5On Anonymity in an Electronic Society:A Survey of Anonymous Communication SystemsMATTHEW EDMAN and B¨ULENT YENERRensselaer Polytechnic InstituteThe past two decades have seen a growing interest in methods for anonymous communication on the Internet,both from the academic community and the general public. Several system designs have been proposed in theliterature, of which a number have been implemented and are used by diverse groups, such as journalists,human rights workers, the military, and ordinary citizens, to protect their identities on the Internet.In this work, we survey the previous research done to design, develop, and deploy systems for enablingprivate and anonymous communication on the Internet. We identify and describe the major concepts andtechnologies in the field, including mixes and mix networks, onion routing, and Dining Cryptographersnetworks. We will also review powerful traffic analysis attacks that have motivated improvements andvariations on many of these anonymity protocols made since their introduction. Finally, we will summarizesome of the major open problems in anonymous communication research and discuss possible directions forfuture work in the field.Categories and Subject Descriptors: C.2.0 [Computer-Communication Networks]: General—Securityand protection (e.g. firewalls); C.2.2 [Computer-Communication Networks]: Network Protocols—Applica-tions (SMTP, FTP, etc.); C.2.4 [Computer-Communication Networks]: Distributed Systems—Distributedapplications; K.4.1 [Computers and Society]: Public Policy Issues—PrivacyGeneral Terms: SecurityAdditional Key Words and Phrases: Anonymity, anonymous communication, DC-nets, mixes, mix networks,onion routing, privacy, traffic analysisACM Reference Format:Edman, M. and Yener, B. 2009. On anonymity in an electronic society: A survey of anonymous communicationsystems. ACM Comput. Surv. 42, 1, Article 5 (December 2009), 35 pages.DOI = 10.1145/1592451.1592456, http://doi.acm.org/10.1145/1592451.15924561. INTRODUCTIONA famous cartoon in The New Yorker once carried the caption, “On the Internet, nobodyknows you’re a dog” [Steiner 1993]. This phrase symbolizes the general public’s percep-tion that the Internet allows one to communicate in relative obscurity and anonymity.Quite the contrary, today’s modern communications infrastructure is indeed capable ofidentifying and recording what we do, where we go, and even sometimes what we sayon the Internet.Author’s address: Department of Computer Science, Rensselaer Polytechnic Institute, 110 8thStreet, Troy,NY 12180; email: {edmanm2,yener}@cs.rpi.edu.Permission to make digital or hard copies of part or all of this work for personal or classroom use is grantedwithout fee provided that copies are not made or distributed for profit or commercial advantage and thatcopies show this notice on the first page or initial screen of a display along with the full citation. Copyrights forcomponents of this work owned by others than ACM must be honored. Abstracting with credit is permitted.To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of thiswork in other works requires prior specific permission and/or a fee. Permissions may be requested fromPublications Dept., ACM, Inc., 2 Penn Plaza, Suite 701, New York, NY 10121-0701 USA, fax +1 (212) 869-0481, or [email protected]2009 ACM 0360-0300/2009/12-ART5 $10.00DOI 10.1145/1592451.1592456 http://doi.acm.org/10.1145/1592451.1592456ACM Computing Surveys, Vol. 42, No. 1, Article 5, Publication date: December 2009.5:2 M. Edman and B. YenerEach time we send an email, visit a Web page, or talk to friends and family viainstant messaging, we send packets of data across the Internet that contain informa-tion regarding where the message is going and who sent it (e.g., IP addresses, SMTPheader information, etc.). As packets are transmitted via several hops from the sourceto their destination, anybody observing a link along that path can roughly identify whois communicating with whom based on information contained in each packet. Even ifthe packet data is encrypted, the source and destination addresses in the packet’s IPheader are still visible to an observer.Research into designing and building anonymity systems seeks to build an infrastruc-ture running on top of the existing Internet protocols that allows people to communicatewith each other without necessarily revealing their personal network identifiers, suchas IP addresses.We can imagine many situations in which someone might desire strong communica-tions privacy and anonymity on the Internet. Law enforcement agencies may want toset up an online anonymous “tip line,” where people can report information they mayhave on unlawful activity without fear of retribution or punishment. Government in-telligence agencies may need to be able to monitor known extremist Web sites withoutrevealing their presence to the Web site operators. Private citizens may want to be ableto freely browse the Web, without advertisers collecting statistics on their personalbrowsing habits and selling that personal information to other companies.Anonymous communication systems can also provide a strong foundation for censor-ship resistance for people living under oppressive regimes that try to limit what theircitizens can say and do on the Internet. There has been significant and interesting workon designing censorship-resistant systems that allow people to anonymously store, pub-lish, and retrieve data, while making it resilient to attack by an adversary [Waldmanand Mazi`eres 2001; Dingledine et al. 2000; Waldman et al. 2000; Anderson 1996; Clarkeet al. 2000]. Concepts from the design of anonymous communication systems have alsobeen applied to the fields of cryptographic voting [Kilian and Sako 1995; Jakobsson et al.2002; Boneh and Golle 2002], electronic currencies [Camenisch et al. 2005], anonymousauctions [Harkavy et al. 1998], and anonymous credentials [Camenisch and Lysyan-skaya 2001; Camenisch et al. 2006].In this work, we survey the previous research done to design, develop, and deploysystems for enabling private and anonymous communication on the Internet. We re-view the major technologies in the field, including mix networks, onion routing andDining Cryptographers networks, as well as improvements and variations on thoseprimitives made since their introduction. We will also summarize some of the powerfulattacks an adversary might conduct in
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