Breaking block and product ciphers applied across byte boundaries.

The security of block and product ciphers is considered using a set theoretic estimation (STE) approach to decryption. Known-ciphertext attacks are studied using permutation (P) and substitution (S) keys. The blocks are formed form three (3) alphabetic characters (metacharacters) where the applications of P and S upon the ASCII byte representation of each of the three characters are allowed to cross byte boundaries. The application of STE forgoes the need to employ chosen-plaintext or known-plaintext attacks.