MIMO Transmissions with Information Theoretic Secrecy for Secret-Key Agreement in Wireless Networks Xiaohua (Edward) Li1 and E. Paul Ratazzi2 1Department of Electrical and Computer Engineering State University of New York at Binghamton xli@binghamton.edu, http://ucesp.ws.binghamton.edu/~xli 2Air Force Research Lab, AFRL/IFGB, paul.ratazzi@afrl.af.mil MILCOM'2005

Contents Introduction Secure MIMO transmission scheme Transmission weights design Transmission secrecy Simulations Conclusions MILCOM'2005

1. Introduction Secure wireless transmission: necessary PHY security techniques for wireless information assurance Wireless transmissions have no boundary, susceptible to listening/analyzing, location, jamming Wireless nodes have severe energy and bandwidth constraints  “light” techniques Unreliable link and dynamic network topology MILCOM'2005

Secure Wireless Transmissions Traditional secure transmission design Data encryption, spread spectrum, etc New idea: use antenna array diversity and array redundancy A completely different approach of secure (LPI) waveform design MILCOM'2005

Significance to Cryptography Provable (information-theoretic) secrecy Inherently secure transmission, no encryption keys involved Comparable to quantum cryptography Provide PHY-layer LPI, and assist higher layer data encryption PHY-layer assisted secret key agreement MILCOM'2005

Secret-Key Agreement Classic Shannon model Alice & Bob try to exchange encryption keys for encrypted data transmission Eve can acquire all (and identical) messages received by Alice or Bob Perfect secrecy impractical under Shannon model Computational secrecy achievable MILCOM'2005

PHY-layer Transmission Secrecy Model Information theoretic secrecy realizable with model different than Shannon’s Eve’s channels, and thus received signals, are different from Alice’s or Bob’s A reality in quantum communication, and wireless transmissions MILCOM'2005

Information-Theoretic Secrecy Wyner’s wire-tap channel: secret capacity Maurer’s common information concept High secret channel capacity requires Eve’s channel being noisier  not practical enough MILCOM'2005

2. Secure MIMO transmission scheme Can we guarantee a large or in practice? Possible: randomized MIMO transmission Basic idea: Use redundancy of antenna array Exploit the limit of blind deconvolution Eve can not estimate channel/symbol blindly MILCOM'2005

Transmission Scheme Alice: antenna array (secure, public, pilot) Does not send training signals Bob: estimate symbols, no channel knowledge required MILCOM'2005

Signal Model and Assumptions Alice, Bob & Eve do not know channels. Alice estimate H by reciprocity Bob need not know channel. Eve depends on blind estimation. MILCOM'2005

MIMO Transmission Procedure Alice select transmit antenna weights so that Bob receives signal By estimating received signal power, Bob can detect signals Key points: No channel information required for Bob, no training required  no training available to Eve Redundancy in selecting weights MILCOM'2005

3. Transmission Weights Design Existing array transmission schemes are susceptible to Eve’s blind deconvolution attack? Eve can easily estimate by blind deconvolution if with optimal transmit beamforming MILCOM'2005

Select Weights with Randomization W1(n): Redundancy in transmitting weights Procedure: MILCOM'2005

4. Transmission Secrecy Eve’s received signal becomes which has distribution Objective: Eve can not estimate channel Hu from xe(n), which relies on Assumption that Eve & Bob’s channels are sufficiently different  wireless channels fade independently when separated a fractional of wavelength Unknown to Eve: MILCOM'2005

Indeterminacy of Blind Channel Estimation Proposition: MILCOM'2005

Indeterminacy of Blind Symbol Estimation Proposition: Result: Eve’s error rate: high Bob’s error rate: low (identical to optimal MIMO eigen-beamforming) Cost paid: higher transmission power MILCOM'2005

Transmission secrecy Weights are selected randomly and unknown to Eve, blind deconvolution is made impossible Weights are selected by Alice, no need to tell Bob  equivalently one-time pad Information theory guarantees high and positive secret channel capacity  provable (information theoretic) secrecy MILCOM'2005

Eve’s Exhaustive Search Attack Eve may exhaustively try all possible channels (both ). The complexity can be at least , according to quantization level Q Low quantization level reduces complexity, but increases symbol estimation error  still makes high positive secret channel capacity possible Example, MILCOM'2005

5. Simulations BER of the proposed transmission scheme J=6. K=4. QPSK. MILCOM'2005

Secret channel capacity with the simulated BER MILCOM'2005

Conclusions Proposed a randomized MIMO transmission scheme Use array redundancy and channel diversity for transmission security Enhance transmission LPI in the PHY-layer by increasing the adversary’s receiving error Proof of secrecy with weight randomization and limit of blind deconvolution MILCOM'2005