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Physical Layer Security Made Fast and Channel-Independent Shyamnath Gollakota Dina Katabi

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What is Physical Layer Security? Introduced by Shannon Sender Receiver Channel Time Variations known only to sender and receiver

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Why is it interesting? No computational hardness assumptions Comes free from wireless channel Combine with cryptography for stronger security

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Past work Much work 2006 – first empirical demonstration [Trappe’06] Effort to increase secrecy rate [Wyner’75], [Csiszar’78], [Johansson‘01], [Shamai’08] [Trappe’08], [Krishnamurthy’09], [Kasera’10] Theory Practice

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But, not fast enough Mobile (44 bits/s) For practical key (2048 bits) 0.75 minutes

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But, not fast enough Static (1 bits/s) Mobile (44 bits/s) For practical key (2048 bits) 0.75 minutes 34 minutes

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Why is it so slow? Existing practical schemes rely on channel changes Sender Receiver Sender transmits, receiver measures channel Receiver transmits, sender measures channel Exploit Channel Reciprocity Generating new secret bits requires channel to change

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How can we make physical security fast? Don’t rely on channel changes Instead, introduce changes by jamming

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Sender repeats its transmission Repetition iJam

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For every sample, receiver randomly jams either the original sample or the retransmission Repetition iJam

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Receiver reconstructs signal by picking clean samples Repetition iJam

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Eavesdropper does not know which samples are clean and hence cannot decode No longer requires channel to change Repetition Generate secret bits faster iJam

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First practical physical layer security that doesn’t rely on channel changes Implemented and empirically evaluated – 3 orders of magnitude more secret bits – Works with both static and mobile channels Contributions

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Challenge 1: Making clean and jammed samples indistinguishable BPSK: ‘0’ bit -1 ‘1’ bit +1 Time Samples +1

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Challenge 1: Making clean and jammed samples indistinguishable BPSK: ‘0’ bit -1 ‘1’ bit +1 Time Samples +1 Jamming should not change structure of transmitted signal

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Solution 1: Exploit characteristics of OFDM X1X1 X2X2 XNXN IFFT Y1Y1 Y2Y2 YNYN.. Time Samples Modulated bits By central limit theorem, transmitted samples approximate Gaussian distribution Time Samples

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Solution 1: Exploit characteristics of OFDM X1X1 X2X2 XNXN IFFT Y1Y1 Y2Y2 YNYN.. Time Samples Modulated bits Time Samples Pick jamming samples using a Gaussian Distribution

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Solution 1: Exploit characteristics of OFDM X1X1 X2X2 XNXN IFFT Y1Y1 Y2Y2 YNYN.. Time Samples Modulated bits Time Samples Jam using a Gaussian Distribution Harder to distinguish between clean and jammed samples

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Challenge 2: Eavesdropper can still exploit signal statistics Transmitted samples Jammed samples Variance of jammed samples greater than clean samples Using hypothesis testing, eavesdropper can guess Probability Distribution

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Solution 2: Use xoring to reduce eavesdropper’s guessing advantage Eavesdropper guessing advantage decreases exponentially.... Secret Bit Sequence 1 Bit Sequence 2 Bit Sequence N

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Challenge 3: Jam effectively independent of eavesdropper’s location Sender Receiver At eavesdropper sender power is larger jamming power Eavesdropper can decode

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Solution 3: Two-way iJam Sender Receiver maskjam mask Receiver transmits a mask which the sender jams with iJam - Sender receives mask, eavesdropper doesn’t

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mask secret Solution 3: Two-way iJam Sender Receiver jam Receiver transmits a mask which the sender jams with iJam Sender transmits XOR of the secret with mask which sender jams mask secret mask secret mask - Sender receives mask, eavesdropper doesn’t - Both receiver and eavesdropper receive the XOR

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Sender Receiver Receiver transmits a mask which the sender jams Sender transmits the XOR of the secret with mask which sender jams mask = secret Receiver can decode secret Eavesdropper can not decode secret Receiver can decode secret Eavesdropper can not decode secret Solution 3: Two-way iJam mask secret mask secret

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Challenge 4: Stitching samples at the receiver First transmission Repetition

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Challenge 4: Stitching samples at the receiver First transmission Repetition Channel may change between the two transmissions

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Challenge 4: Stitching samples at the receiver First transmission Repetition Oscillator phase changes due to lack of synchronization

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Solution 4: Send back-to-back within the same transmission OFDM SymbolOFDM Symbol Repetition Channel for consecutive symbols is the same

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OFDM SymbolOFDM Symbol Repetition Solution 4: Send back-to-back within the same transmission Estimate and correct for oscillator phase

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OFDM SymbolOFDM Symbol Repetition Estimate and correct for oscillator phase Solution 4: Send back-to-back within the same transmission

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Empirical Results

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Implementation USRP/USRP2 Carrier Freq: GHz OFDM and QAM modulations

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Testbed 20-node testbed Each run randomly picks two nodes to be Sender and Receiver Every other node acts as eavesdropper Eavesdropper uses optimal hypothesis testing

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Bit Error Rate at the Eavesdropper Independent of location, Eavesdropper’s BER is close to a random guess

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Can an iJam receiver decode while jamming? Receiver can decode despite jamming

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Prior Work: 1 bit/s Secrecy Rate

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3 orders of magnitude more secret bits than prior schemes Prior Work: 1 bit/s Secrecy Rate

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Conclusion First practical physical layer security that doesn’t rely on channel changes Implemented and empirically evaluated – 3 orders of magnitude more secret bits – Works with both static and mobile channels

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