PHYVOS: Physical Layer Voting for Secure and Fast Cooperation - PowerPoint Presentation

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PHYVOS: Physical Layer Voting for Secure and Fast Cooperation

Bocan

Hu, Yan Zhang, and Loukas Lazos

Department of Electrical and Computer Engineering,

University of Arizona

IEEE CNS 2015, Florence Italy

Slide2

Cooperation in Distributed Wireless Networks

Distributed wireless networks rely on the cooperation principle to coordinate network functions Scalability, availability, fault-tolerance, cost-efficiencyE.g.,

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Cooperative diversity in spectrum sensing for opportunistic networksSpectrum sensing period is specified to 2 sec [IEEE 802.22]

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FC

legacy user

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Cooperation in Distributed Wireless Networks

Distributed wireless networks rely on the cooperation principle to coordinate network functions Scalability, availability, fault-tolerance, cost-efficiencyE.g.,

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Coordination in networked multi-agent systemsConsensus delay specified in msec range

Slide4

Simplest From of Coordination

Exchange of state information – equivalent to a “yes/no” voting processA set of actors u1, u2 ,…, uM cast M binary votes v1, v2 ,…, vME.g., γ = 0 implements a plurality rule

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Voting outcome

threshold

actor votes

u

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u

3

u

2

FC

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A sequential process that scales linearly with the number of votersAdditional delay overhead to guarantee integrity Voter authentication and message integrityDelay is further amplified for coordination of channel access

Secure Message-Based Voting

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time

PHY

header

MAC header

1-bit vote

signature or HMAC

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v

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A simultaneous process that minimizes the communication and delay overheadRobustness: Voting outcome reflects the true outcome in the presence of external attacks and malicious voters

PHYVOS: PHY Layer-Based Voting Scheme

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time

PHY

header

MAC header

1-bit

v

ote

s

ignature or HMAC

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Simultaneous Vote Casting

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time

time

time

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Fusion Center

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5

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f2

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f0

f5

f4

f3

f2

f1

f0

v1

v2

v3

Exploit the subcarrier orthogonality of OFDM

OFDM symbols at the receiver

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Each actor ui is assigned two subcarriers fj and fj+1The FC and ui share random binary sequence based on some pairwise seed siRi(si) = { ri(n) = PRNG(n, si), n = 1,2, . . . }To cast vi, actor ui casts l symbol votes in either fj or fj+1 by transmitting l OFDM symbols ay, k = j + vi XOR ri(n) xk(n) = 0, otherwiseE.g., Ri(si) = { 1, 0, 0, 1 },

Vote Casting Phase

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random symbol

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i

= 1

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Voting Tallying Phase

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The FC computes the vote of each actor and tallies the votes

subcarrier

. . .

Energy

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N-1

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No demodulation

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n-2

fn-1

threshold

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Adversary Model

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Injects OFDM signals to change the voting outcome TDoes not launch DoS attack (i.e., a voting outcome must be computable)Secret vote model: Adversary is unware of the voting intent of each actorOpen vote model: Adversary is aware of the voting intent of each actor

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Modification of a Single Vote

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Adversary injects energy on one of the subcarriers to swing or nullify a vote

 

 

 

 

time

FC

time

 

 

 

 

 

 

 

 

time

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0

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+

adversary

v

oter

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Secret vote model: δ: # of attacked votes μ: winning marginM: # of voters, γ: decision threshold (e.g., γ = 0) p: Pr[vi = e], n1: (M+γ+μ)/2Open vote model:

YES

Modification of the Voting Outcome

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We can

select the number of symbol votes

to drive the above probabilities to any desired value

NO

γ

μ

δ

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Robustness for the Secret Vote Model

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20 voters, plurality vote, adversary attacks δ = N/2 + μ votes

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Robustness for the Open Vote Model

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20 voters, plurality vote, adversary attacks δ = N/2 + μ votes

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MV

: message-based voting using 802.11g (52 subcarriers, 6 bits per symbol, 13 symbol gap between messages)DMV = 20 M – 13 OFDM symbolsPHYVOS: up to 26 votes can votesimultaneously using l OFMD symbolsDPHYVOS = ceil (M/26)

Voting Overhead

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PHYheader

MAC header

1-bitvote

HMAC

CRC

5

30

4

32

1

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Frequency Synchronization

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Carrier frequency offset (CFO) effect on demodulation: Subcarriers are not orthogonal, causing ICI Symbols at each subcarrier appear with arbitrary rotation in the constellation map

PHYVOS:

No preamble

is present to perform CFO synchronization

No

demodulation

necessary, so constellation point rotation does not affect vote tallying

No ICI as long as freq. misalignment stays

below subcarrier spacing

In 802.11g subcarrier spacing is 315Khz while CFO is < 100 KHz

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Time Synchronization

Time misalignment: propagation delay and device clock drifts. Cyclic prefix (0.8 μsec) is applied to normal OFDM symbol.

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Increasing the CP to account for synchronization error.

Cast

a symbol vote in two symbol durations.

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4 NI USRP 2921 devicesCarrier frequency: 2.4 GHz# of subcarriers: 64ConfigurationsTopology A: equidistantTopology B: varying distances

PHYVOS Implementation on USRPs

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Selection of Vote Detection Threshold

Three voters assigned at 1st, 5th and 9th subcarrier casting 1,000 votes using BPSK modulation

Topology A

: voters are placed 5

ft from FC

Topology B

: voters are placed 5/10/15

ft

from FC

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MAC for Multi-channel Networks ( MMAC)

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Received Power Per Subcarrier

Detection threshold set to -80dBm

Topology

A

Topology B

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MAC for Multi-channel Networks ( -MMAC)

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Voting in the Presence of an Adversary

Voter 1 assigned 1st and 2nd subcarrierVoter 2 assigned 5th and 6th subcarrierAdversary injected energy to subcarriers at random

Topology

A

Topology B

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Concluding Remarks

Contributions Summary Designed a PHY-layer based voting scheme for fast coordination Voting is performed without explicit message exchange Significantly lower communication and delay overheads compared to message based voting Robust to vote manipulation without expensive cryptographyFuture Work PHYVOS robustness and correctness under varying channel conditions Extend PHYVOS to accommodate x-ary voting Investigate the application of PHY-layer voting to fully distributed consensus algorithms (No Fusion Center)

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Slide23

Thank you!

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