Exact Random Coding Secrecy Exponents for the Wiretap Channel

TL;DR

This paper precisely characterizes the exponential decay rate of information leakage in wiretap channels using both i.i.d. and constant-composition random codes, revealing that their secrecy exponents can vary and are not universally ordered.

Contribution

It provides the exact secrecy exponents for both i.i.d. and constant-composition random coding ensembles, clarifying their relative performance in wiretap channels.

Findings

Exact secrecy exponent for i.i.d. codes matches the known achievable exponent.

Constant-composition codes can achieve larger secrecy exponents than i.i.d. codes.

No universal ordering exists between the two ensembles' secrecy exponents.

Abstract

We analyze the exact exponential decay rate of the expected amount of information leaked to the wiretapper in Wyner's wiretap channel setting using wiretap channel codes constructed from both i.i.d. and constant-composition random codes. Our analysis for those sampled from i.i.d. random coding ensemble shows that the previously-known achievable secrecy exponent using this ensemble is indeed the exact exponent for an average code in the ensemble. Furthermore, our analysis on wiretap channel codes constructed from the ensemble of constant-composition random codes leads to an exponent which, in addition to being the exact exponent for an average code, is larger than the achievable secrecy exponent that has been established so far in the literature for this ensemble (which in turn was known to be smaller than that achievable by wiretap channel codes sampled from i.i.d. random coding ensemble). We show examples where the exact secrecy exponent for the wiretap channel codes constructed from random constant-composition codes is larger than that of those constructed from i.i.d. random codes and examples where the exact secrecy exponent for the wiretap channel codes constructed from i.i.d. random codes is larger than that of those constructed from constant-composition random codes. We, hence, conclude that, unlike the error correction problem, there is no general ordering between the two random coding ensembles in terms of their secrecy exponent.