (surveying examples of how social-network data is shared).
221.
See Leslie Ann Reis, Personally Identifiable Information, in 2 ENCYCLOPEDIA OF PRIVACY
383–85 (William G. Staples ed., 2006).
222.
Paul M. Schwartz, Reviving Telecommunications Surveillance Law, 75 U. CHI. L. REV. 287,
310–15 (2008) (developing the concept of privacy theater).

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57 UCLA LAW REVIEW 1701 (2010)

Abandoning “Anonymize” and “Deidentify”

We must also correct the rhetoric we use in information privacy debates.
We are using the wrong terms, and we need to stop. We must abolish the wordanonymize;223 let us simply strike it from our debates. A word that shouldmean, “try to achieve anonymity” is too often understood to mean “achieve
anonymity,” among technologists and nontechnologists alike. We need aword that conjures effort, not achievement.
Latanya Sweeney has similarly argued against using forms of the word“anonymous” when they are not literally true.224 Dr. Sweeney instead uses “deidentify” in her research. As she defines it, “[i]n deidentified data, all explicit
identifiers, such as SSN, name, address, and telephone number, are removed,
generalized, or replaced with a made-up alternative.”225 Owing to her influence,
the HIPAA Privacy Rule explicitly refers to the “de-identification of protected
health information.”226
Although “deidentify” carries less connotative baggage than “anonymize,”
which might make it less likely to confuse, I still find it confusing. “Deidentify”
describes release-and-forget anonymization, the kind called seriously into
question by advances in reidentification research. Despite this, many treat
claims of deidentification as promises of robustness,227 while in reality, people
can deidentify robustly or weakly.228 Whenever a person uses the unmodified
word “deidentified,” we should demand details and elaboration.
Better yet, we need a new word for privacy-motivated data manipulationthat connotes only effort, not success. I propose “scrub.” Unlike “anonymize”
or “deidentify,” it conjures only effort. One can scrub a little, a lot, not enough,223.
Anonymize is a relatively young word. The Oxford English Dictionary traces the first useof the word “anonymized” to 1972 by Sir Alan Marre, the UK’s Parliamentary Ombudsman. OXFORDENGLISH DICTIONARY (Additions Series 1997) (“I now lay before Parliament . . . the full but anonymised
texts of . . . reports on individual cases.”). According to the OED, the usage of the word is “chiefly forstatistical purposes.” Id.
224.
Latanya Sweeney, Weaving Technology and Policy Together to Maintain Confidentiality, 25 J.L.
MED. & ETHICS 98, 100 (1997) (“The term anonymous implies that the data cannot be manipulated
or linked to identify an individual.”).
225.
Id.
226.
45 C.F.R. § 164.514(a) (2009) (defining term).
227.
See, e.g., infra Part IV.D.2.a (discussing Google’s weak approach to anonymization of searchengine log files and how the company treats these practices as robust).
228.
For similar reasons, I do not recommend replacing “anonymize” with the parallel construction“pseudonymize.” See Christopher Soghoian, The Problem of Anonymous Vanity Searches, 3 I/S: J.L. &POL’Y FOR INFO SOC’Y 299, 300 (2007) (“In an effort to protect user privacy, the records were
‘pseudonymized’ by replacing each individual customer’s account I.D. and computer network address
with unique random numbers.”). Just as “anonymize” fails to acknowledge reversible scrubbing,“pseudonymize” fails to credit robust scrubbing.

Broken Promises of Privacy

1745

or too much, and when we hear the word, we are not predisposed toward any
one choice from the list. Even better, technologists have been using the word
scrub for many years.229 In fact, Dr. Sweeney herself has created a system shecalls Scrub for “locating and replacing personally-identifying information in
medical records.”230

III.

HALF MEASURES AND FALSE STARTS

Focusing on things other than PII is a disruptive and necessary first step,
but it is not enough alone to restore the balance between privacy and utility
that we once enjoyed. How do we fix the dozens, perhaps hundreds, of laws
and regulations that we once believed reflected a finely calibrated balance,
but in reality rested on a fundamental misunderstanding of science? Before
turning, in Part IV, to a new test for restoring the balance lost, let us firstconsider three solutions that are less disruptive to the status quo but are unfortunately also less likely to restore the balance. Legislators must understand why
these three solutions—which they will be tempted to treat as the only
necessary responses—are not nearly enough, even in combination, to restore
balance to privacy law.
First, lawmakers might be tempted to abandon the preventative move of
the past forty years, taking the failure of anonymization as a signal to return
to a regime that just compensates harm. Even if such a solution involves anaggressive expansion of harm compensation—with new laws defining new
types of harms and increasing resources for enforcement—this is a halfmeasure, a necessary but not sufficient solution. Second, lawmakers might beencouraged to wait for the technologists to save us. Unfortunately, althoughtechnologists will develop better privacy-protection techniques, they will run
up against important theoretical limits. Nothing they devise will share thesingle-bullet universal power once promised by anonymization, and thus any
technical solutions they offer must be backed by regulatory approaches.
Finally, some will recommend doing little more than banning reidentification.
Such a ban will almost certainly fail.

229.
See, e.g., Jeremy Kirk, Yahoo to Scrub Personal Data After Three Months, IDG NEWSSERVICE, Dec. 17, 2008, available at http://www.pcworld.com/article/155610/yahoo_to_scrub_personal_
data_after_three_months.html (reporting Yahoo!’s decision to “anonymize” its databases of sensitive
information ninety days after collection); Tommy Peterson, Data Scrubbing, COMPUTERWORLD,
Feb. 10, 2003, http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=
78230.
230.
Latanya Sweeney, Replacing Personally-Identifying Information in Medical Records, the Scrub
System, in 1996 J. AM. MED. INFORMATICS ASS’N PROC. 333.

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A.

57 UCLA LAW REVIEW 1701 (2010)

Strictly Punish Those Who Harm

If reidentification makes it easier for malevolent actors like identity thieves,
blackmailers, and unscrupulous advertisers to cause harm, perhaps we needto step up enforcement of preexisting laws prohibiting identity theft,231
232
233
extortion, and unfair marketing practices. Anything we do to deter those
who harm and provide remedies for those harmed is, in light of the increased
power of reidentification, imperative. But this is merely a necessary response,
not a sufficient one.
Full retreat to a tort-based privacy regime, which would abandon theforty-year preventative turn in privacy law, would be a grave mistake, because
without regulation, the easy reidentification result will spark a frightening
and unprecedented wave of privacy harm by increasing access to what I callthe “database of ruin.” The database of ruin exists only in potential: It is theworldwide collection of all of the facts held by third parties that can be used
to cause privacy-related harm to almost every member of society. Easy access tothe database of ruin flows from what I call the “accretion problem.”
1.

The Accretion Problem

The accretion problem is this: Once an adversary has linked two anonymized databases together, he can add the newly linked data to his collectionof outside information and use it to help unlock other anonymized databases.
Success breeds further success. Narayanan and Shmatikov explain that “onceany piece of data has been linked to a person’s real identity, any association
between this data and a virtual identity breaks the anonymity of the latter.”234
This is why we should worry even about reidentification events that seem toexpose only nonsensitive information, because they increase the linkability of
data, and thereby expose people to potential future harm.
Because of the accretion problem, every reidentification event, no matter
how seemingly benign, brings people closer to harm. Recall that Narayanan
and Shmatikov linked two IMDb users to records in the Netflix Prize database.
To some online observers, this connection seemed nonthreatening andtrivial235 because they did not care if others knew what movies they had rented.
231.
E.g., 18 U.S.C. § 1028 (2000); CAL. PENAL CODE § 530.5 (1999); MASS. GEN. LAWS ch.
266, § 37E (2002); N.Y. PENAL LAW §§ 190.77–190.84 (2010).
232.
E.g., 18 U.S.C. § 872 (2006) (prohibiting extortion by federal government officials).
233.
E.g., 15 U.S.C. § 45 (2006) (FTC provision regulating unfair competition); CAL. BUS. &PROF. CODE §§ 17200–210 (2008).
234.
Netflix Prize Study, supra note 5, at 119.
235.
E.g., Comment of chef-ele to Netflix Prize Forum, http://www.netflixprize.com/community/

Broken Promises of Privacy

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These people failed to see how connecting IMDb data to Netflix data is a
step on the path to significant harm. Had Narayanan and Shmatikov not been
restricted by academic ethical standards (not to mention moral compunction),they might have connected people to harm themselves.
The researchers could have treated the connections they made between
IMDb usernames and Netflix Prize data as the middle links in chains ofinferences spreading in two directions: one toward living, breathing people
and the other toward harmful facts. For example, they could have tied thelist of movies rated in the Netflix Prize database to a list of movies rated byusers on Facebook. I suspect that the fingerprint-like uniqueness of Netflix
movie preferences would hold for Facebook movie preferences as well.236
They could have also easily extended the chain in the other direction bymaking one reasonable assumption: People tend to reuse usernames atdifferent websites.237 User john_doe20 on IMDb is likely to be john_doe20
on many other websites as well.238 Relying on this assumption, the researchers
could have linked each living, breathing person revealed through Facebook,through the Netflix Prize data, through IMDb username, to a pseudonymous
user at another website. They might have done this with noble intentions.
Perhaps they could have unearthed the identity of the person who had savagely
harassed people on a message board.239 Maybe they could have determined who240
had helped plan an attack on a computer system on a 4chan message board.
But they also could have revealed identities to evil ends. Perhaps they could
have tied identities to the pseudonymous people chatting on a child abuse
victims’ support website, in order to blackmail, frame, or embarrass them.
viewtopic.php?id=809 (Nov. 28, 2007, 09:04:54) (“I think you can find out more about a person by
typing their name into Google; this Netflix data reverse-engineering doesn’t seem to be a bigger threat
than that.”); Comment of jimmyjot to The Physics arXiv Blog, http://arxivblog.com/?p=142, (Feb. 17,
2008) (“Choice of movies also does not tell a whole lot.”). See also various comments to the posting
Anonymity of Netflix Prize Dataset Broken, SLASHDOT, http://it.slashdot.org/article.pl?sid=07/11/27/
1334244&from=rss (Nov. 27, 2007).
236.
Of course, even without the Netflix data release, Narayanan and Shmatikov might havebeen able to connect some records in the IMDb database directly to Facebook records. But recall thatfor many users, the Netflix data contains movies not rated in IMDb. I am assuming that for some ofthe people who use all three services, no direct connection between IMDb and Facebook is possible.
Thanks to Jane Yakowitz for this point.
237.
Arvind Narayanan, Lendingclub.com: A De-Anonymization Walkthrough, 33 BITS OF ENTROPY
BLOG, http://33bits.org/2008/11/12/57 (Nov. 12, 2008) (“Many people use a unique username
everywhere . . . .”); De-Anonymizing Social Networks, supra note 117, at 6–7 (relying on fact that users
tend to reuse usernames on different social networks).
238.
See Narayanan, supra note 237.
239.
Danielle Keats Citron, Cyber-Civil Rights, 89 B.U. L. REV. 61, 71–75 (2009) (discussing
harassing comments on the AutoAdmit internet discussion board).
240.
Mattathias Schwartz, The Trolls Among Us, N.Y. TIMES MAG., Aug. 3, 2008, at MM24
(describing 4chan).

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Imagine a large-scale attack on the pseudonyms used on the social networking
site Experience Project, which tries to connect users to people who have had
similar life experiences.241 If the researchers had access to other, harder-toobtain, outside information, they could have caused even greater harm. With
access to Google’s search query log file, they might have learned the diseases
people had been recently looking up.242 By connecting the IMDb usernames
to Facebook biographies, they might have been able to bypass password
recovery mechanisms for their victims’ online email and bank accounts,
allowing them to steal private communications or embezzle money, just as
somebody broke into Sarah Palin’s email account by guessing that she had met
her husband at “Wasilla high.”243 Other possible mischief is easy to imagine
when one considers databases that track criminal histories, tax payments,
bankruptcies, sensitive health secrets like HIV status and mental healthdiagnoses, and more.
2.

The Database of Ruin

It is as if reidentification and the accretion problem join the data fromall of the databases in the world together into one, giant, database-in-the-sky,
an irresistible target for the malevolent. Regulators should care about the threatof harm from reidentification because this database-in-the-sky contains information about all of us.
Almost every person in the developed world can be linked to at least one
fact in a computer database that an adversary could use for blackmail, discrimination, harassment, or financial or identity theft. I mean more than mereembarrassment or inconvenience; I mean legally cognizable harm. Perhaps it
is a fact about past conduct, health, or family shame. For almost every one ofus, then, we can assume a hypothetical database of ruin, the one containing
this fact but until now splintered across dozens of databases on computers
around the world, and thus disconnected from our identity. Reidentification
has formed the database of ruin and given our worst enemies access to it.

241.
Experience Project, About Us, http://www.experienceproject.com/about.php (last visited
July 5, 2010).
242.
See infra Part IV.D.2.b (discussing the risk to privacy from access to search query logs).
243.
See Posting of Sam Gustin, Alleged Palin Email Hacker Explains, PORTFOLIO.COM TECHOBSERVER BLOG, http://www.portfolio.com/views/blogs/the-tech-observer/2008/09/18/alleged-palinemail-hacker-explains (Sept. 18, 2008).

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1749

Entropy: Measuring Inchoate Harm

But even regulators who worry about the database of ruin will probably
find it hard to care about the reidentification of people to nonsensitive facts
like movie ratings. Until there is completed harm—until the database ofruin is accessed—they will think there is no need to regulate. One way tounderstand the flaw in this is through the concept of entropy.244
In thermodynamics, entropy measures disorder in a system; in information
theory, it tracks the amount of information needed to describe possible
outcomes.245 Similarly, in reidentification science, entropy measures how close
246
an adversary is to connecting a given fact to a given individual. It describes
the length of the inference chains heading in opposite directions, quantifying theremaining uncertainty.
Consider entropy in the children’s game, Twenty Questions.247 At thestart of a game, the Answerer thinks of a subject the Questioner must
discover through yes or no questions. Before any questions have been asked,
entropy sits at its maximum because the Answerer can be thinking of any subject
in the world. With each question, entropy decreases, as each answer eliminates
possibilities. The item is a vegetable; it is smaller than a breadbox; it is notgreen. The Questioner is like the reidentifier, connecting outside information
to the anonymized database, reducing entropic uncertainty about the identityof his target.
Entropy formalizes the accretion problem. We should worry about reidentification attacks that fall short of connecting anonymized data to actual
identities, and we should worry about reidentification attacks that do not reveal
sensitive information. Even learning a little benign information about asupposedly anonymized target reduces entropy and brings an evil adversary
closer to his prey.
Consider one more extended metaphor, which Part IV builds upon toillustrate a prescription.248 Imagine each person alive stands on one side of along hallway specifically dedicated just for him or her. At the other end of
the hallway sits that person’s ruinous fact, the secret their adversary could use
to cause them great harm. In the hallway between the person and the ruinous
244.
Arvind Narayanan, About 33 Bits, 33 BITS OF ENTROPY BLOG, http://33bits.org/about (Sept.
28, 2008) (explaining the concept of entropy).
245.
The concept originated with a seminal paper by Claude Shannon. See C.E. Shannon, A
Mathematical Theory of Communication, 27 BELL SYS. TECH. J. 379 (1948).
246.
Narayanan, supra note 244.
247.
I am indebted to Anna Karion for the analogy.
248.
See infra Part IV.A.

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fact, imagine a long series of closed, locked doors, each lock requiring a different
key, which represent the database fields that must be reconnected or the links
in the inferential chain that must be established to connect the person to thefact. Finally, imagine many other people clutching keys to some of the doors.
Each person represents a database owner, and the keys the person holds
represent the inferences the person can make, using the data they own.
Under the current, now discredited PII approach to privacy regulation,
we tend to hold database owners—the people in the middle of the hallway—
accountable for protecting privacy only if they happen to hold one of two
critical keys. First, if they hold the key that unlocks the first door, the one
closest to the data subject, we regulate them. This is the linkability form of
PII.249 Second, if they hold the key that unlocks the last door, the one closest
to the ruinous fact, we also regulate them. This is the sensitivity form of PII.250
But under our current approach, we tend to immunize all of the databaseowners whose keys unlock only doors in the middle of the hallway.
4.

The Need to Regulate Before Completed HarmIf we fail to regulate reidentification that has not yet ripened into harm,then adversaries can nudge each of us ever closer to the brink of connection
to our personal database of ruin. It will take some time before most people
become precariously compromised, and whether it will take months, years,
or decades is difficult to predict. Because some people have more to hide thanothers, the burden of decreasing entropy will not be distributed equally acrosssociety.251
Once we are finally connected to our databases of ruin, we will be unable
to unring the bell. As soon as Narayanan and Shmatikov tied an IMDb
username to Netflix rental data, they created an inferential link in the chain,and no regulator can break that link. Anybody who wants to can replicate
their result by downloading the Netflix Prize data252 and mining the IMDb
249.
See supra notes 169–171 and accompanying text (explaining difference between sensitiveand linkable forms of PII).
250.
See id.
251.
There are two classes of people who may escape this fate altogether: those with no secrets
and those so disconnected from the grid that databases hold few records about them—including many
residents of lesser-developed countries. In our own advanced society, I tend to believe that the numbersof people in these groups are so small that they are like myths—the unicorns and mermaids of information
privacy. Ultimately, the size of these groups is a difficult empirical question, but one that is not particularly
important. I think most people would agree that large majorities in advanced societies are susceptible
to reidentification harms, making privacy regulation an important question for huge parts of the world.
252.
Since the competition is now over, the data is no longer publicly available, but it has alreadybeen downloaded hundreds of times. Netflix Prize Study, supra note 5, at 119.

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user ratings database. Narayanan and Shmatikov have forever reduced theprivacy of the people whose information they connected. The FBI cannot easilyorder connected databases unconnected, nor can they confiscate every last
copy of a particularly harmful database.
If we worry about the entire population being dragged irreversibly to thebrink of harm, we must regulate in advance because hoping to regulate after
the fact is the same as not regulating at all. So long as our identity is separated
from the database of ruin by a high degree of entropy, we can rest easy. But asdata is connected to data, and as adversaries whittle down entropy, every oneof us will soon be thrust to the brink of ruin.
B.

Wait for Technology to Save Us

Regulators may wonder whether the technologists will save us first. If weview parallel advances in reidentification and anonymization as an arms race,even though the reidentifiers have raced ahead for now, perhaps the anonymizers will regain the advantage through some future breakthrough. Maybe such a
breakthrough will even restore the status quo and shift the privacy laws backinto balance.
We should not expect a major breakthrough for release-and-forget anonymization, because computer scientists have proved theoretical limits of thepower of such techniques. The utility and privacy of data are linked, and solong as data is useful, even in the slightest, then it is also potentially reidentifiable. Moreover, for many leading release-and-forget techniques, the tradeoff
is not proportional: As the utility of data increases even a little, the privacy
plummets.
We might, however, enjoy some help from new technology, although weshould not expect a breakthrough. Computer scientists have devised techniquesthat are much more resistant to reidentification than release-and-forget. Dataadministrators may use some of these techniques—interactive techniques,
aggregation, access controls, and audit trails—to share their data with areduced risk of reidentification. Alas, despite the promise of these techniques,
they cannot match the sweeping privacy promises that once were maderegarding release-and-forget anonymization. The improved techniques tend
to be much slower, more complex, and more expensive than simple anonymization. Worse, these techniques are useless for many types of data analysis
problems. Technological advances like these may provide some relief in a
post-anonymization, post-PII world, but they can never replace the need fora regulatory response.

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57 UCLA LAW REVIEW 1701 (2010)

Why Not to Expect a Major BreakthroughComputer scientists have begun to conclude that in the arms race between
release-and-forget anonymization and reidentification, the reidentifiers holdthe permanent upper hand.
a. Utility and Privacy: Two Concepts at War
Utility and privacy are, at bottom, two goals at war with one another.253
In order to be useful, anonymized data must be imperfectly anonymous.
“[P]erfect privacy can be achieved by publishing nothing at all—but this has
no utility; perfect utility can be obtained by publishing the data exactly as
254
received from the respondents, but this offers no privacy.” No matter what
the data administrator does to anonymize the data, an adversary with theright outside information can use the data’s residual utility to reveal otherinformation. Thus, at least for useful databases, perfect anonymization is impossible.255 Theorists call this the impossibility result.256 There is always somepiece of outside information that could be combined with anonymized data
to reveal private information about an individual.257
258
Cynthia Dwork offers proof of the impossibility result. Although usefuldata can never be perfectly private, it is important to understand the practicallimits of this result;259 some kinds of theoretical privacy breach may concern
policymakers very little. To use Dwork’s example, if a database owner releases
an aggregate statistic listing the average heights of women in the world by
national origin, an adversary who happens to know that his target is precisely
two inches shorter than the average Lithuanian woman may learn a “private”
fact by studying the database.260 Although we would properly say that theutility of the anonymized data revealed a private fact when combined with261
outside information, we would be foolhardy to regulate or forbid the releaseof databases containing aggregated height data to avoid this possibility. In

253.
Shuchi Chawla et al., Toward Privacy in Public Databases, in 2 THEORY CRYPTOGRAPHY
CONF. 363 (2005).
254.
Id. at 364.
255.
Dwork, supra note 122, at 4.
256.
Id.
257.
Dinur & Nissim, supra note 115, at 203 (showing, for a particular model, “tight impossibility
results,” meaning that privacy would require “totally ruining the database usability”).
258.
Dwork, supra note 122.
259.
Id.
260.
Id.
261.
Id.

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1753

this case, the richness of the outside information creates almost all of the privacy
breach, and the statistic itself contributes very little.
Although the impossibility result should inform regulation, it does nottranslate directly into a prescription. It does not lead, for example, to theconclusion that all anonymization techniques are fatally flawed, but instead, as
Cynthia Dwork puts, “to a new approach to formulating privacy’s goals.”262 She
calls her preferred goal “differential privacy” and ties it to so-called interactive
techniques. Differential privacy and interactive techniques are discussed below.
b. The Inverse and Imbalanced Relationship
Other theoretical work suggests that release-and-forget anonymization
techniques are particularly ill-suited for protecting privacy while preserving
the utility of data. Professor Shmatikov, one of the Netflix Prize researchers,
coauthored a study with Justin Brickell that offers some depressing insights
about the tradeoffs between utility and privacy for such techniques. As theresearchers put it, “even modest privacy gains require almost complete destruction of the data-mining utility.”263
The researchers compared several widely used anonymization techniques
to a form of anonymization so extreme no data administrator would ever use
it: a completely wiped database with absolutely no information beyond the single
field of information under study264—for a health study perhaps the diagnoses,
for an education study the grade point averages, and for a labor study thesalaries. We would hope that real-world anonymization would compare very
favorably to such an extreme method of anonymization, of course supplying
worse privacy, but in exchange preserving much better utility.265 Although
the full details are beyond the scope of this Article, consider the intuition
revealed in the following graph:

262.
263.
264.
265.

Id.
Brickell & Shmatikov, supra note 48, at 70, 76.
Id. at 70–71.
See id.

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FIGURE 1: Effects on Privacy and Utility of Anonymization266
Learning the Sensitive Attribute (Marital Dataset)

20
Attacker

18

Researcher
16

Accuracy Gain

14
12
10
8
6
4
2
0
act
Int

10
k=

0
10
k=

00
10
k=

l=2

l=3

l=4

.4
t=0

.3
t=0

.2
t=0

1.2
d=

1.0
d=

0.8
d=

Sanitization Method and Parameter

In Figure 1, the pairs of bars represent the same database transformed
into many different forms using widespread anonymization techniques. For eachpair, the left, black bar represents the privacy of the data, with smaller bars
signifying more privacy. The right, gray bars represent the utility of the data,with longer bars meaning more utility. Anonymization techniques search forways to shorten the left bar without shortening the right bar too much, andthe holy grail of anonymization would be a short, black bar next to a long,gray bar. Even a quick scan of the graph reveals the absence of thiscondition.
The leftmost pair of bars, with a privacy score of almost eighteen and autility score of about eleven, represents the original, unadulterated data. A
score of zero represents the utility or privacy of completely wiped data. Notice
how the first three pairs of bars, the ones labeled with the letter k, describe
techniques that preserve a lot of utility while improving privacy very little.268
266.
This figure has been adapted from a figure in id. at 76. Only the formatting has been changed;
the substance of the figure remains the same.
268.
These bars represent techniques that achieve k-anonymity, a widely embraced metric for strong
anonymity. Id. at 71; Sweeney, supra note 8 (defining k-anonymity).

Broken Promises of Privacy

1755

Although the second trio of bars, those labeled with the letter l,269 show muchgreater improvements in privacy than the first trio, such improvements comeonly at great losses to utility.
These results show that for traditional, widespread, release-and-forgetanonymization, not only are privacy and utility related, but their relationshipis skewed. Small increases in utility are matched by even bigger decreases inprivacy, and small increases in privacy cause large decreases in utility. The
researchers concluded that even the most sophisticated anonymization techniques were scarcely better than simply throwing away almost all of the data
instead.
Thus, using traditional, release-and-forget, PII-focused anonymization
techniques, any data that is even minutely useful can never be perfectly anonymous, and small gains in utility result in greater losses for privacy. Both ofthese relationships cut against faith in anonymization and in favor of otherforms of regulation.
2.

The Prospect of Something Better Than Release-and-Forget

Researchers have developed a few techniques that protect privacy muchbetter than the traditional, release-and-forget techniques. These work byrelaxing either the release or the forget requirement. For example, some data
administrators never release raw data, releasing only aggregated statistics instead.
Every day, USA Today summarizes a survey in a colorful graph on their front
page. Armed only with these survey responses, it would be very difficult for areidentifier to prove that any particular person took part in a USA Today
survey, much less gave a particular response.
Similarly, some researchers favor interactive techniques.270 With thesetechniques, the data administrator answers questions about the data without
ever releasing the underlying data. For example, an analyst might ask, whatpercentage of the people in your database have been diagnosed with this rare
form of cancer? This might prompt the administrator to calculate and return
the answer—say, 2 percent. In most cases, reidentifiers will find it much moredifficult to link answers like these to identity than if they had access to theunderlying raw data.

269.
These bars represent l-diversity, another widely adopted metric. The final six bars represent
t-closeness. Brickell & Shmatikov, supra note 48, at 70–71.
270.
Cynthia Dwork et al., Calibrating Noise to Sensitivity in Private Data Analysis, in 2006 THEORYCRYPTOGRAPHY CONF. 265, 267.

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Researchers can do even better. Using one class of interactive techniques,
those that satisfy a requirement called differential privacy,271 the data administrator never even releases the accurate statistic; instead, she introduces a
carefully calculated amount of random noise to the answer, ensuring
mathematically that even the most sophisticated reidentifier will not be able
to use the answer to unearth information about the people in the database.272
Finally, just as these techniques refer to something less than full release,other techniques refuse to forget—instead, they monitor what happens todata after release. Borrowing from computer security research, these tech273
Using theseniques involve the use of access controls and audit trails.
techniques, data administrators release their data but only after protecting it
using software that limits access and tracks usage. The data analyst who
receives the protected data will be able to interact with it only in limited
ways, and the analyst’s every move will be recorded in the audit trail andreported back to the data administrator or a third-party watchdog.
3.

The Limitations of the Improved Techniques

Unfortunately, these alternatives do not make up for the broken promisesof release-and-forget anonymization. For starters, they tend to be less flexible
than traditional anonymization. Interactive techniques require constant participation from the data administrator. This increases the cost of analysis andreduces the rate of new analysis. Because an analyst must submit requests
and wait for responses, he is not free to simply test theory after theory at the
maximum rate. Even worse, without access to the raw data, he might miss useful
research inquiries that reveal themselves to those who study trends in the data.
Furthermore, even with interactive techniques and aggregation, data
administrators cannot promise perfect privacy. As an example, if an adversary
somehow knows that his target is the only man who visited a hospital clinic
Thursday afternoon, then the aggregated answer to the question, “diagnosesof men who visited the clinic Thursday afternoon” reveals sensitive information tied directly to an identity. As another example, despite decades ofdenials from the Census Bureau, scholars have unearthed proof that theagency provided aggregated, city-block-level data that helped locate JapaneseAmericans who were then sent to internment camps during the Second World
271.
See Dwork, supra note 122, at 8–9.
272.
See Adam & Wortmann, supra note 60, at 540 (describing the “output-perturbation
approach”).
273.
For more information on access controls in the computer security context, see RICK LEHTINENET AL., COMPUTER SECURITY BASICS 66–72 (2006).

Broken Promises of Privacy

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War.274 Even though the data did not identify particular houses or families,
just telling authorities how many Japanese lived on each block gave themenough information to do enormous harm.
Interactive techniques that introduce noise are also of limited usefulness.
For example, a city may want to analyze census data to determine where to
run a bus line to serve elderly residents. Noise introduced to provide privacy
may inadvertently produce the wrong answer to this question.275 Similarly,
law enforcement data miners may find it unacceptable to tell a judge thatthey are using a “noisy” technique to justify asking for a search warrant tosearch a home.276 Techniques that satisfy differential privacy also requirecomplex calculations that can be costly to perform.277
Finally, computer security researchers have thoroughly documented the
problem with creating robust access controls.278 Simply put, even the best computer security solutions are bug-prone, as well as being expensive to createand deploy.279 All of these reasons explain why the vast majority of data
shared or stored today is protected—if at all—by traditional, release-andforget anonymization, not by these more exotic, more cumbersome, and more
expensive alternatives.
Even if computer scientists tomorrow develop a groundbreaking
technique that secures data much more robustly than anything done today—
and this is a very unlikely “if”—the new technique will only work on data
secured in the future; it will do nothing to protect data that has been stored ordisclosed in the past. A database, once released, can become easier to reidentify
but never more difficult. Long chains of inferences from past reidentification
cannot be broken with tomorrow’s advances.
Techniques that eschew release-and-forget may improve over time, but
because of inherent limitations like those described above, they will never supply

274.
William Seltzer & Margo Anderson, Population Association of America, After Pearl Harbor:
The Proper Role of Population Data Systems in Time of War (Mar. 28, 2000) (unpublished paper),available at https://pantherfile.uwm.edu/margo/www/govstat/newpaa.pdf.
275.
See Chawla et al., supra note 253, at 366.
276.
The difficulty of using “noisy” techniques in police work is illustrated by a recent AP story
that documents one instance where the addition of “random material” to a database resulted in repeatedunnecessary police deployments. Cops: Computer Glitch Led to Wrong Address, MSNBC NEWS, Mar.
19, 2010, http://www.msnbc.msn.com/id/35950730.
277.
Jon Kleinberg et al., Auditing Boolean Attributes, in 2000 ACM SYMP. ON PRINCIPLESDATABASE SYS. 86 (proving that particular method supporting interactive technique is NP-hard, meaning
computationally expensive).
278.
BRUCE SCHNEIER, BEYOND FEAR: THINKING SENSIBLY ABOUT SECURITY IN AN
UNCERTAIN WORLD 87–101 (2003).
279.
Id.; cf. FREDERICK P. BROOKS, JR., THE MYTHICAL MAN-MONTH (1975) (discussing how
software engineering principles lead to bugs).

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a silver-bullet alternative. Technology cannot save the day, and regulation must
play a role.
C.

Ban Reidentification

Finally, some have urged simply banning reidentification.280 Lawmakers
can offer a straightforward argument for a ban: By anonymizing data, a data
administrator gives notice of her intent to protect the privacy of her data subjects, who may rely on this notice when consenting to provide her their data.
A reidentifying adversary thwarts this intent and undermines this consent somuch that we might need a law banning the act itself.
A reidentification ban is sure to fail, however, because it is impossible toenforce. How do you detect an act of reidentification?281 Reidentification canhappen completely in the shadows. Imagine that Amazon.com anonymizes
its customer purchase database and transmits it to a marketing firm. Imagine
further that although the marketing firm promises not to reidentify people inAmazon’s database, it could increase profits significantly by doing so. If themarketing firm breaks its promise and reidentifies, how will Amazon or
anybody else ever know? The marketing firm can conduct the reidentificationin secret, and gains in revenue may not be detectable to the vendor.
This problem appears insurmountable, although four forces might help toameliorate it. First, lawmakers might pair a ban with stricter penalties and better
enforcement, for example by declaring reidentification a felony and providing
extra money to the FBI and FTC for enforcement. Second, lawmakers cangive citizens a private right of action against those who reidentify.282 Third,
lawmakers can mandate software audit trails for those who use anonymized
data.283 Finally, a smaller scale ban, one imposed only on trusted recipients ofspecific databases—for example, a ban prohibiting government data-miners
from reidentifying—may be much easier to enforce.284

280.
Earl Lane, A Question of Identity: Computer-Based Pinpointing of ‘Anonymous’ Health Records
Prompts Calls for Tighter Security, NEWSDAY, Nov. 21, 2000, at C8 (quoting Janlori Goldman, headof the Health Privacy Project at Georgetown University as saying: “Our goal has been to get a national
policy making it illegal to re-identify an anonymized database”).
281.
Id. (“As long as the data recipient is discreet, an agency may never learn if its information
is being compromised.” (citing Latanya Sweeney)).
282.
They can model this on the Federal Stored Communications Act, which provides a civil
cause of action to any “person aggrieved by any violation” of the Act. 18 U.S.C. § 2707 (2006).
283.
E.g., 45 C.F.R. § 164.308(a)(1)(ii)(D) (2009) (describing HIPAA Security Rule mandating“Information system activity review” including regular review of “audit logs”).
284.
For another example, see infra Part IV.D.1 (discussing the ban on reidentification for trusted
recipients of health information).

Broken Promises of Privacy

1759

I predict that any of these marginal improvements would still beoutweighed by the inherent difficulty of detecting secret reidentification for
private gain. This significant detection problem makes a ban extremely unlikely
to succeed.

IV.

RESTORING BALANCE TO PRIVACY LAW AFTER THE FAILURE
OF ANONYMIZATION

Once regulators conclude that the three partial solutions discussed above
are not enough to restore balance to privacy law after the failure ofanonymization, they must do more. They should weigh the benefits of unfettered information flow against the costs of privacy harms. They shouldincorporate risk assessment strategies that deal with the reality of easy
reidentification as the old PII model never could. Ultimately, they should
consider a series of factors to identify situations in which harm is likely andwhether it outweighs the benefits of unfettered information flow. When they
identify harm that outweighs these benefits, they should regulate, focusing on
narrow contexts and specific sectors rather than trying to regulate broadly
across industries. To demonstrate how this approach works, this Part ends
with two case studies recommending new strategies for regulating the privacyof health and internet usage information.
A.

Which Database Owners Should We Regulate Anew?

In the search for a new organizing principle to supplement PII, I startfrom the premise that any privacy rule we devise must distinguish between
different types of database owners and different types of databases. Thisapproach might sound like PII, but it is broader. The problem is not that the
PII approach categorizes; the problem is that it focuses on only a few, narrowly
drawn categories that seem insufficient and even somewhat arbitrary in lightof easy reidentification. Recall the hallway metaphor: PII-based rules regulate
only those people with a key to the first door closest to the data subject(those that can link to a user’s identity) or a key to the last door closest to theruinous fact (those holding sensitive information).285 For example, HIPAA
singles out for special treatment social security numbers (linkable data) andmedical diagnoses (sensitive data). PII rules ignore the people who can unlock
doors only in the middle.

285.

See text accompanying supra notes 248–250.

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The power of reidentification demands that we begin to regulate themiddle. But how? It would be logically justifiable but overly aggressive toregulate any entity possessing any fragment of data at any point along thechain of inferences, covering even a person holding only one key. We should
aim to direct scarce regulatory resources at those database owners that mostcontribute to the risk of the database of ruin through well-tuned rules. A rulethat regulates the database owner in the middle that possesses but a single
scrap of unimportant data puts too much regulatory focus on too slight a risk.
Which database owners in the middle most contribute to the risk ofharm and thereby most deserve government scrutiny and regulation? To
the current PII-approach—regulation for those holding linkable data andthose holding sensitive data—I propose we add at least one more categoryof database owners, the “large entropy reducers.”286 Large entropy reducers
are entities that amass massive databases containing so many links between
so many disparate kinds of information that they represent a significant partof the database of ruin, even if they delete from their databases all particularly
sensitive and directly linkable information.
We can justify treating these entities differently using the language of duty
and fault. Because large entropy reducers serve as one-stop shops for adversaries
trying to link people to ruinous facts, they owe their data subjects a heightened
duty of care. When a large entropy reducer loses control of its massive database,it causes much more harm than an entity holding much less data.
Who are large entropy reducers? In the hallway metaphor, they are
the people clutching many keys; imagine the mythical janitor’s keyring, jangling with dozens of different keys. In practice, this category includes large credit
agencies like Experian, TransUnion, and Equifax; commercial data brokers
like ChoicePoint, Acxiom, and LexisNexis; and internet search providers like
Google, Microsoft, and Yahoo. These are among the most important large
entropy reducers, but there are many others, and we should develop a more
precise definition of the category, perhaps one taking advantage of the formal
definition of entropy.
286.
In addition to large entropy reducers, other classes of database owners probably deserve new
regulation to account for the way they increase the risk of harm due to easy reidentification. For one,some database owners can make links between fields of information that can be connected by few otherpeople—they can unlock doors requiring keys held by few people. For example, consider how a cell
phone provider or automobile toll booth administrator can track physical movement and location in
ways that few other providers can. Likewise, some database owners hold fields of data that act as identifiers
on many sites, making them powerful tools for reidentification. Increasingly, email addresses act inthis manner, as websites use them in place of usernames. Perhaps any entity holding an email addressdeserves new regulation. I plan in future work to develop these categories further, and to flesh out thearguments for regulating them more closely.

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We should expand existing privacy laws and enact new privacy laws that
regulate the behavior of companies like these. To be sure, many of these firms
are already obligated to comply with many different privacy laws, but in lightof easy reidentification and the database of ruin, we need to regulate themmore, perhaps with new rules tailored to limiting the type of risk of reidentification such providers represent.
B.

Regulatory Principles

Now that we know whom to regulate—database owners holding linkableor sensitive data (PII) and large entropy reducers—we turn to the content ofregulation. How should regulators respond to the power of reidentificationand the collapse of our faith in anonymization? Before we turn to a list offactors that will guide us to the proper regulation, we need to understand someoverarching principles. This step is necessary because so much of how we regulate privacy depends on our faith in anonymization; stripped of this faith, weneed to reevaluate some core principles.
1.

From Math to Sociology

Regulators need to shift away from thinking about regulation, privacy, andrisk only from the point of view of the data, asking whether a particular fieldof data viewed in a vacuum is identifiable. Instead, regulators must ask abroader set of questions that help reveal the risk of reidentification and threatof harm. They should ask, for example, what has the data administrator done
to reduce the risk of reidentification? Who will try to invade the privacy of thepeople in the data, and are they likely to succeed? Do the history, practices,traditions, and structural features of the industry or sector instill particularconfidence or doubt about the likelihood of privacy?
Notice that while the old approach centered almost entirely on technological questions—it was math and statistics all the way down—the new
inquiry is cast also in sociological, psychological, and institutional terms.
Because easy reidentification has taken away purely technological solutions
that worked irrespective of these messier, human considerations, it follows thatnew solutions must explore, at least in part, the messiness.287

287.
See Chawla et al., supra note 253, at 367 (noting that the relative advantage of one interactive
technique is that “the real data can be deleted or locked in a vault, and so may be less vulnerable to briberyof the database administrator”).

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57 UCLA LAW REVIEW 1701 (2010)

Support for Both Comprehensive and Contextual Regulation

The failure of anonymization will complicate one of the longest-runningdebates in information privacy law: Should regulators enact comprehensive,
cross-industry privacy reform, or should they instead tailor specific regulations
to specific sectors?288 Usually, these competing choices are labeled, respectively,
the European and United States approaches. In a postanonymization world,
neither approach is sufficient alone: We need to focus on particular risks
arising from specific sectors because it is difficult to balance interests comprehensively without relying on anonymization. On the other hand, we need acomprehensive regulation that sets a floor of privacy protection because anonymization permits easy access to the database of ruin. In aiming for bothgeneral and specific solutions, this recommendation echoes Dan Solove, who
cautions that privacy should be addressed neither too specifically nor too
generally.289 Solove says that we should simultaneously “resolve privacy issues
by looking to the specific context,”290 while at the same time using “a generalframework to identify privacy harms or problems and to understand why they291
are problematic.”Thus, the U.S.’s exclusively sectoral approach is flawed, because it allows
entire industries to escape privacy regulation completely based on the illusionthat some data, harmless data, data in the middle of long chains of inferencesleading to harm, is so bland and nonthreatening that it is not likely to lead toharm if it falls into the wrong hands. The principle of accretive reidentification
shatters this illusion. Data almost always forms the middle link in chains ofinferences, and any release of data brings us at least a little closer to ourpersonal databases of ruin. For this reason, there is an urgent need for comprehensive privacy reform in this country. A law should mandate a minimum floorof safe data-handling practices on every data handler in the U.S. Further, it
should require even stricter data-handling practices for every large entropy
reducer in the U.S.
But on the other hand, the European approach—and specifically theapproach the EU has taken in the Data Protection Directive—sets the heightof this floor too high. Many observers have complained about the onerous

288.
See, e.g., Schwartz, supra note 162, at 908–16 (discussing history of sectoral and comprehensive approaches to privacy law).
289.
DANIEL J. SOLOVE, UNDERSTANDING PRIVACY 46–49 (2008).
290.
Id. at 48.
291.
Id. at 49.

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obligations of the Directive.292 It might have made good sense to impose such
strict requirements (notice, consent, disclosure, accountability) on data administrators when we still believed in the power of anonymization because the law
left the administrators with a fair choice: Anonymize your data to escape theseburdens or keep your data identifiable and comply.
But as we have seen, easy reidentification has mostly taken away thischoice, thereby broadening the reach of the Directive considerably. Today,
the EU hounds Google about IP addresses; tomorrow, it can make similararguments about virtually any data-possessing company or industry. A
European privacy regulator can reasonably argue that any database containing
facts (no matter how well scrubbed) relating to people (no matter how indirectly) very likely now falls within the Directive. It can impose the obligationsof the Directive even on those who maintain databases that contain nothingthat a layperson would recognize as relating to an individual, so long as thedata contains idiosyncratic facts about the lives of individuals.
I suspect that some of those who originally supported the Directive
might feel differently about a Directive that essentially provides no exception
for scrubbed data—a Directive covering most of the data in society. The
Directive’s aggressive data-handling obligations might have seemed to strike
the proper balance between information flow and privacy when we thoughtthat they were restricted to “personal data,” but once reidentification science
redefines “personal data” to include almost all data, the obligations of the
Directive might seem too burdensome. For these reasons, the European Union
might want to reconsider whether it should lower the floor of its comprehensive data-handling obligations.
Finally, once the U.S. tackles comprehensive privacy reform and the EU
lowers the burdens of the directive, both governments should expand theprocess of imposing heightened privacy regulations on particular sectors.
What might be needed above the comprehensive floor for health records may
not be needed for phone records, and what might solve the problems ofprivate data release probably will not work for public releases.293 This approachborrows from Helen Nissenbaum, who urges us to understand privacy throughwhat she calls “contextual integrity,” which “couches its prescriptions always
within the bounds of a given context” as better than other “universal”

292.
E.g., DOROTHEE HEISENBERG, NEGOTIATING PRIVACY: THE EUROPEAN UNION, THE
UNITED STATES AND PERSONAL DATA PROTECTION 29, 30 (2005) (calling parts of the Directive “quite
strict” and “overly complex and burdensome”).
293.
Cf. infra Part IV.D (discussing specific rules for health privacy and search engine privacy
contexts).

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57 UCLA LAW REVIEW 1701 (2010)accounts.294 This approach also stands in stark contrast to the advice of otherinformation privacy scholars and activists, who tend to valorize sweeping,society-wide approaches to protecting privacy and say nothing complimentary
about the U.S.’s sectoral approach.
What easy reidentification thus demands is a combination of comprehensive data-protection regulation and targeted, enhanced obligations for
specific sectors. Many others have laid out the persuasive case for a comprehensive data privacy law in the United States, so I refer the reader elsewhere
for that topic.295 The rest of the Article explores how to design sector-specificdata privacy laws, now that we can no longer lean upon the crutch of robust
anonymization to give us balance. What does a post-anonymization privacy
law look like?
C.

The Test

In the post-anonymization age, once regulators pick a target for
regulation—say, large entropy reducers in the healthcare industry—theyshould weigh the following factors to determine the risk of reidentification inthat context. The list is not exhaustive; other factors might be relevant.296 The
factors serve two purposes: They are indicators of risk and instruments for
reducing risk. As indicators, they signal the likelihood of privacy harm. For
example, when data administrators in a given context tend to store massive
quantities of information, the risk of reidentification increases. Regulators
should use these indicative factors like a score card, tallying up the risk ofreidentification.
Once regulators decide to regulate, they should then treat these factors
as instruments for reducing risk—the tuning knobs they can tweak throughlegislation and regulation to reduce the risk of harm. As only one example,
regulators might ban public releases of a type of data outright while declining toregulate private uses of data.

294.
Helen Nissenbaum, Privacy as Contextual Integrity, 79 WASH. L. REV. 119, 154 (2004).
295.
E.g., Solove & Hoofnagle, supra note 161.
296.
The European privacy watchdog, the Article 29 Working Group, offers the following, similar
but not identical, list of factors:
The cost of conducting identification is one factor, but not the only one. The intended purpose,
the way the processing is structured, the advantage expected by the controller, the interests
at stake for the individuals, as well as the risk of organisational dysfunctions (e.g. breachesof confidentiality duties) and technical failures should all be taken into account. On the other
hand [one] . . . should consider the state of the art in technology at the time of the processing
and the possibilities for development during the period for which the data will be processed.
2007 Working Party Opinion, supra note 28, at 15.

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Five Factors for Assessing the Risk of Privacy Harm
Data-Handling Techniques

How do different data-handling techniques affect the risks of reidentification? Experts probably cannot answer this question with mathematicalprecision; it is unlikely we can ever know, say, that the suppression of names
and social security numbers produces an 82 percent risk, while interactive
techniques satisfying differential privacy produce a 1 percent risk. Still, computer scientists could likely provide a rough relative ordering of different
techniques—or at the very least, grade data-handling practices according towhether the risk of reidentification is high, medium, or low.297 For example,
computer scientists might grade favorably a database owner that uses the kindof new interactive techniques described earlier, although remember that suchtechniques are no panacea.
Private Versus Public Release
Regulators should scrutinize data releases to the general public much
more closely than they do private releases between trusted parties. We fear
the database of ruin because we worry that our worst enemy can access it, but
if we use regulation to limit the flow of information to trusted relationships
between private parties, we can breathe a little easier. It is no coincidence
that every case study presented in Part I.B involved the public release ofanonymized data. In each case, the researcher or researchers targeted theparticular data because it was easy to get, and in the AOL search query example
in particular, an army of blogger-reidentifiers acted as a force multiplier,aggravating greatly the breach and the harm.
My argument against public releases of data pushes back against a tide oftheory and sentiment flowing in exactly the opposite direction. Commentatorsplace great stock in the “wisdom of crowds,” the idea that “all of us are smarter
than any of us.”298 Companies like Netflix release great stores of information
they once held closely to try to harness these masses.299

297.
Some computer scientists have already tentatively offered studies that attempt to categorize
the risk of reidentification of different techniques. See, e.g., Lakshmanan et al., supra note 46 (focusing
on anonymization); Adam & Wortmann, supra note 60 (evaluating methods, including conceptual,query restriction, data perturbation, and output perturbation). These studies do not take into accountthe latest advances in reidentification, but they are models for future work.
298.
SUROWIECKI, supra note 15.
299.
See Thompson, supra note 93.

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The argument even throws some sand into the gears of the Obama
Administration’s tech-savvy new approach to governance. Through thelaunch of websites like data.gov300 and the appointment of federal officials likeCTO Aneesh Chopra301 and CIO Vivek Kundra,302 the administration has
promised to release massive databases heralding a twenty-first century modeof government openness.303 Amidst the accolades that have been showered304
upon the government for these efforts, one should pause to consider thecosts. We must remember that utility and privacy are two sides of the samecoin,305 and we should assume that the terabytes of useful data that will soon
be released on government websites will come at a cost to privacy commensurate with, if not disproportionate to,306 the increase in sunlight and utility.
Quantity
307
Most privacy laws regulate data quality but not quantity. Laws dictate
what data administrators can do with data according to the nature, sensitivity,and linkability of the information, but they tend to say nothing about how
much data a data administrator may collect, nor how long the administrator
can retain it. Yet, in every reidentification study cited, the researchers wereaided by the size of the database. Would-be reidentifiers will find it easier to300.
Data.gov, About, http://www.data.gov/about (last visited June 12, 2010) (“The purpose of
Data.gov is to increase public access to high value, machine readable datasets generated by the Executive
Branch of the Federal Government.”).
301.
See Posting of Nate Anderson, Obama Appoints Virginia’s Aneesh Chopra US CTO,
ARSTECHNICA LAW & DISORDER BLOG, http://arstechnica.com/tech-policy/news/2009/04/obamaappoints-virginias-aneesh-chopra-us-cto.ars (Apr. 20, 2009, 13:01 EST).
302.
See Posting of Brian Knowlton, White House Names First Chief Information Officer, N.Y.
TIMES CAUCUS BLOG, http://thecaucus.blogs.nytimes.com/2009/03/05/white-house-names-first-chiefinformation-officer (Mar. 5, 2009, 10:06 EST).
303.
Id. (“Mr. Kundra discussed some of his plans and interests, including his intention . . . to create
a data.gov web site that will put vast amounts of government information into the public domain.”).
304.
E.g., Posting of Clay Johnson, Redesigning the Government: Data.gov, SUNLIGHTLABS.COM,
http://www.sunlightlabs.com/blog/2009/04/16/redesigning-government-datagov (Apr. 16, 2009, 11:52
EST); Posting by Infosthetics, Data.gov: How to Open Up Government Data, INFORMATION AESTHETICS
BLOG, http://infosthetics.com/archives/2009/03/open_up_government_data.html (Mar. 13, 2009, 17:25
EST). But see David Robinson, Harlan Yu, William P. Zeller & Edward W. Felten, Government Dataand the Invisible Hand, 11 YALE J.L. & TECH. 160, 161 (2009) (discussing how the federal government
should structure systems to enable greater internet-based transparency).
The Center for Democracy and Technology has posted a supportive but more cautious memo,
flagging concerns about Data.gov involving deidentification and reidentification. Ctr. for Democracy& Tech., Government Information, Data.gov and Privacy Implications, http://www.cdt.org/policy/
government-information-datagov-and-privacy-implications (July 13, 2009) (“While Data.gov has great
potential, there are important privacy implications associated with data disclosure.”).
305.
See supra Part III.B.1.a.
306.
See supra Part III.B.1.b.
307.
See supra Part II.A.3 (listing privacy statutes that draw distinctions based on data type).

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match data to outside information when they can access many records indicating the personal preferences and behaviors of many people. Thus, lawmakers
should consider enacting new quantitative limits on data collection and retention.308 They might consider laws, for example, mandating data destruction after
a set period of time, or limiting the total quantity of data that may be possessedat any one time.
MotiveIn many contexts, sensitive data is held only by a small number of actors
309
For example, rules governing what
who lack the motive to reidentify.
academic researchers can do with data should reflect the fact that academic
researchers rarely desire to reidentify people in their datasets. A law thatstrictly limits information sharing for the general public—think FERPA(student privacy), HIPAA (health privacy), or ECPA (electronic communications privacy)—might be relaxed to allow researchers to analyze the datawith fewer constraints. Of course, regulators should draw conclusions about
motive carefully, because it is hard to predict who the adversary is likely to
be, much less divine his or her motive.
Regulators should also weigh economic incentives for reidentification.
Although we should worry about our enemies targeting us to learn about our
medical diagnoses, we should worry even more about financially-motivatedidentity thieves looking for massive databases that they can use to target
thousands simultaneously.310
Trust
The flip side of motive is trust. Regulators should try to craft mechanisms
for instilling or building upon trust in people or institutions. While we labored308.
See European Union Article 29 Data Protection Working Party, Opinion 1/2008 on Data
Protection Issues Relating to Search Engines, 00737/EN WP 148, at 19 (April 4, 2008), available at
http://ec.europa.eu/justice_home/fsj/privacy/docs/wpdocs/2007/wp136_en.pdf [hereinafter 2008 Working
Party Opinion] (arguing that search engines should store queries for a maximum of six months).
309.
Cf. EU Data Protection Directive, supra note 3, at recital 26 (noting that “the means likely
reasonably to be used” to identify individuals are relevant to a determination of whether individuals
are “identifiable”).
310.
As one commentator puts it:[T]here’s far less economic incentive for a criminal to go after medical data instead of credit
card information. It’s harder to monetize the fact that I know that Judy Smith of Peoria hasheart disease—by filing false claims in her name, for example—than to have Judy’s credit card
number and expiration date. If I’m a criminal with advanced data skills and I have a day tospend, I’m going to go after financial data and not health data.
Cline, supra note 177.

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under the shared hallucination of anonymization, we trusted the technology,
so we did not have to trust the recipients of data; now that we have lost
trust in the technology, we need to focus more on trust in people. We might,
for example, conclude that we trust academic researchers implicitly, government
data miners less, and third-party advertisers not at all, and we can build these
conclusions into law and regulation.
2.

Applying the Test

By applying the five factors, regulators will have a rough sense of the riskof reidentification of a particular type of provider in a particular context. If therisk is very low, regulators might choose to do nothing. If the risk is very high,regulators should feel inclined to act, imposing new restrictions on data collection, use, processing, or disclosure, and requiring specific data safe-handlingprocedures.
Regulators should perhaps also take into consideration the sensitivity ofthe data. It makes sense to treat medical diagnoses differently than televisionwatching habits, for example, because the path to harm for the former isshorter and more direct than for the latter. But because the database of ruin
can be built almost entirely with nonsensitive data, regulators should beware
not to make too much of this step in the analysis.
Finally, regulators should compare the risk and the sensitivity to the various
benefits of unfettered information flow: for medical privacy, better treatments
and saved lives; for internet privacy, better search tools and cheaper products;
for financial privacy, fewer identity thefts. If the benefits of unfettered information significantly outweigh the costs to privacy in a particular context, theymight decide to surrender.311 Perhaps lawmakers will see reidentification as
the latest example of the futility of attempting to foist privacy on an unappreciative citizenry through ham-handed regulations. Maybe they will conclude
they should just give up and live in a society with very little privacy.
Much more often, regulators will conclude that the costs to privacy
outweigh the benefits of unfettered information flow. When they come tosuch a conclusion, they should consider rules and laws that reduce the risk
by restricting the amount of information flowing through society. Of course,
311.
For example, Harvard’s Personal Genome Project, which is sequencing the DNA of thousandsof volunteers to hunt for genetic markers for disease, has essentially told its volunteers to forget about
privacy. Peter Dizikes, Your DNA Is a Snitch, SALON.COM, Feb. 17, 2009, http://www.salon.com/env/
feature/2009/02/17/genetic_testing (“[T]he Personal Genome Project essentially tells its volunteers toforget about privacy guarantees. ‘I like the Personal Genome Project approach,’ [one scholar] says. ‘It’s
honest. They’re saying, “If you want to take the risks, great.’””).

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such restrictions must be chosen with care because of the important values offree information flow. Regulators should thus try to clamp down on information flow in targeted ways, using the factors listed above in their instrumental
sense as a menu of potential interventions.
If the costs significantly outweigh the benefits of information flow,
regulators might completely ban the dissemination or storage of a particular
type of information. For example, regulators should probably often conclude
that public releases of information—even information that seems benign ornonthreatening—should be banned, particularly because such information
can be used to supply middle links in long chains of inferences. In morebalanced situations, regulators might restrict but not cut off information flow,for example by instituting a quantity cap or a time limit for storage.312 They
might also place even milder restrictions on small classes of trusted people—
academic researchers, for example—while banning the sharing of the datawith anybody else.
D.

Two Case Studies

To demonstrate how a regulator should apply this test, and to highlightthe important roles of context and trust, let us revisit again the case studies
introduced before: health and internet usage information. Debates about theproper regulation of these two classes of data have raged for many years.
Although I cannot capture every nuance of these debates in this space, I revisit
them in order to show how to regulate data privacy after the fall of the robust
anonymization assumption.
1.

Health Information

Once regulators choose to scrap the current HIPAA Privacy Rule—anecessary step given the rule’s intrinsic faith in deidentification—how should
they protect databases full of sensitive symptoms, diagnoses, and treatments?
Consider one class of users of such information in particular: medical
researchers seeking new treatments and cures for disease. In this context,
both the costs and benefits of unfettered use are enormous. On the one hand,
if our worst enemies get hold of our diagnoses and treatments, they can cause
us great embarrassment or much worse. On the other hand, researchers use
this information to cure disease, ease human suffering, and save lives. Regulators
312.
See 2008 Working Party Opinion, supra note 308, at 19 (arguing search engines should store
queries for only six months).

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will justifiably be reluctant to throttle information flow too much in this context
since the toll of such choices might be measurable in human lives lost.
HIPAA tried to resolve this dilemma by trusting the technology ofanonymization. We no longer trust the technology, but we can still rely on adifferent trust: trust in the researchers themselves. Health researchers are
rarely willing to release sensitive data—scrubbed or not—to just anybody
who asks. Instead, they tend to share such data only after verifying the bonafides of the person asking. Regulators should build upon such human networksof trust in a revised HIPAA, allowing data transfer where trust is high andforbidding it where trust is low.
The problem is that today researchers trust one another according to
informal rules and soft intuitions, and to build trust into law, these rules andintuitions must be formalized and codified. Should HIPAA rely only on a
researcher’s certification of trust in another, or should an outside body such as
an Institutional Review Board review the bases for trust?313 Should trust in a
researcher extend also to her graduate students? To her undergraduate lab
assistants? Regulators should work with the medical research community todevelop formalized rules for determining and documenting trusted relationships.
Once the rules of verifiable trust are codified, regulators can free up data
sharing between trusted parties. To prevent abuse, they should require additional safeguards and accountability mechanisms. For example, they canprescribe new sanctions—possibly even criminal punishment—for those whoreidentify. They can also mandate the use of technological mechanisms: bothex ante like encryption and password protection, and ex post review methods
like audit trail mechanisms.
Regulators can vary these additional protections according to the sensitivity of the data. For example, for the most sensitive data such as
psychotherapy notes and HIV diagnoses, the new HIPAA can mandate an
NSA-inspired system of clearances and classifications; HIPAA can requirethat researchers come to the sensitive data rather than letting the data go to
the researchers, requiring physical presence and in-person analysis at the sitewhere the data is hosted. At the other extreme, for databases that containvery little information about patients, perhaps regulators can relax some or allof the additional protections.
While these new, burdensome requirements on their own might stifle
research, they would permit another change from the status quo that mightinstead greatly expand research: With the new HIPAA, regulators should
313.
According to federal rules, federally-funded research involving human subjects must be
approved by an IRB. 45 C.F.R. §§ 46.101–109 (2009).

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rescind the current, broken deidentification rules. Researchers who share dataaccording to the new trust-based guidelines will be permitted to share alldata, even fields of data like birth date or full ZIP code that they cannot access
today.314 With more data and more specific data, researchers will be able toproduce more accurate results, and thereby hopefully come to quicker andbetter conclusions.315
This then should be the new HIPAA: Researchers should be allowed torelease full, unscrubbed databases to verifiably trusted third parties, subject
to new controls on use and new penalties for abuse. Releases to less-trusted
third parties should fall, of course, under different rules. For example, trust
should not be transitive. Just because Dr. A gives her data to trusted Dr. B
does not mean that Dr. B can give the data to Dr. C, who must instead ask Dr.
A for the data. Furthermore, releases to nonresearchers such as the marketing arm of a drug company should fall under very different, much more
restrictive rules.
2.

IP Addresses and Internet Usage Information

Lastly, consider again the debate in the European Union about datacontaining IP addresses. Recall that every computer on the internet, subject
to some important exceptions, possesses a unique IP address that it reveals toevery computer with which it communicates. A fierce debate has raged between
European privacy advocates who argue that IP addresses should qualify as“personal data” under the Data Protection Directive316 and online companies,
317
notably Google, who argue that in many cases they should not. European
318
officials have split on the question, with courts and regulators in Sweden319

314.
It makes sense to continue to prohibit the transfer of some data, such as names, homeaddresses, and photographs that could reveal identity without any outside information at all.
315.
The current HIPAA Privacy Rule has itself been blamed for a reduction in data sharing among
health researchers.
In a survey of epidemiologists reported in the Journal of the American Medical Association,
two-thirds said the HIPAA Privacy Rule had made research substantially more difficult andadded to the costs and uncertainty of their projects. Only one-quarter said the rule had
increased privacy and the assurance of confidentiality for patients.
Nancy Ferris, The Search for John Doe, GOV’T HEALTH IT, Jan. 26, 2009, http://www.govhealthit.com/
Article.aspx?id=71456.
316.
2007 Working Party Opinion, supra note 28, at 21; Electronic Privacy Information Center,Search Engine Privacy, http://epic.org/privacy/search_engine (last visited Apr. 4, 2010).
317.
See sources cited infra note 324.
318.
For a good summary, see Posting of Joseph Cutler, Was That Your Computer Talking to Me?
The EU and IP Addresses as “Personal Data”, P ERKINS C OIE DIGESTIBLE LAW BLOG, http://www.
perkinscoie.com/ediscovery/blogQ.aspx?entry=5147 (June 24, 2008, 23:30 EST).

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and Spain320 deciding that IP addresses fall within the Directive and those in
France,321 Germany,322 and the UK323 finding they do not.
a. Are IP Addresses Personal?
The debate over IP addresses has transcended EU law, as Google hasframed its arguments not only in terms of legal compliance but as the best way
to balance privacy against ISP need.324 In this debate, Google has advanced
arguments that rely on the now discredited binary idea that typifies the PII
mindset: Data can either be identifiable or not. Google argues that data should
be considered personal only if it can be tied by the data administrator to one
single human being. If instead the data administrator can narrow an IP
address down only to a few hundred or even just a few human beings—in otherwords, even if the administrator can reduce the entropy of the data significantly—Google argues that it should not be regulated. By embracing this idea,
Google has downplayed the importance of information entropy, the idea that
we can measure and react to imminent privacy violations before they mature.
Google frames this argument in several ways. First, it argues that IP
addresses are not personal because they identify machines, not people.325
Google’s Global Privacy Officer, Peter Fleischer, offers hypothetical situations
319.
John Oates, Sweden: IP Addresses are Personal . . . Unless You’re a Pirate, REGISTER, June18, 2009, available at http://www.theregister.co.uk/2009/06/18/sweden_ip_law.
320.
AGENCIA ESPAÑOLA DE PROTECCIÓN DE DATOS, STATEMENT ON SEARCH ENGINES(2007), available at http://www.samuelparra.com/agpd/canaldocumentacion/recomendaciones/common/
pdfs/declaracion_aepd_buscadores_en.pdf (opinion of Spanish Data Protection Agency deciding thatsearch engines process “personal data,” relying in part on earlier rulings about IP addresses).
321.
Meryem Marzouki, Is the IP Address Still a Personal Data in France?, EDRI-GRAM, Sept. 12,
2007, http://www.edri.org/edrigram/number5.17/ip-personal-data-fr.
322.
Posting of Jeremy Mittma, German Court Rules That IP Addresses Are Not Personal Data,PROSKAUER PRIVACY LAW BLOG, http://privacylaw.proskauer.com/2008/10/articles/european-union/
german-court-rules-that-ip-addresses-are-not-personal-data (Oct. 17, 2008).
323.
INFO. COMM’R’S OFFICE, DATA PROTECTION GOOD PRACTICE: COLLECTING PERSONALINFORMATION USING WEBSITES 3 (2007), available at http://www.ico.gov.uk/upload/documents/library/
data_ protection/practical_application/collecting_personal_information_from_websites_v1.0.pdf.
324.
Posting of Alma Whitten, Are IP Addresses Personal?, GOOGLE PUBLIC POLICY BLOG,
http://googlepublicpolicy.blogspot.com/2008/02/are-ip-addresses-personal.html (Feb. 22, 2008, 12:31
EST) (tying the discussion to the broad question, “as the world’s information moves online, how should
we protect our privacy?”); Peter Fleischer, Can a Website Identify a User Based on IP Address?, PETERFLEISCHER: PRIVACY . . . ?, http://peterfleischer.blogspot.com/2008/02/can-website-identify-user-basedon-ip.html (Feb. 15, 2008) (“Privacy laws should be about protecting identifiable individuals and theirinformation, not about undermining individualization.”). Mr. Fleischer serves as Google’s Global PrivacyCounsel. Because of this, I cite his blog posts for clues about Google’s views, but I should be clear that
Mr. Fleischer’s blog bears the disclaimer, “these ruminations are mine, not Google’s.”325.
Cf. Fleischer, supra note 324 (An IP address “constitutes by no means an indirectly
nominative data of the person in that it only relates to a machine, and not to the individual who isusing the computer in order to commit counterfeit.” (quoting decision of the Paris Appeals Court)).

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in which many users share one computer with a single IP address, such as “themembers of an extended family each making use of a home pc, a whole
student body utilising a library computer terminal, or potentially thousands ofpeople purchasing from a networked vending machine.”326 Is Fleischer rightto categorically dismiss the threat to privacy in these situations? Is there no
threat to privacy when Google knows that specific search queries can be narrowed down to the six, seven, maybe eight members of an extended family?
For that matter, should regulators ignore the privacy of data that can be
narrowed down to the students on a particular college campus, as Fleischer
implies they should?
Second, in addition to the machine-not-person argument, Google further
ignores the lessons of easy reidentification by assuming it has no access to
information that it can use to tie IP addresses to identity. On Google’s official
policy blog, Software Engineer Alma Whitten, a well-regarded computer scientist, asserts that “IP addresses recorded by every website on the planet without
additional information should not be considered personal data, because thesewebsites usually cannot identify the human beings behind these number
strings.”327 Whitten’s argument ignores the fact that the world is awash in richoutside information helpful for tying IP addresses to places and individuals.
For example, websites like Google never store IP addresses devoid ofcontext; instead, they store them connected to identity or behavior. Google
probably knows from its log files, for example, that an IP address was used toaccess a particular email or calendar account, edit a particular word processing document, or send particular search queries to its search engine. By
analyzing the connections woven throughout this mass of information, Google
can draw some very accurate conclusions about the person linked to any
particular IP address.328
Other parties can often link IP addresses to identity as well. Cable andtelephone companies maintain databases that associate IP addresses directly tonames, addresses, and credit card numbers.329 That Google does not store thesedata associations on its own servers is hardly the point. Otherwise, national

326.
Peter Fleischer, Are IP Addresses “Personal Data”?, PETER FLEISCHER: PRIVACY…?, http://peter
fleischer.blogspot.com/2007/02/are-ip-addresses-personal-data.html (Feb. 5, 2007, 17:18 EST).
327.
Whitten, supra note 324 (emphasis added).
328.
See 2008 Working Party Opinion, supra note 308, at 21 (“The correlation of customer
behaviour across different personalised services of a search engine provider . . . can also be accomplishedby other means, based on . . . other distinguishing characteristics, such as individual IP addresses.”).
329.
Id. at 11, 16.

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ID numbers in the hands of private parties would not be “personal data” because
only the government can authoritatively map these numbers to identities.330
Google can find entropy-reducing information that narrows IP addresses
to identity in many other places: Public databases reveal which ISP owns an
IP address331 and sometimes even narrow down an address to a geographic
region;332 IT departments often post detailed network diagrams linking IP
addresses to individual offices; and geolocation services try to isolate IP addresses
333
to a particular spot on the Earth. In light of the richness of outside information
relating to IP addresses, and given the power of reidentification, Google’s arguments amount to overstatements and legalistic evasions.
Google’s argument that it protects privacy further by deleting a single octet
of information from IP addresses is even more disappointingly facile and incorrect. An adversary who is missing only one of an IP address’s four octets cannarrow the world down to only 256 possible IP addresses.334 Google deserves nocredit whatsoever for deleting partial IP addresses; if there is a risk to storing
IP addresses at all, Google has done almost nothing to reduce that risk, andregulators should ask them at the very least to discard all IP addresses associatedwith search queries, following the practice of their search-engine competitors,
Microsoft and Yahoo.335
b. Should the Data Protection Directive Cover Search Queries?
Not only does the easy reidentification result highlight the flaws in
Google’s argument that IP addresses are not personal, it also suggests that
European courts should rule that the EU Directive covers IP addresses. Recall
that the Directive applies broadly to any data in which a “person . . . can be
identified, directly or indirectly, in particular by reference to an identification
number or to one or more factors specific to his physical, physiological,
330.
Fleischer correctly points out that ISPs are often forbidden from disclosing the user associatedwith an IP address. Fleischer, supra note 324 (“[T]he ISP is prohibited under US law from giving Googlethat information, and there are similar legal prohibitions under European laws.”) This is no different
from any other kind of account number which can be authoritatively tied to identity only by the issuingentity. All other entities must make educated guesses.
331.
E.g., ARIN WHOIS Database Search, http://ws.arin.net/whois (last visited June 12, 2010)(“ARIN’s WHOIS service provides a mechanism for finding contact and registration information forresources registered with ARIN.”).
332.
ERIC COLE & RONALD KRUTZ, NETWORK SECURITY BIBLE 316–18 (2005) (discussing
reverse DNS queries).
333.
E.g., IP2Location.com, http://www.ip2location.com (last visited June 12, 2010); Quova,
http://www.quova.com (last visited June 12, 2010).
8334.
An octet is so named because it contains eight bits of data. 2 = 256.
335.
See supra note 200.

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mental, economic, cultural or social identity.”336 Because websites can often
tie IP addresses to individual people, the Directive should apply to them.
Still, courts in Germany, France, and the UK have held to the contrary. Should
the EU amend the Directive to even more unequivocally cover IP addresses?
The answer is not to expand the Directive to specifically cover IP
addresses, as we might have done when we still organized laws solely around
PII. Instead, the EU should enact new, sectoral regulations that reflect aweighing of costs and benefits for specific problems. In this case, rather
than ask whether any company holding an IP address should bear the burdenof the EU Directive, the EU might ask whether the benefit of allowing searchengines in particular to store and disclose information—including IP addresses
associated with search queries—outweighs the potential harm to privacy.337
I must save for another day a complete response to this question, but todemonstrate the new test for deciding when to regulate after the fall of anonymization, I will outline why I think search engines deserve to be regulated
closely. Compare the benefits and costs of allowing unfettered transfers ofstored search queries to the earlier discussion about health information, taking the benefits first. By analyzing search queries, researchers and companies
can improve and protect services, increase access to information, and tailor
online experiences better to personal behavior and preferences.338 These areimportant benefits, but not nearly as important as improving health and saving
human lives.
On the other side of the ledger, the costs to privacy of unfettered accessare probably as great for search query information as for health information, if
not greater. As the AOL breach revealed, stored search queries often containuser-reported health symptoms.339 In fact, Google takes advantage of this to
track and map influenza outbreaks in the U.S.340 When one considers how often
Google users tell Google about symptoms that never escalate to a visit to thedoctor, one can see how much richer—and thus more sensitive—this information can be than even hospital data.
We reveal even more than health information to search engines, supplying
them with our sensitive thoughts, ideas, and behavior, mixed in of course with

336.
EU Data Protection Directive, supra note 3, art. I(a).
337.
In the EU, the Article 29 Working Group privacy watchdog has proposed similarly special
treatment for search engines. 2008 Working Party Opinion, supra note 308, at 24.
338.
See supra note 201.
339.
Barbaro & Zeller, supra note 69 (“Her search history includes ‘hand tremors,’ ‘nicotine effects
on the body,’ ‘dry mouth’ and ‘bipolar.’ But in an interview, Ms. Arnold said she routinely researched
medical conditions for her friends to assuage their anxieties.”).
340.
Google.org, Flu Trends, http://www.google.org/flutrends (last visited June 12, 2010).

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torrents of the mundane and unthreatening.341 In an earlier article, I argued thatthe scrutiny of internet usage—in that case by Internet Service Providers—
represents the single greatest threat to privacy in society today.342 Regulators
have underappreciated the sensitive nature of this data, but events like the
AOL data release have reawakened them to the special quality of stored
search queries.343
Because the costs of unfettered data access are as high in the searchengine as in the health context, EU and U.S. regulators should consider
enacting specific laws to govern the storage and transfer of this information.
Because the benefits are less than for health information, regulators should be
willing to restrict the storage and flow of search query information even morethan HIPAA restricts health information.
Thus, the EU and U.S. should enact new internet privacy laws that focus
on both the storage and transfer of search queries. They should impose a quantity cap, mandating that companies store search queries for no longer than aprescribed time.344 They should set the specific time limit after considering searchcompanies’ claims that they must keep at least a few months’ worth of data toserve vital business needs. They should also significantly limit third-party accessto search query data.

CONCLUSION
Easy reidentification represents a sea change not only in technology but
in our understanding of privacy. It undermines decades of assumptions aboutrobust anonymization, assumptions that have charted the course for business
relationships, individual choices, and government regulations. Regulators must
respond rapidly and forcefully to this disruptive technological shift, to restore
balance to the law and protect all of us from imminent, significant harm. They
must do this without leaning on the easy-to-apply, appealingly nondisruptive,
but hopelessly flawed crutch of personally identifiable information. This Article
offers the difficult but necessary way forward: Regulators must use the factors
provided to assess the risks of reidentification and carefully balance these risks
against countervailing values.

341.
Cf. Julie Cohen, Examined Lives: Informational Privacy and the Subject as Object, 52 STAN.
L. REV. 1373, 1426 (2000).
342.
Paul Ohm, The Rise and Fall of ISP Surveillance, 2009 U. ILL. L. REV. 1417, 1417.
343.
See 2008 Working Party Opinion, supra note 308, at 8 (“Search engines play a crucial role
as a first point of contact to access information freely on the internet.”).
344.
Cf. id. at 19 (“[T]he Working Party does not see a basis for a retention period beyond 6
months.”).

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Although reidentification science poses significant new challenges, it also
lifts the veil that for too long has obscured privacy debates. By focusing regulators and other participants in these debates much more sharply on the costs
and benefits of unfettered information flow, reidentification will make us
answer questions we have too long avoided. We face new challenges, indeed,
but we should embrace this opportunity to reexamine old privacy questions
under a powerful new light.