SybilCast : Broadcast on the Open Airwaves

1. SybilCast:Broadcast on the Open Airwaves Seth Gilbert, Chaodong Zheng National University of Singapore

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

3. Radios can access many channels Use radio resource testing!   u v x y Base Station !ALERT!  msg channel one Honest users: always pass the test! Malicious users: lose (fake) id with 50% chance! channel two msg [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. Ack for msg

4. 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

5. Overview • Introducing sybil attacks • Model and problem • The SybilCast protocol: • Structure • Why it works

6. Model • Synchronous wireless network: • Single-hop • channels • Users: • One (authenticated) base station • up to real users (unauthenticated) that come and go • Radios: • Everyone has one radio, choose one channel in a round • Transmit or receive Channel one v Channel two … w Channel c Base Station

7. Malicious users • Malicious users: • At most • Colluding • Capabilities: • Create sybilidentities • Jam channels • Spoof messages • Each has only one radio transceiver as well! Channel one x y v q r Channel two Sean  Shirley … w #$%@#%#^@#^@ Base Station Channel c Quit

8. Problem: fair bandwidth access • Basic problem: • Users arrive and request data • Base station delivers data to user • Goal: every user gets a fair share of the bandwidth: • If there are at most users in the system during request • Request gets of the total bandwidth Channel one Channel two Sean  Shirley … data u Base Station Channel c

9. 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 2d-s registered identities d registered identities … … registration phase: at most d new ids registered verification phase: s ids removed data phase: at most 2d ids present one epoch time

10. 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! 2d-s registered identities d registered identities registration phase: at most d new ids registered verification phase: s ids removed data phase: at most 2d ids present … … one epoch time

11. Registration phase • Goal: delivers a final seed to each request: • Long random binary string • Used as a frequency hopping sequence • Hidden from the malicious users • Procedure: • Divide phase into sub-phases of • In each sub-phase, deliver partial seedto user • User takes XOR of all partial seeds … …

12. 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)

13. Data phase random binary string nonce data • 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! Channel one u v Base Station Channel two w Channel three

14. The Power of the NonceTM Most sybil identities miss many nonces: • Many sybil identities → spread on many channels. • Spread on many channels → high likelihood to lose nonces. • We show, if there are sybil identities, after data rounds, of them will lose nonces. Honest identities do not miss many nonces: • For an honest node, it lose each nonce with probability . • After data rounds, each honest node loses nonces. We show , honest nodes win!

15. Verification phase • Procedure: • Users send collected nonces back to base station • (Uncoordinated) frequency hopping to resolve jamming and contention. • Threshold : • Base station eliminates identities without enough nonces • Guarantee: • No honest users are eliminated (w.h.p.) • All but 12t sybil identities are eliminated (w.h.p.)

16. Putting everything together • For a request from honest node • =maximum number of active real nodes • = maximum number of registered identities pfinishes registration pinitiate a request pobtains first partial seed … … epoch i epoch i+1 epoch i+2 epoch j time

17. Putting everything together • finishes reg. time. • However, may count (many) sybil identities! • We need to constrain ! • By the end of any epoch: • remaining identities • at most sybils. • , hence • In next epoch, at most new identities • We have . • finishes registration in time.

18. Putting everything together • finishes registration in time. • Once registered, gets in time. • In total, needs time. • If , this is just time! • I.e., (asymptotically) optimal time!

19. SybilCast’s key property • Theorem: If an honest user requests a data of size , and if there are at most concurrently active real nodes at any point during the request, then the download will complete in time w.h.p. • Corollary: On average, each honest user corresponds to sybil identities, hence each honest user can finish data download in asymptotically optimal time.

20. Conclusion 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!