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Tor – The Onion Router By: David Rollé. What is Tor?  Second generation Onion Routing  Aims to improve on first generation issues  Perfect Forward.

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Presentation on theme: "Tor – The Onion Router By: David Rollé. What is Tor?  Second generation Onion Routing  Aims to improve on first generation issues  Perfect Forward."— Presentation transcript:

1 Tor – The Onion Router By: David Rollé

2 What is Tor?  Second generation Onion Routing  Aims to improve on first generation issues  Perfect Forward Secrecy  Ease of deployability and use  Remove superfluous information  Multiplex streams  Leaky-Pipe Circuit Topology  Congestion Control  Directory Servers  Variable Exit Policies  Integrity Checking End-to-End  Rendezvous Point  Why?

3 Background of Problem  Tracking information throughout the world  China  Is anonymity on the internet really necessary?  Prevalence of cyber crimes?  E.g. – Leverage  Global adversaries versus limited adversaries  Facebook versus your evil cyber-neighbor Bob  How critical is Tor in today’s society?  SOPA and PIPA  Exit Abuse?  Paper is from 2004, dated by several years. Tor has evolved substantially since this paper’s publishing, adding many layers of security.

4 Goals and Non-Goals of Tor Goals  Deployability  Usability  Flexibility  Simple design Deferred Goals  Not Peer-to-Peer  Not Secure from End-to- End attacks  Why wasn’t this emphasized?  Not protocol normalized  No UDP. Good or bad?  Doesn’t conceal who is connected to network.  Why not?

5 Low-Latency vs. High-Latency Low Latency Advantages  Can run regular webpages, with Javascript and JSON technology in near realtime. Low Latency Disadvantages  Can’t obfuscate data too much; data has time limits for expiration High-Latency Pros  Lots of time to obfuscate data, with multiple layers of encryption and reordering of end traffic. High-Latency Cons  Limits the usefulness of the technology, as servers and other important request servers cannot work with materials Which do you think is more efficient at safeguarding anonymity?

6 Tor Design

7 Onion Router  TLS Connection to every other Onion Router  Can interpret CircID’s to send data to another location  Can only see previous router and router ahead  Previously a problem in old architecture. How?  Verified by directory servers to create map  Efficiency problem? Better solutions?  Has identity key to verify its information

8 Onion Proxy  Local software for the user  Fetches Directories  Establishes circuits across network  Handle connections from user applications  Multiplexes TCP streams across circuits  Handles the routing from end to end

9 Cell Technology  Circuit ID (assigned at start, interpreted at router by key)  Control Cells  CircID and CMD  Relay Cells  Includes Relay, StreamID, Digest, Length of cell, as well as the CircID and CMD  Digest critical to Leaky-Pipe algorithm

10 Circuit Technology  Onion Routing with a twist  Construct Circuits  Long time to construct a complete circuit  Short time to add/subtract from  Consider rotating circuits once a minute  Destroy Circuits  Relatively quick, useful for rerouting the circuit through different ORs in case of circuit breakage

11 Circuit Creation  OP connects to OR with TLS secure  New CircID, uses a Control Cell to carry data.  OR responds with the second half of the Diffie- Hellman handshake  OP encrypts additional Control Cell and sends them to OR, waits for response, etc.  End result: Multiple layers of encryption, easily translated by OR. Also, Digest allows multiple exit points along circuit  Build longer circuit than necessary.

12 Streams  OP is asked for a connection via SOCKS  Each stream has random stream ID  Why is this important?  Problems with SOCKS  Applications can pass the hostname to the Tor Client, or pass the IP address first  If DNS reolution performed, Alice reveals location of both ends.  Solutions?

13 Integrity Checking via Digest  The Digest is comprised of encoded bits which verify when the cell is completely decoded  Lynchpin for Leaky-Pipe algorithm  ORs verify stream is not in still in transit  Digest pre-negotiated at circuit creation using SHA-1 digest with derivative of the key  Digest serves Leaky-Pipe topology and Integrity checking

14 Throttle Control  Rate Limiting  Bulk stream versus interactive stream  Fairness  Token Bucket Approach  Enforces average rate of incoming bytes  Permits short term bursts above bandwidth allotment  Cannot always wait for a full cell, send when possible

15 Congestion Control  Circuit Level Throttling  Packaging Window  Delivery Window  Relay sendme cell  Stream Level Throttling  Similar construction to circuit level throttling, just one level up the Open Systems Interconnection (OSI) model

16 Rendezvous Points  Requirements:  Access-Control, Robust, Smear-resistant, Application-Transparent  Introduction Points  Hidden server creates circuits to each introduction point (advertised ORs), and can hide some for only select clients  Rendezvous cookie  Obtained from an RP, given to the introduction point to connect server to client  Rendezvous Point  Server connects with second half of handshake from token, and RP connects two circuits together  Client initiates contact directly, and regular Tor operations commence  Why are these not available from outside of Tor?  Could it be possible to make them available outside of Tor?  Possibly have an OP handle the requests, and translate them into RP?  Con: Makes OP liable to attack from adversaries.

17 Design Defenses  DoS defense  Flow Control and Rate Limiting help, but other ideas need to be implemented.  Exit Policies  Open, Restricted (Some restrictions apply), Middleman (no connection outside Tor), Private (Only connect to local network)  Exit abuse hurts capabilities of Tor’s anonymization.  Directory Servers  Previously in-band updates: Entire network obtained all of the states at varying times.  Directories currently act as policemen of new nodes; new nodes require human intervention.  Directories synchronized and redundant.

18 Attack Methodologies and Defenses

19 Passive Attacks  Observe Traffic Patterns  Multiplexing minimizes damage  Observe User Content  Use of Privoxy  Option Distinguishability  Leads to tracing due to distinct pattern behavior  End-to-end Timing Correlation  Tor does not hide timing (low-latency requirement)  End-to-end Size Correlation  Leaky-Pipe Topology  Website Fingerprinting  New attack as of 2004, semi-defended by mitigation

20 Active Attacks  Compromise Keys  Mitigated by key rotation and redundant multiple layer encryption. Replacing a node via identity key could theoretically avoid this defense.  Iterated Compromise  Short lifetimes for circuits  Run Recipient  Adversary controls end server, which allows him to use Tor to attack the other end. Privoxy would help minimize chance of revealing initiator  Run Onion Proxy  Compromised OPs compromise all information sent through OP  DoS non-observed nodes  Only real defense is robustness  Run hostile OR  Requires nodes at both ends of a circuit to obtain information  Introduce Timing  Similar to timing discussed in passive version

21 Active Attacks continued  Tag Attacks  Integrity check mitigates this  Replay Attacks  Session key changes if replay used  Replace End Server  No real solution, verify that server is actually server with authentication. Similar to Recipient attack  Smear Attacks  Good press and exit policies  Hostile Code Distribution  All Tor releases signed

22 Directory Subversion  Destroy Servers  Directories require majority rule, or human intervention if more than half destroyed.  Subvert Server  At worst, cast tie-breaker vote  Subvert Majority of Servers  Ensure Directories are independent and resistant to attacks  Encourage Dissent in Directory Operators  People problem, not Tor problem.  Trick Directories  Server Operators should be able to filter out hostile nodes.  Convince Directories that OR is Functional  Directory servers should test by building circuit and streams to OR.

23 Rendezvous Point Attacks  Many Introduction Point Requests  IP can block requests with authorization tokens, or require certain amounts of computation per request.  Attack Introduction Point  Server re-advertises on different IP, or advertise secretly. Attacker must disable all IPs.  Compromise Introduction Point  Servers should occasionally verify their IPs, and close circuits that flood them.  Compromise Rendezvous Point  Similar to active attacks against ORs

24 Other Attacks?

25 Food For Thought  Global adversaries: Paper never touches on adversaries with large programming armies behind them. Can Tor be useful and efficient in environments such as China?


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