SybilCast: Broadcast on the Open Airwaves SETH GILBERT, CHAODONG ZHENG National University of Singapore

Base Station u u v v Sunday afternoon in Starbucks  v2 v3 v1 v4 v7 v8 v6 v9 v5 We have a Sybil attack!  Sybil identities: AliceSean B/2 B/10 …

Radios can access many channels u u msgAck for msg x x  msg  channel one channel two Honest users: always pass the test! Malicious users: lose (fake) id with 50% chance! Use radio resource testing! [1] N. James, E. Shi, D. Song, and A. Perrig. The sybil attack in sensor networks: Analysis & defenses. [2] D. Mónica, J. Leitão, L. Rodrigues, and C. Ribeiro. On the use of radio resource tests in wireless ad-hoc networks. Base Station v v y y !ALERT!

Challenges  Colluding:  Malicious users can cover more than one channel  Other malicious behavior:  Malicious user jam channels, and/or spoof messages  Continuous nature of the system:  Cannot run a set of tests and then stick to normal data deliver protocols  Efficiency of detection:  Overhead for detecting sybil identities must be low

Overview 1. Introducing sybil attacks 2. Model and problem 3. The SybilCast protocol:  Structure  Why it works

Model Base Station v v w w Channel two Channel c … Channel one

Channel two Channel c … Channel one Malicious users Base Station v v w w Sean Shirley x x y y q q r r Quit

Channel two Channel c … Channel one Problem: fair bandwidth access u u Sean Shirley data Base Station

Introducing SybilCast  Three phases per epoch:  Registration phase: new users join the network  Data phase: registered users receive data and authentication information  Verification phase: base station checks registered users time … d registered identities registration phase: at most d new ids registered data phase: at most 2d ids present verification phase: s ids removed 2d-s registered identities … one epoch

Why those lengths?  Balance sybil identities’ admission rate and honest identities’ admission rate:  Fast admission → Low registration overhead  However: Fast admission → More sybil identities → Low throughput  Registered identities at most double! time … d registered identities registration phase: at most d new ids registered data phase: at most 2d ids present verification phase: s ids removed 2d-s registered identities … one epoch

Registration phase … …

Challenges and Tools  Avoid jamming  Random uncoordinated frequency hopping  Authenticating nodes (to counter spoofing):  Hash chain  Avoid contention among nodes:  Backoff protocol (ensures delivery of single partial seed)  Registration list (ensures enough partial seeds)

Structure of SybilCast  Three phases per epoch:  Registration phase: new users join the network  Data phase: registered users receive data and authentication information  Verification phase: base station checks registered users time … d registered identities registration phase: at most d new ids registered data phase: at most 2d ids present verification phase: s ids removed 2d-s registered identities … one epoch

Channel one Channel two Channel three Data phase  Goal: deliver data and nonces to registered identities  Procedure for each round:  Base station chooses a random registered identity  Send a packet on the pre-agreed channel with data and nonce  Intended receiver get the data  All nodes on that channel record the nonce! Base Station u u v v w w random binary string datanonce

The Power of the Nonce TM

Verification phase

p finishes registration Putting everything together time … p initiate a request … epoch iepoch i+1epoch i+2epoch j p obtains first partial seed

Putting everything together

SybilCast’s key property

THIS IS IT!  SybilCast solves fair bandwidth allocation despite:  Sybil attacks! Jamming! Spoofing!  Combination of existing tools:  Radio resource testing, frequency hopping, hash chain, …  And innovations:  Admission rate control, deferred verification, …  Distri-SybilCast?  If you have questions, now is the time! Conclusion