Tor and blocking-resistance Roger Dingledine The Tor Project - PowerPoint PPT Presentation

Tor and blocking-resistance Roger Dingledine The Tor Project https://www.torproject.org/ 1

Tor: Big Picture ● Freely available (Open Source), unencumbered. ● Comes with a spec and full documentation: Dresden and Aachen implemented compatible Java Tor clients; researchers use it to study anonymity. ● 1500 active relays, 200000+ active users, >1Gbit/s. ● Official US 501(c)(3) nonprofit. Eight funded developers, dozens more dedicated volunteers. ● Funding from US DoD, Electronic Frontier Foundation, Voice of America, Human Rights Watch, Google, NLnet, ...you? 2

Anonymity serves different interests for different user groups. Anonymity Private citizens “It's privacy!” 3

Anonymity serves different interests for different user groups. Businesses Anonymity “It's network security!” Private citizens “It's privacy!” 4

Anonymity serves different interests for different user groups. “It's traffic-analysis resistance!” Businesses Governments Anonymity “It's network security!” Private citizens “It's privacy!” 5

Anonymity serves different interests for different user groups. “It's reachability! Blocked users “It's traffic-analysis resistance!” Businesses Governments Anonymity “It's network security!” Private citizens “It's privacy!” 6

Threat model: what can the attacker do? Alice Anonymity network Bob watch Alice! watch (or be!) Bob! Control part of the network! 7

Anonymity isn't cryptography: Cryptography just protects contents. “Hi, Bob!” “Hi, Bob!” <gibberish> Alice attacker Bob 8

Anonymity isn't just wishful thinking... “You can't prove it was me!” “Promise you won't look!” “Promise you won't remember!” “Promise you won't tell!” “I didn't write my name on it!” “Isn't the Internet already anonymous?” 9

The simplest designs use a single relay to hide connections. Bob1 Alice1 E(Bob3,“X”) “Y” Relay Alice2 “Z” Bob2 E(Bob1, “Y”) ) “X” ” Z “ , 2 b o B ( E Bob3 Alice3 (example: some commercial proxy providers) 10

But a single relay (or eavesdropper!) is a single point of failure. Bob1 Alice1 E(Bob3,“X”) “Y” Evil Alice2 Relay “Z” Bob2 E(Bob1, “Y”) ) “X” ” Z “ , 2 b o B ( E Bob3 Alice3 11

So, add multiple relays so that no single one can betray Alice. Bob Alice R1 R3 R5 R4 R2 12

A corrupt first hop can tell that Alice is talking, but not to whom. Bob Alice R1 R3 R5 R4 R2 13

A corrupt final hop can tell that somebody is talking to Bob, but not who. Bob Alice R1 R3 R5 R4 R2 14

Alice makes a session key with R1 ...And then tunnels to R2...and to R3 Bob Alice R1 R3 Bob2 R5 R4 R2 15

Tor gives three anonymity properties ● #1 : A local network attacker can't learn, or influence, your destination. – Clearly useful for blocking resistance. ● #2 : No single router can link you to your destination. – The attacker can't sign up relays to trace users. ● #3 : The destination, or somebody watching it, can't learn your location. – So they can't reveal you; or treat you differently. 16

Attacker's goals (1) ● Restrict the flow of certain kinds of information – Embarrassing (rights violations, corruption) – Opposing (opposition movements, sites that organize protests) ● Chill behavior by impression that online activities are monitored 17

Attacker's goals (2) ● Complete blocking is not a goal. It's not even necessary. ● Similarly, no need to shut down or block every circumvention tool. Just ones that are – popular and effective (the ones that work) – highly visible (make censors look bad to citizens -- and to bosses) 18

Attacker's goals (3) ● Little reprisal against passive consumers of information. – Producers and distributors of information in greater danger. ● Censors (actually, govts) have economic, political, social incentives not to block the whole Internet. – But they don't mind collateral damage. 19

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Governments and other firewalls could block the whole Tor network. S Alice S S X Alice X S 21

Alice Alice Alice Blocked Alice Alice User R3 Alice Blocked R4 Bob User Alice Alice R2 Blocked User Alice R1 Alice Blocked Alice User Alice Blocked Alice User Alice Alice 22

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Tor is only a piece of the puzzle ● Assume the users aren't attacked by their hardware and software – No spyware installed, no cameras watching their screens, etc ● Assume the users can fetch a genuine copy of Tor: from a friend, via PGP signatures, etc. 24

Sustainability ● Tor has a community of developers and volunteers. ● Commercial anonymity systems have flopped or constantly need more funding for bandwidth. ● Our sustainability is rooted in Tor's open design: clear documentation, modularity, and open source. 25

Using Tor in oppressed areas ● Common assumption: risk from using Tor increases as firewall gets more restrictive. ● But as firewall gets more restrictive, more ordinary people use Tor too, for more mainstream activities. ● So the “median” use becomes more acceptable? ● (Of course, that doesn't mean they won't try to block it.) 26

Publicity attracts attention ● Many circumvention tools launch with huge media splashes. (The media loves this.) ● But publicity attracts attention of the censors. ● We threaten their appearance of control, so they must respond. ● We can control the pace of the arms race. 27

Next steps ● Technical solutions won't solve the whole censorship problem. After all, firewalls are socially very successful in these countries. ● But a strong technical solution is still a critical puzzle piece. ● We'd love to help teach people about Tor -- to help users and to make Tor better. 28

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Research components ● How do we measure anonymity? Many attacks and defenses need analysis. ● Safe user metrics ● Tor is slow: lots of systems questions. ● Better blocking-resistance? ● Application-level anonymity; safe SSL ● Usability, user education 30