Tandem Coding and Cryptography on Wiretap Channels: EXIT Chart Analysis

TL;DR

This paper explores how combining channel and cryptographic coding on wiretap channels, analyzed through EXIT charts, can improve information-theoretic security by reducing the eavesdropper's mutual information and predicting attack success.

Contribution

It introduces an EXIT chart-based analysis of cryptographic and channel coding interplay on wiretap channels, enhancing understanding of security and attack prediction.

Findings

EXIT charts effectively predict eavesdropper decoding success.

Mutual information correlates with attack effectiveness.

Channel coding can reduce information leakage.

Abstract

Traditional cryptography assumes an eavesdropper receives an error-free copy of the transmitted ciphertext. Wyner's wiretap channel model recognizes that at the physical layer both the intended receiver and the passive eavesdropper inevitably receive an error-prone version of the transmitted message which must be corrected prior to decryption. This paper considers the implications of using both channel and cryptographic codes under the wiretap channel model in a way that enhances the \emph{information-theoretic} security for the friendly parties by keeping the information transfer to the eavesdropper small. We consider a secret-key cryptographic system with a linear feedback shift register (LFSR)-based keystream generator and observe the mutual information between an LFSR-generated sequence and the received noise-corrupted ciphertext sequence under a known-plaintext scenario. The effectiveness of a noniterative fast correlation attack, which reduces the search time in a brute-force attack, is shown to be correlated with this mutual information. For an iterative fast correlation attack on this cryptographic system, it is shown that an EXIT chart and mutual information are very good predictors of decoding success and failure by a passive eavesdropper.