EXIT charts applied to tandem coding and cryptography in a wiretap scenario

Security in a communications system is traditionally determined by the strength of a cryptographic layer in the protocol stack. Wyner used a wiretap channel model to show that information-theoretic security is possible by exploiting characteristics of the physical layer. This paper discusses the enhancement of security in a communications system by considering jointly the physical layer, along with the cryptographic layer within a wiretap scenario assuming strictly positive secrecy capacity. In this paper, the coupling effect of the security attributed to the two layers is shown in the context of a stream cipher with an LFSR keystream generator as the cryptographic layer. A well-known iterative attack is analyzed by means of extrinsic information transfer (EXIT) charts which are formed using two different methods. The first uses hard-decision information, while the second estimates densities in a message-passing algorithm. The EXIT analysis provides concrete evidence of the enhanced security achievable through properly exploiting characteristics of the physical layer by means of channel coding.