Tor: Anonymous Communications for the Dept of Defense ... and you. - PowerPoint PPT Presentation

Tor: Anonymous Communications for the Dept of Defense ... and you. Roger Dingledine The Free Haven Project http://tor.eff.org/ 1

Tor: Big Picture ● Freely available (Open Source), unencumbered. ● Comes with a spec and full documentation: German universities implemented compatible Java Tor clients; researchers use it to study anonymity. ● Chosen as anonymity layer for EU PRIME project. ● 200000+ active users. ● PC World magazine named Tor one of the Top 100 Products of 2005. 2

Formally: anonymity means indistinguishability within an “anonymity set” Alice1 Alice2 Alice3 Bob Alice4 Alice5 .... Attacker can't tell which Alice Alice6 Alice7 is talking to Bob! Alice8 3

We have to make some assumptions about what the attacker can do. Alice Anonymity network Bob watch Alice! watch (or be!) Bob! Control part of the network! Etc, etc. 4

Anonymity isn't cryptography: Cryptography just protects contents. “Hi, Bob!” “Hi, Bob!” <gibberish> Alice attacker Bob 5

Anonymity isn't steganography: Attacker can tell that Alice is talking; just not to whom. Bob1 Alice1 Anonymity network Alice2 Bob2 ... AliceN (Strong high-bandwidth steganography may not exist.) 6

Anonymity serves different interests for different user groups. Governments Businesses “It's traffic-analysis “It's network security!” resistance!” Anonymity “It's privacy!” Private citizens 7

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 Alice “Hey, they aren't Indifferent Bob Sick my secrets.” Alice Name, address, Consumer age, friends, (the network can track too) Alice interests .... (medical, financial, etc), Oppressed unpopular opinions, Alice illegal opinions.... 8

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?” 9

Law enforcement needs anonymity to get the job done. Investigated “Why is alice.localpolice.gov reading suspect my website?” “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 Anonymous “Are they really going to ensure Alice tips my anonymity?” 10

Governments need anonymity for their security “What will you bid for a list of Baghdad Untrusted IP addresses that get email from .gov?” ISP Agent Alice Compromised “ What does the CIA Google for? ” service Shared “Do I really want to reveal my Coalition network internal network topology?” member Alice Defense in “What about insiders?” Depth 11

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!” 12

... so, anonymity loves company! Citizen “???” ... Alice Officer Investigated Alice Shared “???” suspect anonymity net AliceCorp Competitor “???” 13

Yes, bad people need anonymity too. But they are already doing well. Compromised botnet Stolen mobile phones Evil Criminal Alice Open wireless nets ..... 14

Current situation: Bad people on the Internet are doing fine Trojans Viruses Exploits Botnets Zombies Espionage Phishing DDoS Spam Extortion 15

IP addresses can be enough to bootstrap knowledge of identity. Alice Hotlinked ad 18.244.x.x Amazon account Wikipedia post 16

Tor is not the first or only design for anonymity. Low-latency High-latency Single-hop Chaum's Mixes proxies (1981) Crowds (~96) anon.penet.fi (~91) V1 Onion ZKS Routing (~96) “Freedom” Remailer networks: (~99-01) cypherpunk (~93), mixmaster (~95), Java Anon Proxy mixminion (~02) (~00-) Tor (01-) 17 ...and more!

Low-latency systems are vulnerable to end-to-end correlation attacks. match! Low-latency: Alice1 sends: xx x xxxx x Bob2 gets: xx x xxxx x Alice2 sends: x x xx x x Bob1 gets: x x x x x x match! Time High-latency: Alice1 sends: xx x xxxx Alice2 sends: x x xx x x Bob1 gets: xx xxxx ..... Bob2 gets: x xxxxx ..... These attacks work in practice. The obvious defenses are expensive (like high-latency), useless, or both. 18

Still, we focus on low-latency, because it's more useful. Interactive apps: web, IM, VOIP, ssh, X11, ... # users: millions? Apps that accept multi-hour delays and high bandwidth overhead: email, sometimes. # users: tens of thousands at most? And if anonymity loves company....? 19

The simplest designs use a single relay to hide connections. Bob1 Alice1 Bob3,“X” “Y” Relay Alice2 Bob2 “Z” Bob1, “Y” “X” ” Z “ , 2 b o B Bob3 Alice3 (ex: some commercial proxy providers) 20

But an attacker who sees Alice can see what she's doing. Bob1 Alice1 Bob3,“X” “Y” Relay Alice2 Bob2 “Z” Bob1, “Y” “X” ” Z “ , 2 b o B Bob3 Alice3 21

Add encryption to stop attackers who eavesdrop on Alice. Bob1 Alice1 E(Bob3,“X”) “Y” Relay Alice2 Bob2 “Z” E(Bob1, “Y”) “X” ) ” Z “ , 2 b o B ( E Bob3 Alice3 (ex: some commercial proxy providers) 22

But a single relay is a single point of failure. Bob1 Alice1 E(Bob3,“X”) “Y” Evil Alice2 Bob2 Relay “Z” E(Bob1, “Y”) “X” ) ” Z “ , 2 b o B ( E Bob3 Alice3 Eavesdropping the relay works too. 23

So, add multiple relays so that no single one can betray Alice. Bob Alice R1 R3 R5 R4 R2 24

A corrupt first hop can tell that Alice is talking, but not to whom. Bob Alice R1 R3 R5 R4 R2 25

A corrupt final hop can tell that somebody is talking to Bob, but not who. Bob Alice R1 R3 R5 R4 R2 26

Alice makes a session key with R1 Bob Alice R1 R3 R5 R4 R2 27

Alice makes a session key with R1 ...And then tunnels to R2 Bob Alice R1 R3 R5 R4 R2 28

Alice makes a session key with R1 ...And then tunnels to R2...and to R3 Bob Alice R1 R3 R5 R4 R2 29

Alice makes a session key with R1 ...And then tunnels to R2...and to R3 Bob Alice R1 R3 R5 R4 R2 30

Can multiplex many connections through the encrypted circuit Bob Alice R1 R3 Bob2 R5 R4 R2 31

Tor anonymizes TCP streams only: it needs other applications to clean high-level protocols. SSH S O C K S SOCKS Web T T P H Web browser scrubber Tor network Tor client SOCKS IRC client 32

We added a control protocol for external GUI applications. (GUI contest!) SSH SOCKS HTTP S O Web C Web K S browser scrubber Tor client Control protocol Controller GUI (Change configuration, report errors, manage circuits, etc.) 33

Usability for server operators ● Rate limiting: eating too much bandwidth is rude! ● Exit policies: not everyone is willing to emit arbitrary traffic. allow 18.0.0.0/8:* allow *:22 allow *:80 reject *:* 34

Server discovery must not permit liars to impersonate the whole network. 1. Alice says, “Describe the network!” Evil Alice1 Server E.S. E.S. E.S. Evil Alice1 Server E.S. E.S. E.S. 2. Alice is now in trouble. 35

Server discovery is hard because misinformed clients lose anonymity. Known to Alice1 S S Bob1 Alice1 S S S S Alice2 S Bob2 S S Known to Alice2 36

Early Tor versions used a trivial centralized directory protocol. cache S1 Trusted directory Alice S2 Alice downloads any signed list Trusted directory cache Authorities S3 publish signed Servers publish lists of all descriptors self-signed descriptors. 37

We redesigned our directory protocol to reduce trust bottlenecks. cache Evil S1 Trusted directory Alice S2 Alice downloads all statements; Trusted directory cache believes the majority; Authorities S3 downloads Servers publish publish signed descriptors as self-signed statements about needed. descriptors. descriptors. 38 (Also uses less bandwidth!)

Tor implements responder anonymity with hidden services. Directory 2 ? . ” “ n P o i K n o , ! . ) ” S K ) i P 1 g S ( H n ( n ( “ g S . i 3 S 1 ) , K ” P “ 1. “Sign(PK)” S1 Bob Alice All these connections are anonymized. 39

Tor implements responder anonymity with hidden services. Directory 5' E(“Meet me at S2”,T) 5 . P K , E ( “ M e e t m e a t S 2 ” , T ) S1 Bob Alice 4. “Wait for T, ” ! T “ handshake” . e 6 k a h s d n a h S2 All these connections are anonymized. 40