The Problem with Regular Multiple Byte Block Boundaries in Encryption

Product ciphers composed of multi-byte permutations and substitutions are generally considered to be more secure than the substitution ciphers. However, this is not the case. In fact, product ciphers using the same boundaries can be isomorphically reduced to substitution ciphers. These product block ciphers are characterized by block definitions having the same defined block sizes. This also means that such product ciphers can be attacked using the same methodology as can be applied to all substitution (S) ciphers. We show that Permutation-Substitution-Permutation (PSP) ciphers are insignificantly more secure than a substitution cipher and propose a chosen plaintext attack for solving these product ciphers.