Читать книгу Security Engineering - Ross Anderson - Страница 163

The design of the S-boxes also affects the number of rounds required for security, and studying bad choices gives us our entry into the deeper theory of block ciphers. Suppose that the S-box were the permutation that maps the inputs (0,1,2,…,15) to the outputs (5,7,0,2,4,3,1,6,8,10,15,12,9,11,14,13). Then the most significant bit of the input would come through unchanged as the most significant bit of the output. If the same S-box were used in both rounds in the above cipher, then the most significant bit of the input would pass through to become the most significant bit of the output. We certainly couldn't claim that our cipher was pseudorandom.