Anonymity With Tor. The Onion Router. July 5, It s a series of tubes. Ted Stevens. Technische Universität München
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1 Anonymity With Tor The Onion Router Nathan S. Evans Christian Grothoff Technische Universität München July 5, 2012 It s a series of tubes. Ted Stevens
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 Review: Mixing David Chaum s mix (1981) and cascades of mixes are the traditional basis for destroying linkability:
8 Review: Mixing David Chaum s mix (1981) and cascades of mixes are the traditional basis for destroying linkability:
9 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 Si until circuit of desired length n constructed
10 Onion Routing Example Client sets up symmetric key K S1 with server S 1 Exchange K S1 Client S 1 S 2
11 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
12 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
13 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
14 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
15 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
16 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
17 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
18 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)
19 Hidden Services Example 1
20 Hidden Services Example 2
21 Hidden Services Example 3
22 Hidden Services Example 4
23 Hidden Services Example 5
24 Hidden Services Example 6
25 Types of Attacks on Tor Exit Relay Snooping Website fingerprinting Traffic Analysis Intersection Attack DoS
26 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
27 Key Tor Properties 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
28 Our Basis for Deanonymization Target user is running Tor from 2009 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) 3. Paths of arbitrary length through the network can be constructed
29 Regular Path Example Client Tor Node 3 Tor Node 1 Tor Node 2 Server
30 Circular Path Example 1/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server
31 Circular Path Example 2/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server
32 Circular Path Example 3/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server
33 Circular Path Example 4/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server
34 Circular Path Example 5/5 Client Tor Node 3 Tor Node 1 Tor Node 2 Server
35 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
36 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
37 Queue example 1 (3 circuits) A B C B5 B4 B3 B2 B1 A0 B0 C1 Output Queue t = 0 C0
38 Queue example 2 (3 circuits) A B C B5 B4 B3 B2 B1 B0 C1 Output Queue t = 1 t = 0 A0 C0
39 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
40 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
41 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
42 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
43 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
44 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
45 Queue example 1 (15 circuits) A A0 A1 A2 A3 B B0 B1 B2 B3 C C1 C2 C3 C4 D D0 D1 D2 D3 D4 D5 E E0 E1 E2 E3 E4 F G G0 G1 H H0 H1 I I0 I1 I2 I3 I4 J J0 J1 K K0 L L0 L1 L2 L3 M M0 M1 N N0 N1 N2 N3 N4 N5 N6 O O0 O1 O2 O3 O4 O5 C0 t = 0 Output Queue
46 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
47 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
48 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
49 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
50 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
51 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
52 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
53 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
54 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
55 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
56 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
57 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
58 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
59 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
60 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
61 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)
62 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)
63 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)
64 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)
65 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
66 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
67 What We Actually Achieve We do identify the entire path through the Tor network We do achieve this on the 2009 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
68 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)
69 Fixes Don t use a fixed path length (or at least make it longer) Don t allow infinite path lengths (this is fixed in Tor now!) 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
70 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
71 Conclusion Initial Tor implementation allowed arbitrary length 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
72 Questions?
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