Crowds: Anonymity for Web Transactions Michael Reiter and Avi Rubin 1998

Privacy Online Supreme Court Justice Louis Brandeis defined privacy as "the right to be let alone", which he said was one of the rights most cherished by Americans. The Internet represents previously inconceivable opportunities to monitor your actions and personal information! Just imagine the McCarthy hearings now.

Strong Privacy Online NSA, FBI, etc. Consumer databases, Axciom, and Hackers What about *Bad Guys*? Beware the Four Horsemen of the Information Apocalypse: terrorists, drug dealers, kidnappers, and child pornographers. Seems like you can scare any public into allowing the government to do anything with those four. - Bruce Schneier Good Guys: CIA, Undercover Cops, Biz., etc.

Opportunities for Exploitation Your computer’s IP address uniquely identifies you across web sites. Nothing illegal about cross-referencing.

Conclusions: Free Exchange The Internet’s benefit increases directly with –the number of resources online –the privacy people having in obtaining it –The privacy people have in serving it Anonymity is a promising technology for providing user privacy.

Why Anonymity? Today, only 20% of web sites meet the FTC’s fair information practices. Anonymity is a technical means to privacy –Without cooperation of the receiver. Legitimate social uses on the Net –Allow for safe “whistle blowing” –Privacy in medical issues or psychological counseling –Web surfing privacy –Web serving privacy

Anonymous Routing Anonymity is the state of being indistinguishable from other members of some group. Our goal is to provide mechanism for routing that hides initiator’s IP address Not trying to protect content of message. –Can use end-to-end encryption for that. That said... –Does not protect higher-level protocols/data. –Doesn’t make sense to send “I’m Matt and my SSN is...” anonymously.

Anonymizer.com Lucent personalized web assistant. You must trust the proxy! In fact, now they are in a position to monitor everything you do. Anon.penet.fi and the Church of Scientology Single Proxy I R P

Key Contributions? Crowds

Decentralized P2P solution Anonymous within the Crowd Jondo (John Doe) –Proxy –User Path based

Path-based Initiator Anonymity R X Y Z I Packets are passed from the initiator, I, to the proxies which then deliver the packet to the responder R.

Crowds Paths R X Y Z I Weighted Coin Flip Spinner

Does it work? Threat models: –Responder (end server): Beyond Suspicion! –Local eavesdropper –Malicious (collaborating) Jondos Types of attacks: –Timing attacks –Passive logging –Traceback

Degree of Anonymity Not a Boolean question! –Rarely undetectable –Difficult to prove ID unless signed Range: Absolute Privacy Beyond Suspicion Probable Innocence Possible Innocence Exposed Provably Exposed

Eavesdropping Messages are encrypted between jondos –Otherwise complete exposure Information available –Message timing –Initiator? –Messages to responders (but path length > 0 proxies) R1 A B Jondo

Malicious Jondos Giving information –Your IP address is seen by the next node in the path –Being on the path means you might be the initiator Many attackers –Ratio of attackers (c) to total (n) is important –So is weight of the coin flip (p f ) Innocent? –If p f = 3/4 and n  3(c+1), probable innocence –Higher p f implies greater resilience to attackers I R

Performance Path length –A function of p f : larger = longer paths Latency –note: all local nodes, no error info. –note 2: older machines; encryption is more expensive –latency of up to 13.5 seconds! (8.6 for 1-hop) –No 0-hop tests

Scalability How many paths will node X be on? –Spse. ave. path length is l –n nodes, so n l positions on the path –chance of picking node X = 1/n –thus, expectation of l times on a path Independent of n

End of Crowds

Strengths Performance & Scaling Security against weak attackers –single operators generally fail ISP, web site, your neighborhood eavesdropper, one person with a few jondos Parameter to trade off security/performance

Usability Weaknesses Must disable Java & ActiveX More generally, a good proxy required –clean all traces –could be bypassed? Group membership –keeping a full list may be hard/expensive –centralizing it provides a way to attack –(intersection attack) Delay in joining Group size –required to have either small or large groups Network delays

Security Weaknesses Problem –strong eavesdroppers exist –Sybil attacks (many bad peers) –Combined attacks possible (e.g. local eavesdropper + responder) Collaborating members –increasing bad peers guarantees compromise –growing threat over time DOS + Sybil attack –always changing non-sending members

Security Weaknesses Possible eavesdrop –When many peers use the same ISP (cable modem, DSL), a full path may be controlled by the ISP. Exposure of information –a path of nodes that sees all –info. can allow attackers to guess at initiators –can change web requests