SybilCast:Broadcast on the Open AirwavesSETH GILBERT, CHAODONG ZHENGNational University of Singapore

Base Station

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Sunday afternoon in Starbucks

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v1

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We have a Sybil attack!

Sybil identities:

Alice Sean

B/2 B/2

B/10 B/10 B/10…

Radios can access many channels

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msgAck for msg

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 Stationv 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 attacks2. Model and problem3. The SybilCast protocol:

Structure Why it works

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

Base Station

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Channel two

Channel c

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Channel one

Channel two

Channel c

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Channel one

Malicious users Malicious users:

At most Colluding

Capabilities: Create sybil identities Jam channels Spoof messages

Each has only one radio transceiver as well!

Base Station

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Sean Shirley

x y q r

#$%@#%#^@#^@Quit

Channel two

Channel c

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Channel one

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

u Sean Shirley

dataBase 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

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d registered identities

registration phase:at most d new ids registered

2(( )log )Nd c

data phase:at most 2d ids present

2(( )log )d c N

verification phase:s ids removed(( )log )d c N

2d-s registeredidentities

…

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

2(( )log )Nd c 2(( )log )d c N (( )log )d c N

2d-s registeredidentities

…

one epoch

2(( )log )Nd c 2(( )log )d c N (( ) log )d c N

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 seed to user User takes XOR of all partial seeds

2(( )log )x c N …

(log )N…

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)

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!

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¿𝑚𝑢∨𝑟1>¿ ¿𝑚𝑢∨𝑟1>¿ ¿×∨𝑟 1>¿

¿𝑚𝑤∨𝑟2>¿ ¿𝑚𝑤∨𝑟2>¿ ¿×∨𝑟 1,𝑟2>¿

random binary string

data nonce

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!

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.)

p finishes registration

Putting everything together For a request from honest node

= maximum number of active real nodes = maximum number of registered identities

time

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p initiate a request

…epoch i epoch i+1 epoch i+2 epoch j

p obtains first partial seed

𝑂 ((𝑛∗+𝑐 )𝑐 log3𝑁 ) 𝑂 ((𝑑∗+𝑐 ) log2𝑁 )

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.

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!

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.

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