DES-X

DES-X, also written DESX, is a variant of the Data Encryption Standard block cipher designed to make brute-force key search harder. The original DES, specified in 1976, has a 56-bit key, and critics worried that large governments could exhaust that key space. DES-X, proposed by Ron Rivest in May 1984, lengthens the key without changing DES itself by a technique called key whitening: an extra 64-bit key is combined with the plaintext by exclusive-or before DES is applied, and a second extra 64-bit key is combined with the output afterwards. The total key size becomes 184 bits, but the effective security is only about 119 bits minus the logarithm of the number of plaintext and ciphertext pairs an attacker can obtain. DES-X also raises the cost of differential and linear cryptanalysis, though far less than it raises the cost of brute force. The best known attack is a slide attack by Biryukov and Wagner needing about 2 to the power 32.5 known plaintexts. RSA Security has included DES-X in its BSAFE library since the late 1980s.