U NIVERSITY OF CRETE Department of Computer Science

1. UNIVERSITY OF CRETE Department of Computer Science Low –Resource Routing Attacks Against Anonymous Systems Fragkiadaki Georgia AM681

2. Outline • A short overview of the Tor system • Analysis of Tor’s router selection algorithms • A description of Tor’s envisioned attack model • A overview of the common anonymity metrics • Attack and description attack model • Experiments about attack and which the results • A detailed analysis of proposed defenses • Comparison attack with previous attacks against Tor

3. Overlay mix-networks • used widely • provide low-latency anonymous communication services • provide perfect anonymity • Tor is one of the most popular privacy enhancing systems • provide optimal performance for interactive application • Goal: the system provide anonymity against non-global adversaries

4. Introduction • Research Themes: • how can we minimize the requirements necessary for any adversary to compromise the anonymity of a flow • how can we harden Tor against our attacks. • Attack Overview: • The adversary need few malicious nodes to break the anonymity of clients. • Context • Additional Improvements and Other Attacks • Prevention • Broader Implications

5. Tor System contain : • Onion router (OR) : the server component of the network – is responsible for traffic • Onion proxy (OP) : is the client part of the network • A circuit : is a path of three onion routers through the Tor from OP • entrance router • mix router • exit router

7. Understanding Tor at a High Level • There is a circuit switched network at the core of Tor • The originator of the circuit knows the complete path • Uses standard cryptographic methods , like TLS (Transport Layer Security) • Onion routing: encrypted cells use a layered scheme. Each hop removes a layer until the last node, so it is fully decrypted. • Onion proxy: forward a local user’ s traffic through the network

8. Selection Algorithms (SA) Which onion router select Tor to include in a circuit? • Entrance Router SA: • select a set of onion routers that are marked by servers as fast and stable • protect the first hop of a circuit and use more reliable and trustworthy nodes • Non-Entrance Router SA • select non-entrance nodes, which optimize onion router selection for bandwidth and uptime • not always choose the best nodes every time

9. Common Anonymity Metrics • We define entropy , a metric to measure the amount of anonymity • p(xi) : probability i-th node being including on path • N : the number of routers in the Tor network

10. Attack Model • We focus on : • Attacking the anonymity of clients, running default configurations • attacking clients, which join the network, after first phase of attack • Phases: • Setting Up • Linking Paths

11. Phase One: Setting Up • The basic Attack: the adversary’s setup procedure is to enroll or compromise a number of high-bandwidth, high-uptime Tor servers • Resource Reduction: decrease the resource requirements for the malicious nodes • low bandwidth connections • What Happens Next: a goal is to provide a low-latency service about a new client joins the network

12. Phase Two: Linking Paths • When the full path has been populated with malicious nodes • Each malicious router logs information for each cell received, according to • location of circuit path, • timestamp, • circuit ID, • IP address (previous, next) and • ports

13. Experiments (1/2) • Experimental Setup • We set up an isolated Tor deployment • not run in real environment, due to probably destructive effects. • determine test with real sizes about Bandwidth Class • create two isolated Tor networks, with 40 and 60 nodes, each running

14. Experiments • Traffic Generation • would make it more realistic ,but there are not much data available • Malicious Node Configuration • an attacker can correlate, each malicious router advertises a read and write bandwidth capability of 1.5 MB/s and a high uptime

15. Results of Experiment • There is difference between the analytical expectation and the experimental results • The attack is effective in influencing Tor’s routing mechanisms • Entropy decrease in network when we added malicious nodes • shows the global impact that our attack has on decreasing Tor’s anonymity

16. Attack Extension • Compromising Existing Clients • Improving Performance Under the Resource-Reduced Attack • Selective Path Disruption • Displacing Honest Entry Guards. • Compromising Only the Entry Node

17. Previous Attacks Against Tor • There have been published three attacks in the past parts of the Tor protocol: • Murdoch and Danezis: a low cost traffic – analysis technique • Overlier and Syverson : attack located hidden servers inside the Tor network • Murdoch: the key to this attack is the observation that when a server is idle, its CPU runs at a cooler temperature

18. Conclusion (1/2) • We introduce a low-resource traffic analysis attack , which is highly practical • an attacker can compromise the anonymity of a large amount of the communication channels through the network • Tor is vulnerable to attacks from non-global adversaries that control only a few high-resource nodes, or nodes that are perceived to be high-resource. • the system utilizes a preferential routing algorithm that attempts to optimize for performance and this fact exploits the attack

19. Conclusion (2/2) • an adversary can, with high probability, compromise the entrance and exit servers on a route for new clients

20. Questions? Thank you for attention