TorScan: Tracing Long-lived Connections and Differential Scanning Attacks
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1 TorScan: Tracing Long-lived Connections and Differential Scanning Attacks A. Biryukov, I. Pustogarov, R.P. Weinmann University of Luxembourg September 5, 2012 A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
2 Overview What is Tor and How it works A classification of published attacks on Tor Revealing Tor topology information Topology-based attacks Evaluation of the attacks A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
3 Tor anonymity network Tor relays Internet servers running Tor software Both relays and clients run the same software Authorities 9 most trusted relays which maintain and distribute the list of Tor relays (the Consensus) Clients route their traffic through a chain of Tor relays Tor relays do not delay traffic nor use padding Is the most popular anonymity network (>3000 Tor relays; 400,000 users/day). It is fast. A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
4 Tor anonymity network A user chooses three Tor relays guard,middle,exit and builds a circuit: exchanging symmetric keys and updating the relays Tor routing tables Telescoping is used to exchange symmetric keys between the client and the relays Traffic flows down the circuit in fixed-size cells, which are unwrapped by a symmetric key at each node (like the layers of an onion) TLS connections between relays are used as the transport for Tor cells One TLS connection carries many circuits TLS TLS TLS Client Guard Middle Exit Server A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
5 Classification of published attacks server exit relay middle relay guard node Passive traffic analysis attacks. Cell counting, time correlation, Website fingerprinting Active traffic analysis attacks Watermarking, node clogging, etc. Attacks based on information leakage from specific applications. user attacker Bittorrent leaking IP addresses, etc. A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
6 Intersection attack and Guard nodes server exit relay middle relay guard node user attacker Controlling the entry node and sniffing the server locating the client If the entry node is chosen randomly, as the client makes many different circuits over time, then the probability that the attacker will see a sample of the traffic goes to 1 It does not happen if the entry nodes are fixed. Each client has a pool of three guard nodes which are actual for 1 month A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
7 The attack goal We do not reveal the actual IP address of a client We do reveal the guard nodes of a client Guard nodes are the next point to attack. Guard nodes are specific to the user and can be considered as his signature A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
8 The key to our attack Before the Tor network was considered as a fully connected graph We have found ways to probe the connectivity of a Tor relay. We found how topology leakage can be used to trace back a user from an exit relay to a small set of potential entry nodes. A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
9 Revealing Tor connectivity (1/5) Each node is uniquely identified by its RSA public key (fingerprint) Use CREATE/CREATED cells to exchange a Diffie-Hellman key. Use EXTEND cell to relay a CREATE cell EXTEND cell contains the fingerprint and IP address of the next relay EXTENDED cell indicates the success; DESTROY cell indicates a failure CREATE Guard CREATED Middle Exit Server Bill A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
10 Revealing Tor connectivity (2/5) Each node is uniquely identified by its RSA public key (fingerprint) Use CREATE/CREATED cells to exchange keys. Use EXTEND cell to relay a CREATE cell EXTEND cell contains the fingerprint and IP address of the next relay EXTENDED cell indicates the success; DESTROY cell indicates a failure (1) EXTEND[CREATE] To FP_A@ Bill Guard (4) EXTENDED[CREATED] (2) CREATE FP_A@ (3) CREATED Middle Exit Server A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
11 Reavealing Tor connectivity (4/5) Each pair of Tor relays tries to keep just one TLS connection A connections to Relay A is Canonical if IP A and fingerprint A of relay A are from the Consensus document The canonical connection will be used for all subsequent circuit extension requests to the relay with fingerprint A. IP address is ignored What happens if I indicate a fingerprint from the consensus and as IP FP_A@ Bill DESTROY EXTEND[CREATE] To FP_A@ Steve A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
12 Reavealing Tor connectivity (3/5) Each pair of Tor relays tries to keep just one TLS connection A connections to Relay A is Canonical if IP A and fingerprint A of relay A are from the Consensus document The canonical connection will be used for all subsequent circuit extension requests to the relay with fingerprint A. IP address is ignored What happens if I indicate a fingerprint from the consensus and as IP FP_A@ Bill EXTENDED EXTEND[CREATE] To FP_A@ Steve A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
13 Reavealing Tor connectivity (5/5) It takes less time to extend a circuit over already existing connection 18 additional steps at least 18 RTT time slower A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
14 Show me! One scan takes 30 seconds. Can be easily parallelized. 89 USD for scanning the whole network for a day (4 minutes between scans). A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
15 The attack scenarios 1 We will trace Long-lived connections: long-lived SSH sessions, very large files downloads, file-sharing applications and communications over instant messaging networks. 2 We will trace recurrent connections: Gmail establishes new connections every 2 minutes; web sites with auto-refresh contents. Pseudonymous user is identified by a cookie or a login credential. A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
16 Tracing Long-lived connections. A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
17 Tracing Long-lived connections. Evaluation How long should we wait for other connections to disappear? What is the asymptotic behaviour of the decay curve? bw-router, Control-Port-measurements 1500-bw-router, Canonical-Connections-Probing bw-router, Control-Port-Measurements bw-router, Canonical-Connections-Probing Feb14:18 12Feb14:18 13Feb14:18 14Feb14:18 15Feb14:18 A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
18 Tracing Long-lived connections. Evaluation How long should we wait for other connections to disappear? What is connections duration distribution? Are there immortal connections? Incoming circuit rate for this connections is very high What is the asymptotic behaviour of the decay curve? 100 Connections medium-to-high bandwidth Connections medium-to-medium bandwidth Connections, % % of routers BW distribution Percentage of immortal connections Connection duration, hours Bandwidth weights from consensus A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
19 Differential scanning 1 Tor Specification: a circuit (route) lives 10 minutes 2 Assume the client establishes recurrent connection to the server 3 The client is identified by his cookie or pseudonym 4 If we kill connection A, there is a probability that connection B will drop as well A B C D PPrivCom052 spftor3 A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
20 Differential scanning. Evaluation 1 What is the probability that connections B drops (Signal)? 2 What is the probability that other connections drops (Noise)? 3 How many tries should we make? C37B234FAD013453B90375EB55864FEBC876104A: 58 (PPrivCom052) bw=36500 CA1CF70F4E6AF9172E6E743AC5F1E918FFE2B476: 35 (spftor3) bw= Signal Noise Number of differentials Bandwidth weight A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
21 Some remarks For many existing attacks: exhaustive probing of each link in the Tor network is required Existing attacks can become practical again since the amount of links to be probed is significantly reduced A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
22 Conclusion All prior research on Tor assumed opacity of the Tor network topology meaning that the attacker had to assume a fully connected graph. In practice, the real degree of a node in this graph is substantially smaller than its maximum at any given point in time. For the first time, we have shown methods to determine the real connectivity of relays in the Tor network and the dynamics of the topology of the whole Tor network. Based on this, we described several novel attacks that use this information to deanonymize the entry points of the users into the Tor network. A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
23 Thank you. A. Biryukov, I. Pustogarov, R.P. Weinmann (UNI LU) Torscan September 5, / 23
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