Fmj Frans Willems

Biometric Systems: Privacy and Secrecy Aspects

This paper addresses privacy leakage in biometric secrecy systems. Four settings are investigated. The first one is the standard Ahlswede-Csiszar secret-generation setting in which two terminals observe two correlated sequences. They form a common secret by interchanging a public message. This message should only contain a negligible amount of information about the secret, but here, in addition, we require it to leak as little information as possible about the biometric data. For this first case, the fundamental tradeoff between secret-key and privacy-leakage rates is determined. Also for the second setting, in which the secret is not generated but independently chosen, the fundamental secret-key versus privacy-leakage rate balance is found. Settings three and four focus on zero-leakage systems. Here the public message should only contain a negligible amount of information on both the secret and the biometric sequence. To achieve this, a private key is needed, which can only be observed by the terminals. For both the generated-secret and the chosen-secret model, the regions of achievable secret-key versus private-key rate pairs are determined. For all four settings, the fundamental balance is determined for both unconditional and conditional privacy leakage.

Information Leakage in Fuzzy Commitment Schemes

In 1999, Juels and Wattenberg introduced the fuzzy commitment scheme. This scheme is a particular realization of a binary biometric secrecy system with chosen secret keys. It became a popular technique for designing biometric secrecy systems, since it is convenient and easy to implement using standard error-correcting codes. This paper investigates privacy- and secrecy-leakage in fuzzy commitment schemes. The analysis is carried out for four cases of biometric data statistics, i.e., memoryless totally symmetric, memoryless input-symmetric, memoryless, and stationary ergodic. First, the achievable regions are determined for the cases when data statistics are memoryless totally symmetric and memoryless input-symmetric. For the general memoryless and stationary ergodic cases, only outer bounds for the achievable rate-leakage regions are provided. These bounds, however, are sharpened for systematic parity-check codes. Given the achievable regions (bounds), the optimality of fuzzy commitment is assessed. The analysis shows that fuzzy commitment is only optimal for the memoryless totally symmetric case if the scheme operates at the maximum secret-key rate. Moreover, it is demonstrated that for the general memoryless and stationary ergodic cases, the scheme leaks information on both the secret and biometric data.

Coding for a binary independent piecewise-identically-distributed source

Two weighting procedures are presented for compaction of output sequences generated by binary independent sources whose unknown parameter may occasionally change. The resulting codes need no knowledge of the sequence length T, i.e., they are strongly sequential, and also the number of parameter changes is unrestricted. The additional-transition redundancy of the first method was shown to achieve the Merhav lower bound, i.e., log T bits per transition. For the second method we could prove that additional-transition redundancy is not more than 3/2 log T bits per transition, which is more than the Merhav bound; however, the storage and computational complexity of this method are also more interesting than those of the first method. Simulations show that the difference in redundancy performance between the two methods is negligible.

Bit-Interleaved Coded Modulation in the Wideband Regime

The wideband regime of bit-interleaved coded modulation (BICM) in Gaussian channels is studied. The Taylor expansion of the coded modulation capacity for generic signal constellations at low signal-to-noise ratio (SNR) is derived and used to determine the corresponding expansion for the BICM capacity. Simple formulas for the minimum energy per bit and the wideband slope are given. BICM is found to be suboptimal in the sense that its minimum energy per bit can be larger than the corresponding value for coded modulation schemes. The minimum energy per bit using standard Gray mapping on M-PAM or M 2 -QAM is given by a simple formula and shown to approach - 0.34 dB as M increases. Using the low SNR expansion, a general tradeoff between power and bandwidth in the wideband regime is used to show how a power loss can be traded off against a bandwidth gain.

Pilot-Aided Angle-Domain Channel Estimation Techniques for MIMO-OFDM Systems

Early multiple-input multiple-output with orthogonal frequency division multiplexing (MIMO-OFDM) channel estimation techniques treat channels as spatially uncorrelated. However, in many situations, MIMO-OFDM channels tend to be spatially correlated, for example, due to limited scattering. For such channels, estimation performance can be improved through exploitation of prior knowledge of the channel spatial correlation, for example, by means of the linear multiple mean square error (MMSE) technique. This knowledge is, however, not always available. As an alternative, we investigate techniques in the angle domain, where the MIMO-OFDM channel model lends itself to a physical interpretation. Our theoretical analysis and simulation results indicate that the proposed angle-domain approximated MMSE (AMMSE) channel estimation technique performs well in terms of the mean square error (mse) for various channel models representing different indoor environments. When a suitable threshold is chosen, we can use the angle-domain most-significant-taps selection technique instead of the angle-domain AMMSE technique to simplify the channel estimation procedure with little performance loss.

Bit-interleaved coded modulation revisited: A mismatched decoding perspective

We revisit the information-theoretic analysis of bit-interleaved coded modulation (BICM) by modeling the BICM decoder as a mismatched decoder. The mismatched decoding model is well defined for finite, yet arbitrary, block lengths, and naturally captures the channel memory among the bits belonging to the same symbol. We give two independent proofs of the achievability of the BICM capacity calculated by Caire et al., where BICM was modeled as a set of independent parallel binary-input channels whose output is the bitwise log-likelihood ratio. Our first achievability proof uses typical sequences, and shows that due to the random coding construction, the interleaver is not required. The second proof is based on the random coding error exponents with mismatched decoding, where the largest achievable rate is the generalized mutual information. Moreover, the generalized mutual information of the mismatched decoder coincides with the infinite-interleaver BICM capacity. We show that the error exponent-and hence the cutoff rate-of the BICM mismatched decoder is upper-bounded by that of coded modulation and may thus be lower than in the infinite-interleaved model; for binary reflected Gray mapping in Gaussian channels the loss in error exponent is small. We also consider the mutual information appearing in the analysis of iterative decoding of BICM with extrinsic information transfer (EXIT) charts: if the symbol metric has knowledge of the transmitted symbol, EXIT mutual information admits a representation as a pseudo-generalized mutual information, which is in general not achievable. A different symbol decoding metric, for which the extrinsic side information refers to the hypothesized symbol, induces a generalized mutual information lower than the coded modulation capacity. In this case, perfect extrinsic side information turns the mismatched-decoder error exponent into that of coded modulation.

Temporal Stability of a Transparent Mode Group Diversity Multiplexing Link

The temporal behavior of a mode group diversity multiplexing (MGDM) link is examined. MGDM has been proposed as a way of creating parallel, independent communication channels over multimode fiber (MMF) links using different groups of modes. When the effect of dispersion can be neglected, the output electrical signals of an MGDM link are related to the input ones via a real-valued transmission matrix. The transmission matrix is vulnerable to changes in the modal spectrum of the propagating mode groups as well as to the coupling of light at the input and output of the MMF. Measurements of the transmission matrix of a two-input two-output system over 12.7 h show that an MGDM link can be very stable

Privacy leakage in biometric secrecy systems

Motivated by Maurer [1993], Ahlswede and Csiszar [1993] introduced the concept of secret sharing. In their source model two terminals observe two correlated sequences. It is the objective of both terminals to form a common secret by interchanging a public message (helper data), that should contain only a negligible amount of information about the secret. Ahlswede and Csiszar showed that the maximum secret key rate that can be achieved in this way is equal to the mutual information between the two source outputs. In a biometric setting, where the sequences correspond to the enrollment and authentication data, it is crucial that the public message leaks as little information as possible about the biometric data, since compromised biometric data cannot be replaced. We investigate the fundamental trade-offs for four biometric settings. The first one is the standard (Ahlswede-Csiszar) secret generation setting, for which we determine the secret key rate - privacy leakage region. Here leakage corresponds to the mutual information between helper data and biometric enrollment sequence conditional on the secret. In the second setting the secret is not generated by the terminals but independently chosen, and transmitted using a public message. Again we determine the region of achievable rate - leakage pairs. In setting three and four we consider zero-leakage, i.e. the public message contains only a negligible amount of information about the secret and the biometric enrollment sequence. To achieve this a private key is needed which can be observed only by the terminals. We consider again both secret generation and secret transmission and determine for both cases the region of achievable secret key rate - private key rate pairs.

Low-Complexity LMMSE-Based MIMO-OFDM Channel Estimation Via Angle-Domain Processing

In many situations, multiple-input-multiple-output with orthogonal frequency division multiplexing (MIMO-OFDM) channels tend to be spatially correlated due, for example, to limited scattering. Prior knowledge of this channel spatial correlation and the channel frequency correlation can be exploited by using the linear minimum-mean-square-error (LMMSE) technique. However, the complexity of the 2-D LMMSE technique, which fully utilizes both the channel spatial and frequency correlation is quite high. To solve this problem, this paper presents and analyzes several low-complexity, suboptimal, approximated LMMSE channel estimation techniques in the angle domain, where the channel model lends itself to a physical interpretation. The choice of angle-domain techniques is largely dependent on the extent of channel stochastic information (e.g., channel correlation or power) that is available to the receiver. Nevertheless, all the proposed angle-domain techniques have much lower complexity compared to the 2-D LMMSE technique. Further, all the angle-domain techniques improve over the conventional least square (LS) technique for all the typical MIMO-OFDM models under consideration. More importantly, our simulation results show that the angle-domain quasi 1-D (Ql-D) LMMSE technique can achieve similar performance compared to the 2-D LMMSE technique for all typical MIMO-OFDM models with significantly lower complexity.

Achieving Secure Fuzzy Commitment Scheme for Optical PUFs

Home security surveillance systems, using stable and fully equipped monitors, may reduce losses caused by burglary and increase home safety. In this paper, we present the implementation of a quick and cost-effective wireless connected home security surveillance system using Lego Mindstorms NXT robot tool kit and JAVA language. The system implemented uses Bluetooth protocol to record data monitored.