This Demonstration shows an example of the DES (Data Encryption Standard) algorithm, which is the most widely used encryption algorithm in the world. DES encrypts groups of 64 message bits and requires a key of 64 message bits. If a message or key is less than 64 message bits, it is padded with zeros to the right. All of the example messages are padded to the right with zeros to make a message with groups of 64 message bits.
DES operates on 64-bit blocks with keys that permute 56 bits. That is, it uses permutation matrices that switch one 64-bit block for another, holding 8 bits fixed. The permutation matrices used for the subkeys are those presented in  and appear to be the standard matrices; however, other permutation matrices could be applied. There is a maximum of 16 permutation steps done with these matrices. By choosing the number of steps, you can limit this number to fewer steps. This Demonstration shows the codes at each even step, up to and including the number of steps chosen.