One technique, called symmetric encryption, uses the same key for both encryption and decryption and relies on safe communication lines for exchanging the key. However, most users do not have access to a secure communication line and, to be really secure, the keys need to be as long as the message (Leiss, 1982). However, most working systems are based on user keys shorter than the message. One scheme used for encryption is the Data Encryption Standard (DES), which is a standard encryption algorithm developed by IBM. This scheme uses one key for both encryption and decryption, which must be kept secret, although the algorithm need not be. The algorithm transforms each 64-bit block of