  This paper proposes a generic approach for providing enhanced security to
communication systems which encode their data for reliability before encrypting
it through a stream cipher for security. We call this counter-intuitive
technique the {\em encoding-encryption} paradigm, and use as motivating example
the standard for mobile telephony GSM. The enhanced security is based on a
dedicated homophonic or wire-tap channel coding that introduces pure
randomness, combined with the randomness of the noise occurring over the
communication channel. Security evaluation regarding recovery of the secret key
employed in the keystream generator is done through an information theoretical
approach. We show that with the aid of a dedicated wire-tap encoder, the amount
of uncertainty that the adversary must face about the secret key given all the
information he could gather during different passive or active attacks he can
mount, is a decreasing function of the sample available for cryptanalysis. This
means that the wire-tap encoder can indeed provide an information theoretical
security level over a period of time, but after a large enough sample is
collected the function tends to zero, entering a regime in which a
computational security analysis is needed for estimation of the resistance
against the secret key recovery.
