Letters

Stopping Junk Email by Using
Conditional ID Technology: privango
Katsumi Takahashi†, Tsuyoshi Abe,and Masahisa Kawashima
Abstract
We have developed a conditional ID technology called privango, which enables users to set usage conditions freely, and a privango email system, which provides a conditional email address containing usageconditions, e.g., expiration date and a specific sender. Using a conditional email address, the system prevents unsolicited email from being sent to users. In this system, the conditions for receiving an email are
cryptographically embedded in the email address itself, so no database is needed to store filter rules forthe addresses. The practical usability of the presented system has been tested with major Internet services that require email addresses. Besides email, we are studying various other possible uses for privango.

1. Introduction
Needs for email utilization becomes more and moresophisticated. Today, the exchange of email is not
limited to family members, acquaintances, and colleagues but can occur with anyone, and often via
websites. However, unsolicited (junk or spam) email
is a major nuisance for all email users. Once junkemail starts arriving at a specific address it is very difficult to stop it from continuing to arrive.
One way to avoid junk email is to selectively use
different email addresses depending on the email
sender, the email purpose, and/or the timeframe inwhich email
arrives. NTT
Information
Sharing
Platform
Laboratories
has developed a system that provides email† NTT Information Sharing Platform Laboratories
Musashino-shi, 180-8585 Japan
E-mail: takahashi.katsumi@lab.ntt.co.jp

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addresses containing usage condition informationsuch as an expiration date and the email sender.
Emails sent from such an addressing system arecalled privango* mail. An email sent to a conditional
email address, i.e., a privango mail address, is either
sent to the recipient’s real email address or blocked
depending on whether the conditions are met or not.
Obtaining an email address with user-defined rejection conditions, such as expiration date, purpose, andsender, allows the user to control the reception ofunsolicited email.
2. Problems and solutions
Some well-known measures for stopping junkemail include:• Setting rejection conditions for receiving email:specifying email addresses/domains from whichemails should be accepted because they originate* Privango: This is a coding technology developed by NTT Information Sharing Platform Laboratories to protect email addresses
and other private information by using encryption and other security technologies. The main feature of privango is the encoding ofaccess information into the ID itself in such a way that the ID iscompact enough for the user to enter it easily. The word “privango” is derived from “privacy” and “ango” (a Japanese word forencryption).

NTT Technical ReviewLettersfrom a reliable sender or rejected because they
originate from an unknown sender. This is also
known as setting up a white list and/or black list.
• Using disposable email address: using an email
address until junk email starts arriving and thenswitching to a different email address.
• Filtering by content: analyzing the content of an
incoming email and determining whether it isjunk or not.
Setting rejection conditions is fairly effective whenthe email addresses sending junk email are known.
However, it is not a completely satisfactory solution
because there is usually a limit on the number ofaddresses/domains that can be registered for blocking
emails. Also, it is difficult to select a completelyworkable list of addresses/domains because many
junk email senders use address spoofing, which offers
them an almost infinite number of possible addresses.
Using disposable email addresses has the drawbackthat it is too anonymous, so receivers easily forget theuser’s identity.
Filtering by content is fairly effective for stopping

junk mail; however, although the accuracy of the filtering methods is improving, the filters are not 100%
reliable and occasionally mail that is not junk is filtered by mistake.
Considering these problems, we developed privango mail, which offers a means of stopping junk emailthat is free from the above limitations and combines
the advantages of the conventional measures mentioned above. Rather than define rejection conditions
for the user’s real address, privango mail allows
selective use of a number of email addresses that havethe conditions for receiving an email embedded in theemail address itself. Such conditions may include thepossible senders to these email addresses and how
these email addresses are used. In addition, just as
with disposable email addresses, the user can create
multiple email addresses and attach conditions toeach of them. The limitation where the email sender’sidentity is lost has been solved by assigning a uniqueand permanent nickname to each privango mail member, which is included in a privango mail address.

• Entering a contest

Privango center

Privango email address
with an expiry date conditionof one month is issued.
Issuing server

Mr. Sato

Enters a contest on the Web using the conditional email addressContest website• Notifying the winner
Email is forwarded tothe real email address.

Email notifying the winneris sent (within one month).
Mail server

MailboxContest website

Mr. Sato• After expiry of the contest period

After one month, email to theconditional address is blocked.

Mail server
MailboxMalicious user

Mr. Sato

Fig. 1. Example of temporary use of various services, such as email and Web service.

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Letters

3. Procedure for using privango mail system
3.1 Registration at a website
User interaction with the privango mail system isexplained below using Fig. 1. Suppose that Mr. Satofinds an advertisement for a new product giveaway
campaign on the Internet. When he visits the specified website, he finds that he must enter his email
address to participate in the campaign, but he is worried about receiving unsolicited advertisement emailor that his email address will be released to third parties. In such a case, Mr. Sato can use the privango
mail service to obtain a privango mail address that has
an expiration date. He can enter this privango mailaddress instead of his real email address (Fig. 1(a)).
Any email sent to a privango mail address is checkedby the privango center for any matching conditions
contained in the email. If the email meets the conditions, it is forwarded to Mr. Sato’s real email address(Fig. 1(b)). If email arrives after the expiration date, itis blocked by the privango center so that Mr. Sato will
not receive it (Fig. 1(c)). By using the privango mailservice, Mr. Sato takes precautionary measures toavoid receiving unsolicited email or having his realemail address released to unknown parties.

3.2 Exchanging email between acquaintances
Figure 2 shows a case where a person wants toexchange email with a specific person and avoid
receiving unsolicited email. Mr. Yamada and Mr.
Suzuki always contact each other by email via theirmobile phones. Since they often receive junk email,they change their mobile phone email addresses fromtime to time, but this is troublesome because theymust change their mail settings and notify each otherof the changes. They can avoid such inconvenienceby using the privango mail service, which allows
them to use email addresses that guarantee only email
from the other party will be received. To set up thisprotected communication system, Mr. Yamada
obtains a privango mail address from the privango
mail service, specifying that email to that address canonly be from Mr. Suzuki. Similarly, Mr. Suzuki
obtains a privango mail address and specifies thatemail to that address can only be from Mr. Yamada.
By using these email addresses, Mr. Yamada and Mr.
Suzuki can contact each other, confident that they
will not receive junk email.
4. Main features
The privango mail system is an email forwarding

Email is sent to Mr. Suzuki’sconditional email address.
Email is sent to Mr. Suzuki’sconditional email address.
Privango center
Mr. Yamada
Mr. Suzuki’s conditional
email address is issued.

Issuing server

Mail server

Mr. Yamada’s conditional
email address is issued.
Malicious user

Mr. Suzuki

Sender’s email address ischecked and any email
from a third party is blocked.

Fig. 2. Example of blocking unsolicited email and allowing email exchange with only specific email addresses.

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NTT Technical ReviewLetterssystem that receives an email sent to an alias address(an address different from the recipient’s realaddress) and forwards it to the real address. Thus, aprivango mail address is an alias. The privango center that receives an email to a privango mail address
checks whether the email meets the conditions specified in the address and forwards it to the real address
only when it satisfies the conditions. The user can use
his or her ISP-provided email address, office email
address, or mobile phone email address as his or her
real address. A privango mail address has the following features:(1) Encryption
The usage conditions are embedded in the email
address using encryption. The result is an email
address consisting of the nickname and usage conditions (Fig. 3). The nickname can be selected without
any association to the real address and is anunchangeable character string unique among themembers registered with the privango mail service.
Since the usage conditions are encrypted, no third
parties can read or change them. In addition, since noequipment is required to hold the information about
the issued email addresses or their usage conditions,an infinite number of safe email addresses can begenerated at a low cost.
(2) Versatile configuration
Any or all of the following four conditions can beembedded in an email address:• Expiration date (e.g., March 1, 2005)• Sender email address (e.g., only email from suzuki@ntt.co.jp is accepted)• Sender’s email address domain (e.g., only email
with the domain name ntt.co.jp is accepted)• Specific key words in the subject (e.g., only email
whose subject includes “ABC” is accepted)(3) Automatic conversion of sender address

Nickname part
Unique among
registeredmembers

When a user replies to an email sent to his or her
privango mail address, the privango mail system
automatically converts this reply address to the privango mail address used by the original email. Since
the user need not change the reply address, he or she
can exchange privango mail just like ordinary email.
5. Field tests
The privango mail system was implemented on aLinux-based server and operated experimentally inan Internet environment. About thirty users participated in this field test, which lasted from August2003 to March 2004. During the test, privango mailaddresses were used for contacting acquaintances,
registering on electronic commerce websites, sending
Internet auction bidding notifications, posting messages on Internet bulletin boards, and publishingemail addresses on websites. In spite of the short period of the field test, several unsolicited emails, most ofwhich were addressed to email addresses publishedon websites, arrived at the privango center. Thesejunk emails were discarded by the privango center inaccordance with the expiration dates included in theprivango mail addresses. Thus, the system protected
the users from receiving junk mail.
6. Future work
In addition to the email application explained above[1], we are developing a series of other applications
for safely accessing information (schedule information, etc.) using email [2] and an URL application forsafely accessing websites [3]. We are also creating asoftware development kit to facilitate the development of privango-based systems.

Usage condition part
Each set of conditions is represented by a different
set of characters.
Expiration date and specific sender email
addresses are defined in a set of conditions.

Fig. 3. Example of a privango email address.

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LettersReferences
Katsumi Takahashi
[1]

[2]

[3]

T. Abe, M. Kawashima, K. Takahashi, and J. Miyake, “Conditionsembedded email address using encryption technology,” Proceedingsof the 2004 IEICE Communication Society Conference, B-16-6, 2004
(in Japanese).
K. Fukami, T. Abe, M. Kawashima, and K. Takahashi, “Secure information access method using conditions-embedded email address,”
Proceedings of the 2004 IEICE Communication Society Conference,B-16-7, 2004 (in Japanese).
M. Sakuma, A. Shirakami, T. Abe, and K. Takahashi, “User authentication method using conditional URI,” Proceedings of the 2004
IEICE Communication Society Conference, B-7-56, 2004 (in Japanese).

Senior Research Engineer, Supervisor, Software Architecture Project, NTT Information
Sharing Platform Laboratories.
He received the B.S. degree in mathematics
from Tokyo Institute of Technology, Tokyo in
1988. Since then he has been in NTT Laboratories, Tokyo, Japan. His research interests are
Internet/mobile information services, data mining, data storage system, and security and privacy. He is currently a Ph.D. student at the University of Tokyo. He won the Information Processing Society of Japan (IPSJ) Best Paper Award in
2000. He is a member of IPSJ.

Tsuyoshi Abe
Research Engineer, Software Architecture Project, NTT Information Sharing Platform Laboratories.
He received the B.E. and M.E. degrees inmechanical engineering from Waseda University, Tokyo in 1993 and 1995, respectively. In
1995, he joined the NTT Network Service Systems Laboratories. He has been engaged in R&D
of file transfer and sharing systems. He is now
working on collaboration and communication
service systems. He is a member of the Instituteof Electronics, Information and Communication
Engineers (IEICE) of Japan.

Masahisa Kawashima
Senior Research Engineer, Supervisor, Application Platform SE Project, NTT Information
Sharing Platform Laboratories.
He received the B.E, M.E., and D.E. degrees inelectrical engineering from Waseda University,
Tokyo in 1989, 1991, and 1994, respectively.
Since then he has been in NTT Laboratories,
Tokyo, Japan, working on communication services over the Internet. He received the Study
Group Award from the IEICE-J SSE study groupin 1991. He received Awards for Outstanding
Personal Contribution from DAVIC, an international consortium for digital video distribution,in 1995 and 1999.

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