Dynamic Substitution Model
In this paper, we present the Dynamic Substitution Model (DSM) and its variant the Static Substitution Model (SSM). In DSM and SSM, the secret encryption key is divided into a primary key and a secondary key. DSM is a model that allows any block cipher to accept a variable length secondary key, this is achieved by substituting some bits of the cipher's expanded key with the secondary key. SSM is a variant of DSM, where the secondary key length and the positions of the replaced bits of the subkeys are determined in the design time. We used the Advanced Encryption Standard (AES) to demonstrate the usage of DSM and SSM models.