Tor and circumvention: Lessons learned Nick Mathewson The Tor Project https://torproject.org/ 1
What is Tor? Online anonymity 1) open source software, 2) network, 3) protocol Community of researchers, developers, users, and relay operators Funding from US DoD, Electronic Frontier Foundation, Voice of America, Google, NLnet, Human Rights Watch, NSF, US State Dept, SIDA, ... 2
The Tor Project, Inc. 501(c)(3) non-profit organization dedicated to the research and development of tools for online anonymity and privacy Not secretly evil. 3
Estimated ~250,000? daily Tor users 4
Anonymity in what sense? “Attacker can’t learn who is talking to whom.” Bob Alice Anonymity network Bob Alice Alice Bob 5
Threat model: what can the attacker do? Alice Anonymity network Bob watch Alice! watch (or be!) Bob! Control part of the network! 6
Anonymity isn't cryptography: Cryptography just protects contents. “Hi, Bob!” “Hi, Bob!” <gibberish> Alice attacker Bob 7
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?” 8
Anonymity serves different interests for different user groups. Anonymity Private citizens “It's privacy!” 9
Anonymity serves different interests for different user groups. Businesses Anonymity “It's network security!” Private citizens “It's privacy!” 10
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!” 11
Anonymity serves different interests for different user groups. “It's reachability!” Human rights “It's traffic-analysis activists resistance!” Businesses Governments Anonymity “It's network security!” Private citizens “It's privacy!” 12
Regular citizens don't want to be watched and tracked. “I sell the logs.” Blogger Hostile Bob Alice “Oops, I lost the logs.” 8-year-old Incompetent Bob The AOL fiasco Alice “Hey, they aren't Indifferent Bob Sick my secrets.” Alice Name, address, (the network can track too) Consumer age, friends, Alice interests .... (medical, financial, etc), unpopular opinions, Oppressed illegal opinions.... Alice 13
Businesses need to keep trade secrets. “Oh, your employees are reading Competitor our patents/jobs page/product sheets?” “Hey, it's Alice! Give her the 'Alice' version!” Competitor AliceCorp “Wanna buy a list of Alice's suppliers? Compromised What about her customers? network What about her engineering department's favorite search terms?” 14
Law enforcement needs anonymity to get the job done. “Why is alice.localpolice.gov reading Investigated my website?” suspect “Why no, alice.localpolice.gov! Officer Sting I would never sell counterfeits on ebay!” Alice target “Is my family safe if I Organized go after these guys?” Crime Witness/informer “Are they really going to ensure Anonymous Alice my anonymity?” tips 15
Governments need anonymity for their security “What will you bid for a list of Baghdad IP addresses that get email from .gov?” Untrusted ISP Agent “ Somebody in that hotel room just Alice checked his Navy.mil mail! ” Compromised “ What does FBI Google for? ” service Shared “Do I really want to reveal my Coalition network internal network topology?” member Alice Defense in “What about insiders?” Depth 16
Journalists and activists need Tor for their personal safety Monitoring “Did you just post to that website?” ISP Activist/ Whistleblower “Where are the bloggers connecting from?” Alice Monitored “I run livejournal and track my users” website “Of course I tell China about my users” “What does the Global Voices website Filtered say today?” Blocked website “I want to tell people what's going on Alice in my country” Monitored “I think they're watching. I'm not even network going to try.” 17
You can't get anonymity on your own: private solutions are ineffective... Alice's small Citizen “One of the 25 users ... anonymity net Alice on AliceNet.” Officer Municipal Investigated Alice “Looks like a cop.” anonymity net suspect AliceCorp AliceCorp “It's somebody at Competitor anonymity net AliceCorp!” 18
... so, anonymity loves company! Citizen “???” ... Alice Officer Investigated Alice Shared “???” suspect anonymity net AliceCorp Competitor “???” 19
Yes, bad people need anonymity too. But they are already doing well. Compromised botnet Stolen mobile phones Evil Criminal Alice Open wireless nets ..... 20
Current situation: Bad people on the Internet are doing fine Trojans Viruses Exploits Botnets Zombies Espionage Phishing DDoS Spam Extortion 21
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) 22
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 23
... or a single point of bypass. Bob1 Alice1 E(Bob3,“X”) “Y” Irrelevant Alice2 Relay “Z” Bob2 E(Bob1, “Y”) ) “X” ” Z “ , 2 b o B ( E Bob3 Alice3 Timing analysis bridges all connections through relay ⇒ An attractive fat target 24
So, add multiple relays so that no single one can betray Alice. Bob Alice R1 R3 R5 R4 R2 25
A corrupt first hop can tell that Alice is talking, but not to whom. Bob Alice R1 R3 R5 R4 R2 26
A corrupt final hop can tell that somebody is talking to Bob, but not who. Bob Alice R1 R3 R5 R4 R2 27
Alice makes a session key with R1 ...And then tunnels to R2...and to R3 Bob Alice R1 R3 Bob2 R5 R4 R2 28
What we spend our time on Performance and scalability Maintaining the whole software ecosystem Blocking-resistance (circumvention) Basic research on anonymity Reusability and modularity Advocacy, education, and trainings around the world Metrics, data, and analysis 29
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Another Iran user count Talked to chief security officer of one of the web 2.0 social networking sites: 10% (~10k) of their Iranian users in June 2009 were coming through Tor 90% (~90k) were coming from proxies in the Amazon cloud 31
Iran and DPI We made Tor's TLS handshake look like Firefox+Apache. When Iran kicked out Smartfilter in early 2009, Tor's old (non-TLS) directory fetches worked again! Jan 2011, Iran blocked Tor by DPI for SSL and filtering our Diffie-Hellman parameter. Socks proxy worked fine the whole time. 32
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Relay versus Discovery There are two pieces to all these “proxying” schemes: a relay component: building circuits, sending traffic over them, getting the crypto right a discovery component: learning what relays are available 34
The basic Tor design uses a simple centralized directory protocol. cache S1 Trusted directory Alice S2 Alice downloads consensus and Trusted directory cache descriptors from anywhere Authorities S3 publish a consensus Servers publish list of all descriptors self-signed descriptors. 35
Attackers can block users from connecting to the Tor network By blocking the directory authorities By blocking all the relay IP addresses in the directory By filtering based on Tor's network fingerprint By preventing users from finding the Tor software 36
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 37
“Bridge” relays Hundreds of thousands of Tor users, already self-selected for caring about privacy. Rather than signing up as a normal relay, you can sign up as a special “bridge” relay that isn't listed in any directory. No need to be an “exit” (so no abuse worries), and you can rate limit if needed Integrated into Vidalia (our GUI) so it's easy to offer a bridge or to use a bridge 38
How do you f i nd a bridge? 1) https://bridges.torproject.org/ will tell you a few based on time and your IP address 2) Mail bridges@torproject.org from a gmail address and we'll send you a few 3) We mail some to a friend in Shanghai who distributes them via his social network 4) You can set up your own private bridge and tell your target users directly 39
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