Equivocations for the simple substitution cipher with erasure-prone ciphertext

Abstract

In this paper, we analyze an attack scenario for the simple substitution cipher using the wiretap channel model, where the attacker only has access to error-prone ciphertext at the output of a packet erasure channel (PEC). Each packet is comprised of exactly one symbol of ciphertext, and hence, the attacker's channel could be viewed as a symbol erasure channel. Information-theoretic cryptanalysis provides key and message equivocations for the cipher in general, and then gives the results as functions of the error-free ciphertext equivocations. The findings characterize the increase in equivocation that might be expected if encrypted data were further encoded using wiretap codes that introduce symbol erasures to passive eavesdroppers.