DES has several known weaknesses including complements and semi-weak keys. Notice
that two messages m and m are called complements if m ⊕ m = 11 . . . 1. The weakness
related to complements shows that if a message m is encrypted with a particular key k, the
complement of the resulting ciphertext will be the encryption of the complementmusing the
complement key k. Semi-key weaknesses mean that there are specific pairs of keys having
identical decryption, implying that two keys decrypt the messages encrypted by the other
key. Other common symmetric algorithms likely to be used (and therefore replace DES) in
the computer industry are triple DES, and the advanced encryption standard (AES). Triple
DES is a variant of DES that decreases the risk from brute force attack by using longer keys.
This gives an effective key length of roughly 168-bit. AES serves as an eventual successor to
DES, as selected by NIST. The latter has conducted a selection process involving 13 secret
key algorithms. AES is based on a powerful substitution-linear transformation network with
10, 12, or 14 rounds, depending on the key size and with variable block sizes of up to 256
bits (NIST, 2001).