S-boxes substitute or transform input bits into output bits . In particular , the length of the output should be the same as the length of the input ( the picture on the right has S-boxes with 4 input and 4 output bits ) , which is different from S-boxes in general that could also change the length , as in DES ( Data Encryption Standard ) , for example . An S-box is usually not at all just a permutation of the bits .