Principles and Performance of Cryptographic Algorithms
By Bart Preneel, Vincent Rijmen, and Antoon Bosselaers
Dr. Dobb's Journal December 1998
Figure 2: Two rounds of the DES, the most famous block cipher. It has the Feistel structure. E denotes the linear expansion of the 32 input bits to 48 input bits, 
denotes the bitwise XOR with the round key, S is the nonlinear substitution, and P is the bit permutation.
Copyright © 1998, Dr. Dobb's Journal
