Tor – The Onion RouterBy: David Rollé

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?

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.

Goals and Non-Goals of TorGoals 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?

Low-Latency vs. High-LatencyLow 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 email servers and other important request servers cannot work with materials

Which do you think is more efficient at safeguarding anonymity?

Tor Design

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

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

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

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

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.

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?

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

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

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

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.

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.

Attack Methodologies and Defenses

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

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

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

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.

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

Other Attacks?

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?