Anonymity With Tor. The Onion Router. July 21, Technische Universität München

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1 The Onion Router Nathan S. Evans Christian Grothoff Technische Universität München July 21, 2011

2 Overview What is Tor? Motivation Background Material How Tor Works Hidden Services Attacks Specific Attack Summary

3 What is Tor? Tor is a P2P network of Chaum inspired low-latency mixes which are used to provide anonymous communication between parties on the Internet.

4 What is Tor? Sender anonymity for low latency applications Common usage: Web browsing Sender anonymity Web server cannot identify client Advanced usage: Hidden services (send/receive anonymity) Filesharing IRC Any application that communicates using TCP Tor provides users with a service that effectively hides their identity on the Internet.

5 Motivation Internet packets travel from A to B transparently A knows B, and B knows A (by IP address) Routers, etc. can determine that A and B are communicating This may reveal unintended information (e.g. person X s bank) Encryption For example, TLS (HTTPS) Provides Data anonymity Does not hide routing information

6 Motivation - Routing Example

7 Motivation Who needs anonymous communication? Political dissidents Employees Whistleblowers Military, police, governments Terrorists, criminals You, me, everybody! There are times when people wish to communicate (or browse naughty web sites) without fear of reprisal, ridicule or punishment.

8 Background - Mixes A mix server takes requests from sources and sends them to destinations Hides the sender from the receiver Provides Sender Anonymity Web Server 1 Mix Server Web Server 2 Web Server 3

9 Background - Mixes May delay or batch messages Usually used in series to provide better anonymity (why??) Mix cascade Web Server 1 Mix Server 1 Mix Server 2 Mix Server 3 Web Server 2 Web Server 3

10 Background - Proxies Similar to single mix server Single server Handles requests from clients Returns results to clients Anonymizing proxy mix server Difference to mix server Proxies generally used for different purposes Caching responses for web pages Corporate or educational blocking Logging access

11 Background - Onion Routing Multiple mix servers Subset of mix servers chosen by initiator Chosen mix servers create circuit Initiator contacts first server S 1, sets up symmetric key K S1 Then asks first server to connect to second server S 2 ; through this connection sets up symmetric key with second server K S2... Repeat with server S i until circuit of desired length n constructed

12 Background - Onion Routing Example Assume n = 2, the initiator takes a message m to be sent along the circuit The initiator encrypts the message thusly, K S1 (K S2 (m)) and sends it to S 1 S 1 decrypts the message with K S1 and sends the result to S 2 S 2 decrypts the message revealing m S 2 then performs whatever action is required on m

13 Background - Onion Routing Example Client sets up symmetric key K S1 with server S 1 Exchange K S1 Client S 1 S 2

14 Background - Onion Routing Example Via S 1 Client sets up symmetric key K S2 with server S 2 Exchange K S2 Client S 1 S 2

15 Background - Onion Routing Example Client encrypts m as K S1 (K S2 (m)) and sends to S 1 K S1 (K S2 (m)) Client S 1 S 2

16 Background - Onion Routing Example S 1 decrypts, sends on to S 2, S 2 decrypts, revealing m K S1 (K S2 (m)) K S2 (m) Client S 1 S 2

17 Tor - How it Works Low latency P2P Network of mix servers Designed for interactive traffic (https, ssh, etc.) Directory Servers store list of participating servers Contact information, public keys, statistics Directory servers are replicated for security Clients choose servers randomly with bias towards high BW/uptime Clients build long lived Onion routes circuits using these servers Circuits are bi-directional Circuits are hard coded at length three

18 Tor - How it Works - Example Example of Tor client circuit Client Tor Node 1 Tor Node 2 Tor Node 3 Tor Node 4 Tor Node 5 Tor Node 6 Server Tor Node 7 Tor Node 8 Tor Node 9

19 Tor - How it Works - Servers Servers connected in full mesh All servers exchange symmetric keys Allows fast sending between servers, regardless of which circuits Allows combining of multiple messages with same next-hop New servers publish information to directory servers Once online for a certain period, they are added to the live list They are then available for use by clients

20 Tor - How it Works - Servers Servers are classified into three categories for usability, security and operator preference Entry nodes (aka guards) - chosen for first hop in circuit Generally long lived good nodes Small set chosen by client which are used for client lifetime (security) Middle nodes - chosen for second hop in circuit, least restricted set Exit nodes - last hop in circuit Visible to outside destination Support filtering of outgoing traffic Most vulerable position of nodes

21 Hidden Services in Tor Hidden services allow Tor servers to receive incoming connections anonymously Can provide access to services available only via Tor Web, IRC, etc. For example, host a website without your ISP knowing Uses a Rendezvous point to connect two Tor circuits Uses Introduction points, which allow outside peers to contact hidden server (while keeping it hidden) Publishes Intro. point addresses to Lookup server Client gets Introduction point address from lookup server, sends random rendezvous point to hidden server Data travels a total of 7 hops (once established)

22 Hidden Services Example 1

23 Hidden Services Example 2

24 Hidden Services Example 3

25 Hidden Services Example 4

26 Hidden Services Example 5

27 Hidden Services Example 6

28 Types of Attacks on Tor Exit Relay Snooping Website fingerprinting Traffic Analysis Intersection Attack DoS

29 Why attack Tor? Tor is the most popular and widely used free software P2P network used to achieve anonymity on the Internet: Tor has a large user base The project is well supported Generally assumed to give users strong anonymity Our results: All the Tor nodes involved in a circuit can be discovered, reducing Tor users level of anonymity and revealing a problem with Tor s protocol

30 Tor General Information Tor stands for The onion router Encrypts data multiple times and is decrypted as it travels through the network a layer at a time: like peeling an onion Tor is a P2P network of mixes Routes data through network along a circuit Data is encrypted as it passes through nodes (until the last hop)

31 Routing Data is forwarded through the network Each node knows only the previous hop and the next hop Only the originator knows all the hops Number of hops is hard coded (currently set to three) Key security goal: No node in the path can discover the full path

32 Routing Example Client Tor Node 1 Tor Node 2 Tor Node 3 Tor Node 4 Tor Node 5 Tor Node 6 Server Tor Node 7 Tor Node 8 Tor Node 9

33 Previous work Murdoch and Danezis wrote Low Cost Traffic Analysis of Tor Goal is to discover all the Tor routers involved in a given circuit Based on being able to tell the added load of one normal Tor connection Send a certain sequence down a tunnel, monitor each Tor router to see if it is involved Their attack worked reasonably well with the 13 Tor routers they used in 2005 (with 15% false negative rate)

34 Problems With Previous Work Too inaccurate with today s routers Must identify all the separate routers in the circuit Attempting to measure small effects, large fluctuations that occur in actual current network give false positives We replicated their experiments, found method to be much less effective on today s network

35 M and D Results - With Attack Control Run Attack Run Latency measurement graph xbota with attack Latency variance (in seconds) Sample number

36 M and D Results - Without Attack Latency measurement graph chaoscomputerclub42 no attack 2 Control Run Attack Run Latency variance (in seconds) Sample number

37 M and D Testing Used same statistical methods for correlation Used same source code for attacks In our tests, highest correlations seen with false positives Attack may be viable for some Tor nodes Improved statistical methods may improve false positives

38 Our Basis for Deanonymization Target user is running Tor with default settings Three design issues enable users to be deanonymized 1 No artificial delays induced on connections 2 Path length is set at a small finite number 3 Paths of arbitrary length through the network can be constructed

39 Regular Path Example Client Tor Node 3 Tor Node 1 Tor Node 2 Server

40 Circular Path Example 1/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server

41 Circular Path Example 2/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server

42 Circular Path Example 3/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server

43 Circular Path Example 4/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server

44 Circular Path Example 5/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server

45 Attack Implementation Exit node injects JavaScript ping code into HTML response Client browses as usual, while JavaScript continues to phone home Exit node measures variance in latency While continuing to measure, attack strains possible first hop(s) If no significant variance observed, pick another node from candidates and start over Once sufficient change is observed in repeated measurements, initial node has been found

46 Attack Example Client Tor Node 1 - Unknown Node Malicious Client Tor Node 3 - Our Exit Node Tor Node 2 - Known High BW Tor Node 1 Server High BW Tor Node 2 Malicious Server

47 Queue example 1 (3 circuits) A B C B5 B4 B3 B2 B1 A0 B0 C1 Output Queue t = 0 C0

48 Queue example 2 (3 circuits) A B C B5 B4 B3 B2 B1 B0 C1 Output Queue t = 1 t = 0 A0 C0

49 Queue example 3 (3 circuits) A B C B5 B4 B3 B2 B1 C1 Output Queue t = 2 t = 1 t = 0 B0 A0 C0

50 Queue example 4 (3 circuits) A B C B5 B4 B3 B2 B1 Output Queue t = 3 t = 2 t = 1 t = 0 C1 B0 A0 C0

51 Queue example 5 (3 circuits) A B C B5 B4 B3 B2 Output Queue t = 4 t = 3 t = 2 t = 1 t = 0 B1 C1 B0 A0 C0

52 Queue example 6 (3 circuits) A B C B5 B4 B3 Output Queue t = 5 t = 4 t = 3 t = 2 t = 1 t = 0 B2 B1 C1 B0 A0 C0

53 Queue example 7 (3 circuits) A B C B5 B4 Output Queue t = 6 t = 5 t = 4 t = 3 t = 2 t = 1 t = 0 B3 B2 B1 C1 B0 A0 C0

54 Queue example 8 (3 circuits) A B C B5 Output Queue t = 7 t = 6 t = 5 t = 4 t = 3 t = 2 t = 1 t = 0 B4 B3 B2 B1 C1 B0 A0 C0

55 Queue example 1 (15 circuits) A B C D E F G H I J K L M N O N6 D5 N5 O5 D4 E4 I4 N4 O4 A3 B3 C4 D3 E3 I3 L3 N3 O3 A2 B2 C3 D2 E2 I2 L2 N2 O2 A1 B1 C2 D1 E1 G1 H1 I1 J1 L1 M1 N1 O1 A0 B0 C1 D0 E0 G0 H0 I0 J0 K0 L0 M0 N0 O0 Output Queue t = 0 C0

56 Queue example 2 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 E4 I4 N4 O4 A3 B3 C4 D4 E3 I3 L3 N3 O3 A2 B2 C3 D3 E2 I2 L2 N2 O2 A1 B1 C2 D2 E1 G1 H1 I1 J1 L1 M1 N1 O1 A0 B0 C1 D1 E0 G0 H0 I0 J0 K0 L0 M0 N0 O0 Output Queue t = 1 t = 0 D0 C0

57 Queue example 3 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 I4 N4 O4 A3 B3 C4 D4 E4 I3 L3 N3 O3 A2 B2 C3 D3 E3 I2 L2 N2 O2 A1 B1 C2 D2 E2 G1 H1 I1 J1 L1 M1 N1 O1 A0 B0 C1 D1 E1 G0 H0 I0 J0 K0 L0 M0 N0 O0 Output Queue t = 2 t = 1 t = 0 E0 D0 C0

58 Queue example 4 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 I4 N4 O4 A3 B3 C4 D4 E4 I3 L3 N3 O3 A2 B2 C3 D3 E3 I2 L2 N2 O2 A1 B1 C2 D2 E2 H1 I1 J1 L1 M1 N1 O1 A0 B0 C1 D1 E1 G1 H0 I0 J0 K0 L0 M0 N0 O0 Output Queue t = 3 t = 2 t = 1 t = 0 G0 E0 D0 C0

59 Queue example 5 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 I4 N4 O4 A3 B3 C4 D4 E4 I3 L3 N3 O3 A2 B2 C3 D3 E3 I2 L2 N2 O2 A1 B1 C2 D2 E2 I1 J1 L1 M1 N1 O1 A0 B0 C1 D1 E1 G1 H1 I0 J0 K0 L0 M0 N0 O0 Output Queue t = 4 t = 3 t = 2 t = 1 t = 0 H0 G0 E0 D0 C0

60 Queue example 6 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 N4 O4 A3 B3 C4 D4 E4 I4 L3 N3 O3 A2 B2 C3 D3 E3 I3 L2 N2 O2 A1 B1 C2 D2 E2 I2 J1 L1 M1 N1 O1 A0 B0 C1 D1 E1 G1 H1 I1 J0 K0 L0 M0 N0 O0 Output Queue t = 5 t = 4 t = 3 t = 2 t = 1 t = 0 I0 H0 G0 E0 D0 C0

61 Queue example 7 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 N4 O4 A3 B3 C4 D4 E4 I4 L3 N3 O3 A2 B2 C3 D3 E3 I3 L2 N2 O2 A1 B1 C2 D2 E2 I2 L1 M1 N1 O1 A0 B0 C1 D1 E1 G1 H1 I1 J1 K0 L0 M0 N0 O0 Output Queue t = 6 t = 5 t = 4 t = 3 t = 2 t = 1 t = 0 J0 I0 H0 G0 E0 D0 C0

62 Queue example 8 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 N4 O4 A3 B3 C4 D4 E4 I4 L3 N3 O3 A2 B2 C3 D3 E3 I3 L2 N2 O2 A1 B1 C2 D2 E2 I2 L1 M1 N1 O1 A0 B0 C1 D1 E1 G1 H1 I1 J1 L0 M0 N0 O0 Output Queue t = 7 t = 6 t = 5 t = 4 t = 3 t = 2 t = 1 t = 0 K0 J0 I0 H0 G0 E0 D0 C0

63 Queue example 9 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 N4 O4 A3 B3 C4 D4 E4 I4 N3 O3 A2 B2 C3 D3 E3 I3 L3 N2 O2 A1 B1 C2 D2 E2 I2 L2 M1 N1 O1 A0 B0 C1 D1 E1 G1 H1 I1 J1 L1 M0 N0 O0 Output Queue t = 8 t = 7 L0 K0 t = 6 J0 t = 5 I0 t = 4 t = 3 t = 2 t = 1 H0 G0 E0 D0 t = 0 C0

64 Queue example 10 (15 circuits) A B C D E F G H I J K L M N O N6 N5 O5 D5 N4 O4 A3 B3 C4 D4 E4 I4 N3 O3 A2 B2 C3 D3 E3 I3 L3 N2 O2 A1 B1 C2 D2 E2 I2 L2 N1 O1 A0 B0 C1 D1 E1 G1 H1 I1 J1 L1 M1 N0 O0 Output Queue t = 9 t = 8 t = 7 t = 6 t = 5 t = 4 t = 3 t = 2 t = 1 t = 0 M0 L0 K0 J0 I0 H0 G0 E0 D0 C0

65 Attack Example Client Tor Node 1 - Unknown Node Malicious Client Tor Node 3 - Our Exit Node Tor Node 2 - Known High BW Tor Node 1 Server High BW Tor Node 2 Malicious Server

66 Attack Implementation Modified exit node Modified malicious client node Lightweight malicious web server running on GNU libmicrohttpd Client side JavaScript for latency measurements Instrumentation client to receive data

67 Gathered Data Example (1/8) Latency measurement graph freedomsurfers Latency variance (in seconds) Control Run Attack Run Downloaded Data Bytes expended by attacker (in kb) Sample number

68 Gathered Data Example (2/8) Latency variance (in seconds) Control Run Attack Run Downloaded Data Latency measurement graph bloxortsipt Bytes expended by attacker (in kb) Sample number

69 Gathered Data Example (3/8) Latency variance (in seconds) Control Run Attack Run Downloaded Data Latency measurement graph carini Bytes expended by attacker (in kb) Sample number

70 Gathered Data Example (4/8) Latency measurement graph carini Latency variance (in seconds) Control Run Attack Run Downloaded Data Bytes expended by attacker (in kb) Sample number

71 Gathered Data Example (5/8) Histogram of latency measurements for freedomsurfers Control Run Attack Run Control Run Regression Line Attack Run Regression Line Number of measurements in range Range of measurements (in seconds)

72 Gathered Data Example (6/8) Histogram of latency measurements for bloxortsipt Control Run Attack Run Control Run Regression Line Attack Run Regression Line Number of measurements in range Range of measurements (in seconds)

73 Gathered Data Example (7/8) Histogram of latency measurements for carini Control Run Attack Run Control Run Regression Line Attack Run Regression Line Number of measurements in range Range of measurements (in seconds)

74 Gathered Data Example (8/8) Histogram of latency measurements for carini Control Run Attack Run Control Run Regression Line Attack Run Regression Line Number of measurements in range Range of measurements (in seconds)

75 Statistical Analysis Use modified χ 2 test Compare baseline distribution to attack distribution High χ 2 value indicates distribution changed in the right direction Product of χ 2 confidence values over multiple runs Iterate over suspect routers until single node stands out

76 Cumulative Product of χ 2 p-values Product of Confidence Values 1-1x x10-10 Rattensalat SEC wie6ud6b hamakor yavs auk dontmesswithme cthor Raccoon eponymousraga BlueStar88a wranglerrutgersedu conf555nick mf62525 miskatonic WeAreAHedge anon1984n2 c bond server3 Product of Confidence Values 1-1x Privacyhosting c DieYouRebelScum1 ArikaYumemiya auk mrkoolltor TorSchleim myrnaloy judas Doodles123 tin0 baphomet kallio diora aquatorius Einlauf dontmesswithme askatasuna century Number of Runs Number of Runs

77 What We Actually Achieve We do identify the entire path through the Tor network (same result as Murdoch and Danezis) We do achieve this on the modern, current Tor network Attack works on routers with differing bandwidths This means that if someone were performing this attack from an exit node, Tor becomes as effective as a network of one-hop proxies

78 Why Our Attack is Effective Since we run the exit router, only a single node needs to be found Our multiplication of bandwidth technique allows low bandwidth connections to DoS high bandwidth connections (solves common DoS limitation)

79 Fixes Don t use a fixed path length (or at least make it longer) Don t allow infinite path lengths Induce delays into connections (probably not going to happen) Monitor exit nodes for strange behavior (been done somewhat) Disable JavaScript in clients Use end-to-end encryption

80 Attack Improvements/Variants Use meta refresh tags for measurements instead of JavaScript Parallelize testing (rule out multiple possible first nodes at once) Improved latency measures for first hop to further narrow possible first hops

81 Conclusion Current Tor implementation allows arbitrary length paths Current Tor implementation uses minimally short paths Arbitrary path lengths allow latency altering attack Latency altering attack allows detection of significant changes in latency Significant changes in latency reveal paths used

82 Questions?

83 R. Dingledine, N. Mathewson, and P. Syverson. Tor: The second-generation onion router. In Proceedings of the 13th USENIX Security Symposium, August N. S. Evans, R. Dingledine, and C. Grothoff. A practical congestion attack on tor using long paths. In 18th USENIX Security Symposium, pages USENIX, 2009.

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