Entropy model for symmetric key cryptography algorithms based on numerical methods

Cryptography is used to protect information content communicated over a network from being accessed by adversaries. This is achieved by transforming (encrypting) plain text before transmission in such a way so that its contents can only be disclosed upon application of a reverse transformation (decryption). Transformations often involve some key used in the process. This paper focuses on estimating the security of a symmetric key cryptography algorithms based on solving nonlinear equations using numerical methods. Security is estimated using Shannon´s information theory, in particular, the concept of key equivocation taken as a security measure. The aim of this paper is to investigate the security of the substitution cipher and to recommend ways to improve it.