Unlocking PDF
guaranteed password recovery for Adobe Acrobat

Copyright (c) 2007 ElcomSoft Co.Ltd.

Contents

Going digital

3

What’s good about PDF?

4

PDF documents protection

5

Cui bono?
PDF documents protection methods

Data access loss

7

Solving a puzzle

8

A few words about passwords
Password recovery methods
Choosing a solution“ElcomSoft” solution – secure access to PDF-files

12

Advanced PDF Password Recovery
ElcomSoft Distributed Password Recovery

About ElcomSoft

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

172

Going digital
Rapid development of digital technologies and electronic communication, availability of mobile
PCs and smart phones, abundance of tools for creating documents or high-quality presentations
coupled with constantly growing volumes of information and yearning for doing business more
effectively leads to switching to electronic exchange of information.
Not only enterprises, but state organizations are introducing new electronic systems of documents circulation. Online-readers of e-newspapers and e-magazines outnumber those, who stillread print media. Some titles exist on the net only. Those who own PDAs prefer e-books ande-magazines over paper ones. For instance, more than 80,000 e-books can be bought at the
Amazon1 online store.
Exchanging e-documents means compatible platforms, applications and software versions. Everyone has faced a “can’t open the file” situation. What does a file recipient use – PC or Mac,
Windows Vista or Windows XP, Microsoft Word or Corel WordPerfect? The solution is using auniversal format, which depends neither on software nor on hardware.

1 PDF books and documents. Data of 12.09.07

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

3What’s good about PDF?
Such format does exist. This is an immensely popular PDF (Portable Document Format) — crossplatform format for e-documents, designed by Adobe.
Documents saved as PDF files can be viewed on virtually any system. Though platform, operating system, installed fonts or software may vary, a document with original text formatting, fontsettings, pictures or layout can still be opened, read or printed correctly.
PDF is widely used in publishing and printing, distributing e-versions of media, publishing reports,
reference information or documents, and exchanging data. Google search results for PDF files
amounts to 2 360 000 pages!
It is really important that the software needed to open a PDF is free (from Adobe or a third party).
These figures show the popularity of PDF format: Acrobat Reader is downloaded from CNET
Downloads for nearly 35 billion times per month2.
Versatile PDF format has quite a number of extra features: easy browsing of large documents witha help of a cross-reference system; proper viewing of a document with handheld devices running
Palm OS, Symbian OS etc; text availability for search engines; compact files (small file size) and –the feature we are most interested in – wide range of document protection methods.

2 Over a period 08.08.07-13.09.07

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

4

PDF documents protection
Cui bono?
Let’s see who needs to protect a PDF document and why. Today’s world assumes that possessing information is a competitive edge. Leak of confidential data may cause direct financial harm
to an enterprise, indirect losses of opportunities and other unfavorable or unpredictable effects.
Obviously, much attention is paid to data protection. IT Security is a rapidly developing branchof Information Technology industry. The easiest method of data protection is locking with a password. One has an ability to set the password (to open and/or edit the document, or use particularfeatures like hi-resolution printing) in most desktop applications, including Adobe Acrobat.
Proper document password protection should keep the balance between security and usability.
When setting a protection to a PDF document, one should keep the following points in mind:
1.  target audience (all users, or just a certain group);
2.  version of Adobe Acrobat / Acrobat Reader available;
3.  storage location or publishing location of the document (company web site, intranet, or printing-house);
4.  data type (text, graphics, multimedia);
5.  presumable usage of the document (viewing, filling forms, sending by mail, editing or reviewing).

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

5

PDF documents protection methods
Adobe Acrobat features two levels of PDF password protection. Protecting the document withaccess restriction (“owner”, so-called “security” or “master”) password does not affect a user’s
ability to open and view the PDF file, but prevents user from editing (changing) the file, printing it,
selecting text and graphics (and copying them into the Clipboard), adding/changing annotations
and form fields etc (in any combination). Also, there are “open” (so-called “user”) passwords. If
one is set, the file is encrypted with strong algorithm, and cannot be opened at all, if the password or encryption key is not known.
Adobe Acrobat uses RC4 encryption algorithm (stream cipher, widely-used by various data protection systems), while Adobe Acrobat 7.0 and upwards can also use AES (Advanced Encryption
Standard). Originally, 40-bit encryption has been used, but version 5.0 and above uses 128bitencryption, which makes it much more difficult to find a password (40-bit encryption involves
240 values, while 128-bit encryption involves 2128 values).
In addition, using security certificates allows creating different access/usage rights for different
user groups. For instance, some users will be allowed to fill in the forms, while users from anothergroup will be able to edit the document as well.
Certificate-based protection for Adobe Acrobat is based on two keys: public key and private key.
The former one is used to decrypt the file, and the last one is used for document encryption and/or signing a document.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

6

Data access loss
Good document protection is a double-edged weapon. Why?
Confidential data, such as sales reports, market research results or analytic reports, is storedin PDF files, and all operations (editing, printing and even opening a file) could be passwordprotected.
Obviously, the weakest link of any protection technology is a human. Password protection is asubject of various “flaws”. How many times have you forgotten a password? Once you may needto access a document created by your colleague or partner, but that person could have quitted
the job or taken a leave. What should be done? You can’t cease a project because of such ahindrance.
And another problem is more likely to occur when working with PDF files. Imagine you need to
extract a piece of information from a report to prepare tender documents, but you fail to selectand copy some text from a file. You obey copyright and always refer to a source of information,
but you have to prepare the documents for tomorrow’s tender as soon as possible.
And here is the rub: you need to remove restrictions set for a PDF file, or decrypt it (if it requires
password at opening) to get access to the information and solve current business tasks.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

7

Solving a puzzle
A few words about passwords
Since the problem of password loss first occurred the day password protection was invented,
software developers have considered a way of tackling it. As the result, a number of password
recovery technologies are available now.
Putting aside the issue of nowadays password recovery methods, let’s start with basic knowledge
about passwords, password types and information, which may assist you in finding a password.
English language passwords generally use the following symbols: 26 lowercase letters (a…z), 26
uppercase letters (A…Z), 10 digits (0…9) and 33 specific characters (!@#$%^ etc), which makes
95 symbols for any combinations. Sometimes specific symbols are excluded from the group, which
decreases the number of possible combinations. Moreover, password may be short or long, whichis crucial when one cannot retrieve or reset a password, but has to use the brute-force attack.
Understanding human mind also counts on a quest for a password. In spite of numerous restrictions forced on users to secure password protection, some users still neglect the most simple
security tips. Such phenomenon proves that human is a weak link, a dangerous breach in systemsecurity.
The majority of popular passwords are nothing but words, derived from a mother tongue of a user.
Sometimes words, used as passwords, can be found in user’s daily life: birth year, pet name, phone
number, credit card number etc. A new password can be a slightly modified previous password.
This is the way most users solve the problem of regular password changing, prescribed by security
policy. And the most important hint, people tend to keep a password note right at a work desk or ina file on a PC. Though, such a remedy undermines the whole idea of password protection.
Thus, being aware of basic password security requirements (such as password structure or length)or having some information about a user may help finding a password. Technologies, applied by
specific software to recover passwords, enable using such kind of information.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

8Password recovery methods
The basic methods of password recovery are brute force, mask attack, dictionary search, encryption key search (less possible combinations in comparison with brute force) and so-called“rainbow attack”. Sometimes other methods of restoring access to a file are used, for example,
known-plaintext attack. Let’s review some of the methods briefly.

Brute force attack
Brute force attack is simple: in search for a password a program tries every possible combination of symbols. The search may be restricted to a certain length and character set (letters, digits
and other symbols).
But how much time does the brute force attack need to recover a password? It depends onthe factors mentioned above (password length, character set), performance of the PC used forpassword recovery task, and the file type.
Of course, a correct password may be found quickly and a program won’t have to try all the possible combinations. But you shouldn’t count on that. The task can take years, if ran on an average
PC. The brute force attack, as the most time-consuming method, may be resorted to, when noother methods are at the hand.

Mask attack
In the case you created a password by yourself, you may try to recover a password with a helpof mask attack by limiting the search range. You might remember the length of a password orsome of the symbols. Any information may be of use to you.
For example, you are quite sure, that you used only digits and lowercase Latin letters. Then,
when setting search parameters, you may exclude specific symbols and uppercase letters. Thatwould be great if you also knew a certain position of a symbol in a password. For example, apassword consists of 10 symbols, starts with a letter “a” and ends with “2007”, than you can use
a “a?????2007” search pattern. Unknown symbols are designated with questions marks in thepattern.
Mask attack is making sense: a program has to try fewer combinations, so a password will befound in less time.
It’s a pity, but any details about a password are rarely known, thus mask attack cannot be usedgenerally. Fortunately, there is one more efficient password recovery method.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

9Dictionary attack
Let’s assume that you possess some information about possible words or names that could be
used in a password. In this case you may use the dictionary attack.
Users tend resort to common words for creating passwords. Generally, these are English words
like “open”, “access” or “letmein”. In comparison with chaotic combinations of letters and digits
such passwords are easier to remember. In fact, such passwords are as easily forgotten as anyother passwords, but they are easier to recover.
Where the dictionary (or the word list) should be taken from? First, it may be included into password recovery program package. Second, you may search for it on the Internet. Various lists ofcommon words, thematic lists (fauna, football teams etc), abbreviation lists are commonly available. Third, you can create a dictionary manually.
The method has a number of apparent advantages. The list of common words, generally usedin passwords, is limited; it never contains more than a hundred thousand words. Trying hundred thousand combinations is an easy task for a modern PC. It turns out that dictionary attack
method should be implemented in the first place. It may do well.

Rainbow table attack
Obviously, the most important criterion of password search is the criterion of time, consumed by
the search. Brute force attack tries all possible combinations, and recovery of complex passwords
may take too much time. If the search may take up months or years, than its practical use is zero.
A method employing rainbow tables (rainbow attack) is used to eliminate the problem. The basisof the method is using precomputations of password variants for a certain set of symbols.
The idea of replacing resource-intensive computations with a search by a lookup table, that was
prepared beforehand, is not brand new. Lookup tables are used when data is easier extracted
from the memory, rather than created. The main drawback of a lookup table is its size: not every
enterprise can afford storing terabytes of data. That’s why rainbow tables, or optimized lookup
tables, came into being. The size of a rainbow table is much less, than of a lookup one.
Generating rainbow tables may have preset probability of password or key recovery, suggested
time of attack, and time of table’s generation. Adjusting the settings and finding a good balance
between the attack time and probability of password/key recovery should be considered separately. As a result, the tables that help to quickly find the password/key from a certain range witha high probability are created in a reasonable time.
In comparison with simple lookup tables, the probability of password recovery using rainbow
attack is slightly lower than 100%, but the result is still worth trying. For example, rainbow attack
based on a table for 7 alphanumeric symbols (built within a week) allows recovering any password consisting of seven alphanumeric symbols within 20-30 seconds. Brute force attack would
take up more than 24 hours. The advantage is obvious.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

10Choosing a solution
Thus there’re no more doubts in buying password recovery tool (for PDF files) or not. Obviously,
every system administrator should have such tool at hand. The expenses will be repaid a hundredfold when the first password is missed.
What points should be considered in this case?
First, the probability of password recovery claimed by a software provider. The criterion is crucial
for estimating solution efficiency. This is why you buy it. Of course, 100% probability may beguaranteed in the absence of time constraints, but this picture is not good enough for you. As ageneral rule, access to a document has to be restored as soon as possible: time counts.
Second, the next point to consider is a range of supported operating systems, application versions, file formats, languages and encodings. It’s hard to tell what version of Adobe Acrobat you’ll
have to deal with when recovering a password. Make sure you know how to get an upgrade withsupport of newer versions, and what timeframe such upgrade will be available in.
Third, consider the time needed for password recovery. Of course, it may depend on performance of your PC, but a software provider usually gives some “average” data.
And the last point is whether distributed computing is ever possible. This method of solving complex (CPU-hungry) problems suggests using the united performance of a computer group, forexample, computers connected locally or remotely. The method is employed when hacking apassword. Some passwords for documents or applications may be recovered in a short periodof time with a help of a single computer (for example, ICQ password saved locally, or a password
to WordPerfect document), but for many others, the recovery has much higher requirements.
For example, PGP passwords are so safe that hacking them could be possible using distributed
computing only.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

11

“ElcomSoft” solution – secure access to PDF-files
Russian company ElcomSoft offers a wide range of password-recovery solutions for virtually
any system and file format: from applications and instant messengers to archives and Windows
logon passwords.
ElcomSoft takes advantage of unique technologies and staff experienced in the field of cryptography. It allows creating high-quality password recovery software. Depending on password
length, complexity and encryption method the probability of password recovery amounts intonearly 80%. But in most cases, 100% success rate is guaranteed.
ElcomSoft has developed special software to restore access to Adobe Acrobat files – Advanced
PDF Password Recovery. Moreover, you can get Elcomsoft Distributed Password Recovery,
which allows to take advantage of distributed computing if the password is long and well selected.

Advanced PDF Password Recovery
Advanced PDF Password Recovery supports all versions and encryption methods used by
Adobe Acrobat 3.0 – 8.0, allowing to unlocking PDF files and remove restrictions.
Features provided by the program depend on the version: Standard, Professional, and Enterprise. Standard edition allows to removing editing and printing restrictions. Professional edition
can find a password to open a file. Enterprise edition finds an encryption key (DVD with rainbow
tables is supplied) with a help of improved rainbow attack.
Let’s review basic features of the product.
A “user” password (required to open the file) is often known or not set. So the only thing to search
for is an “owner” password, which restricts editing and printing options. Advanced PDF Password Recovery uses unique technology to solve this problem: it does not have to try password
combinations at all, but allows to decrypting the document regardless of encryption algorithm
and key length (see pic 1).
If “user” is set but unknown, Advanced PDF Password Recovery uses a number of methods:
brute force attack, mask attack, dictionary search and exclusive technology of key search (seepic 2). The probability of password recovery by common methods (brute force attack or dictionary search) generally amounts to 80%. In the meantime, the key search attack provides 100%
success rate, but can be applied only to files with 40-bit protection.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

12Advanced PDF Password Recovery
Advanced PDF Password Recovery supports all versions and encryption methods used by
Adobe Acrobat 3.0 – 8.0, allowing to unlocking PDF files and remove restrictions.
Features provided by the program depend on the version: Standard, Professional, and Enterprise. Standard edition allows to removing editing and printing restrictions. Professional edition
can find a password to open a file. Enterprise edition finds an encryption key (DVD with rainbow
tables is supplied) with a help of improved rainbow attack.
Let’s review basic features of the product.
A “user” password (required to open the file) is often known or not set. So the only thing to search
for is an “owner” password, which restricts editing and printing options. Advanced PDF Password Recovery uses unique technology to solve this problem: it does not have to try password
combinations at all, but allows to decrypting the document regardless of encryption algorithm
and key length (see pic 1).
If “user” is set but unknown, Advanced PDF Password Recovery uses a number of methods:
brute force attack, mask attack, dictionary search and exclusive technology of key search (seepic 2). The probability of password recovery by common methods (brute force attack or dictionary search) generally amounts to 80%. In the meantime, the key search attack provides 100%
success rate, but can be applied only to files with 40-bit protection.

Picture 1. Removing restrictions from a PDF file.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper13

If “user” is set but unknown, Advanced PDF Password Recovery uses a number of methods:
brute force attack, mask attack, dictionary search and exclusive technology of key search (seepic 2). The probability of password recovery by common methods (brute force attack or dictionary search) generally amounts to 80%. In the meantime, the key search attack provides 100%
success rate, but can be applied only to files with 40-bit protection.
Apart from probability of password recovery, the speed of recovery process counts as well. Advanced PDF Password Recovery has the best results: an average PC needs only several days torestore access to documents protected by user-password and encrypted with 40-bit RC4.
Enterprise edition employs rainbow attack to solve the problem in just a few minutes. Program
uses pre-computed hash tables (supplied on a DVD). Thunder Tables™ 3 technology, developed
by ElcomSoft, allows reaching 100% probability of finding the key (and so decrypting the file).
The technology uses lookup tables and rainbow tables jointly, which, on one hand, guarantees
success (as when using simple lookup tables) and, on the other hand, takes only a few minutes
to complete. Other parties may guarantee such results only when using full search for an encryption key (takes several days) or using bulky lookup tables (a few terabytes).

3 Patent technology

has been claimed

Picture 2. Selecting file decryption method.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

14

The amount of decrypted files depends on attack period (see pic 3). Half of the files are opened
within 10 seconds. The maximum attack time is 15 minutes, and the minimum is a fraction of
a second; 25 seconds on an average. Using modern computer with multi-core processor (forexample, Intel® Core™ 2 Duo) and reading tables from a USB flash card (instead of DVD) is
recommended.
The program also allows to restore access to PDF documents protected by 128-bit encryption(including Advanced Encryption Standard) with a help of brute force attack, mask attack anddictionary search. The technology of key search is not applied to files of this type.
The program offers the best working pattern depending on a configuration of your processor(Non-MMX processors, Intel PII/PIII/Celeron, AMD Athlon, Intel P4 SSE2 are enlisted). The highest
performance for Core, Core Duo or Core 2 Duo processors is gained by selecting Intel PII/PIII/
Celeron from the list.
Trial version of the program can help to estimate its features. Though the functionality is limited(recovery of 4-symbol password and decrypting first 10% of document pages), it still gives acomplete image of the product.

100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
1 sec

2 sec

5 sec

10 sec20 sec30 sec

1 min

2 min

5 min

10 min15 minPicture 3. The amount of decrypted PDF files Vs period of attack.

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

15

ElcomSoft Distributed Password Recovery
The advantages of using distributed computing when solving complex tasks have been discussed earlier. When talking about PDF documents, you may also face such tasks, especially
if you deal with considerable amount of paperwork, safe passwords and advanced encryption
algorithms.
A program consists of three components: server, agent and console. Server (see pic 4), installedon the computer in the local network, controls the brute force attack. Agents, which try certain
sets of password combinations sent by the Server, may be installed on all computers in the net.
Console may be launched from any computer and allows driving the Server, adding new tasks
and viewing statistics. Server and Agent have trial versions.

Picture 4. Main window of Elcomsoft Distributed Password Recovery (server component).

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

16About ElcomSoft
Founded in 1990 in Moscow, Russia, ElcomSoft is a leader in the password/system recovery andforensics market. Thanks to one-of-a-kind technologies, ElcomSoft’s products have garneredwide recognition both in Russia and abroad.
ElcomSoft’s clients include many well known international companies from the following sectors:
High Tech: Microsoft, Adobe, IBM, Cisco
Governmental: FBI, CIA, US Army, US Navy, Department of Defence
Consulting: Andersen Consulting, Deloitte & Touche, Ernst and Young, KPMG, PricewaterhouseCoopers
Finance: Bank of America, Citibank, Equifax, HSBC, Wells Fargo, J.P.Morgan, Credit Suisse
Telecommunications: France Telecom, BT, AT&T
Insurance: Allianz, Mitsui Sumitomo
Retail: Wal-Mart, Best Buy, Woolworth
Media&Entertainment: Sony Entertainment
Manufacturing: Volkswagen, Siemens, Boeing
Energy: Lukoil, Statoil
Pharmaceuticals: Johnson&Johnson, Pfizer, GlaxoSmithKline, Novartis
ElcomSoft is a Microsoft Gold Certified Partner, Intel Software Partner, as well as a member ofthe Russian Cryptology Association, the Computer Security Institute (CSI), and the Associationof Shareware Professionals (ASP).
ElcomSoft is an acknowledged expert in the password/system recovery and forensics market.
The company’s technological achievements and opinion leadership is quoted in many authoritative publications. For example: “Microsoft Encyclopedia of Security”, “The art of deception”(Kevin Mitnick), “IT Auditing: Using Controls to Protect Information Assets” (Chris Davis), “Hacking exposed” (Stuart McClure).
Visit our website to find out more.

Address:

ElcomsoftZvezdny bulvar 21, office 541
129085 Moscow, Russian Federation

Fax:

US (toll-free): +1 (866) 448-2703
United Kingdom: +44 (870) 831-2983
Germany: +49 18054820050734

Websites:

http://www.elcomsoft.ru
http://www.elcomsoft.com
http://www.elcomsoft.de
http://www.elcomsoft.jp
http://www.elcomsoft.fr

Unlocking PDF: guaranteed password recovery for Adobe Acrobat
whitepaper

17

Copyright (c) 2007 ElcomSoft Co.Ltd.
All right reserved.
This document is provided for information purposes only
and the contents hereof are subject to change without notice. This document is not warranted to be error-free, nor
subject to any other warranties or conditions, whether expressed orally or implied in law, including implied warranties
and conditions of merchantability or fitness for a particularpurpose. We specifically disclaim any liability with respect
to this document and no contractual obligations are formed
either directly or indirectly by this document. This document
may not be reproduced or transmitted in any form or by anymeans, electronic or mechanical, for any purpose, without
our prior written permission.
Microsoft and Windows are registered trademarks of Microsoft Corporation. Intel and Intel logo are registered trademarksof Intel Corporation. Elcomsoft and Elcomsoft logo are trademarks or registered trademarks of ElcomSoft Co.Ltd. Other
names may be trademarks of their respective owners.