Preserving Caller Anonymity in Voice-over-IP Networks Mudhakar Srivatsa, Ling Liu and Arun Iyengar Presenter: Bo Wu

Agenda Voice-over-IP Caller Anonymity Threat Models Defending Methods Experimental Evaluation Conclusion

Phone. The history…

PSTN PSTN- stands for Public Switched Telephone Network PSTN- stands for Public Switched Telephone Network Circuit-based means reserving resources for each user Circuit-based means reserving resources for each user Kind of expensive Kind of expensive

Voice-over-IP: another choice Voice over Internet Protocol Voice over Internet Protocol “A method for taking analog audio signals, like the kind you hear when you talk on the phone, and turning them into digital data that can be transmitted over the Internet. “ “A method for taking analog audio signals, like the kind you hear when you talk on the phone, and turning them into digital data that can be transmitted over the Internet. “ Also known as: Also known as: Voice over Packet (VoP) Voice over Packet (VoP) IP Telephony (IPT) IP Telephony (IPT)

Benefits #1. SAVING MONEY! #1. SAVING MONEY! Routing phone calls over existing data networks to avoid the need for separate voice and data networks. VOIP offer features and services for free (or at little cost)

Benefits Increased Agility Increased Agility Tactical Advantages Tactical Advantages Integrate things like: s, phone, instant messages, etc. Integrate things like: s, phone, instant messages, etc.

VoIP is popular

Characteristics of VoIP network P2P topology Internet peer peer peer peer peer

Characteristics of VoIP network Additional QoS requirement ITU (International Telecommunication Union) recommends up to 250ms one-way latency for interactive voice communication. People go mad due to bad quality

Anonymity in VoIP networks What is anonymity? NO leakage of information about identity Why is it important? Human rights Sensitive applications

Where is the caller? Source privacy Hot topic in many kinds of networks: Ad hoc, Sensor networks, Mesh networks, …… Papers published in: Infocom, ICDCS, CCS, Securecomm, S&P…

What’s the difficulties? Strong ability of attackers Content analysis Timing analysis Fully distributed Link latency ……

How VoIP works? Establish routes: Unstable topology Routes across different ASPs Sending messages Comply to different application protocols Confidentiality Hop-by-hop encryption End-to-end encryption

Establishing routes InitSearch: Bo Zhenhua

How does it work? ProcessSearch Bo Zhenhua

How does it work? FinSearch Bo Zhenhua

What’s the problem? Bad guys are there… Bo Bad guy: Mr. X Bad guy: Mr. Y

Zhenhua What’s the problem? Bad guys are there… Bo Bad guy: Mr. X Bad guy: Mr. Y

Zhenhua What’s the problem? What if Zhenhua is surrounded by bad guys? Bo Bad guy: Mr. X Bad guy: Mr. Y Bad guy: Mr. W Bad guy: Mr. Z

Threat model Composed by assumptions and formulations Three threat models: Deterministic Triangulation Attack Statistical Triangulation Attack Differential Triangulation Attack

Deterministic Triangulation Attack “Deterministic” means fixed latency for each link Exploit two properties of the route set up protocol: 1. It establishes the shortest route between the two nodes src and dst. 2. Any node can estimate its distance from src => Each bad guy has the knowledge of its distance from any other node in the network

Deterministic Triangulation Attack BoMr. Y Mr. X

Deterministic Triangulation Attack

For each bad guy p i in network If Calculate the final score:

Statistical Triangulation Attack “Statistical” means link latency follows some probabilistic distribution, say Gaussian distribution Exploit one nice property of Gaussian distribution X, Y follow Gaussian distribution If Z = X + Y THEN E(Z) = E(X)+E(Y) When calculating scores, use mean value

Differential Triangulation Attack The mentioned two attacks relies on the time stamp in search packet to make the first estimation. What if the source remove time stamp? The attackers can still cooperate……

Differential Triangulation Attack Bo Mr. Y Zhenhua Dist(Bo, X)-Dist(Bo,Y) < Dist(Zhenhua, X)-Dist(Zhenhua, Y)

Topology discovery All of the three threat models require global information like topology and link latency Malicious nodes can collude to collect such information Send ping messages with small TTL Infer local topology and link latency through pong messages

Attack efficiency Deterministic Triangulation Statistical Triangulation

Attack efficiency Differential Triangulation

Defending algorithms General idea: break the tight correlation of timing and distance Random walk Search Algorithm Best anonymity, worst QOS Hybrid route set up Tradeoff between anonymity and QOS

Random walk search algorithm Basic idea: Randomly select a neighbor to forward search request instead of broadcasting (Random walk is used in tens of papers to defend against traffic analysis.) Why it works? According to random walk theory:

Hybrid Route set up protocol Controlled random walk Two phases Random walk search phase Search dest node by random walk Broadcast search phase Search dest node by broadcast One kind of probabilistic routing: Start at random walk search phase Remain in this phase with probability of p Transfer to Braodcast search phase with probability of 1-p

Hybrid Route set up protocol Multi-Agent Random Walk Send out w search messages instead of one Every search message performs random walk Route established when the first search message arrives at dest node Tradeoff when setting w Bigger w means smaller latency Bigger w also increases attacking efficiency

Simulation results Latency study:

Simulation results Anonymity study:

Comments Brilliant Threat models Capture key properties of broadcast A small percentage of nodes can attack very accurately Not quite novel defending methods Random walk has been used by tens of (if not hundreds of) papers No deep analysis of the performance

Conclusion VoIP is gaining more and more popularity Three threat models directly target at caller’s anonymity Introduce randomness to defend against timing attack Lesson: challenging problem to protect privacy as well as providing QoS

Questions?