DESX

DES-X is a variant of the Data Encryption Standard (DES) designed to strengthen DES against brute-force attacks by adding extra key material. The scheme encrypts by first XORing the plaintext with a 64-bit key K1, then encrypting the result with DES under a 56-bit key K2, and finally XORing the DES output with another 64-bit key K3 to produce the ciphertext. Decryption reverses this sequence: XOR with K3, decrypt with DES under K2, then XOR with K1.

Key structure and operation details: K1 and K3 are 64-bit keys, and K2 is the 56-bit DES key (parity bits kept for compatibility). The total amount of key material is 184 bits, but the practical security advantage comes from making brute-forcing more costly. The effective security level is not the sum of the key lengths; it is typically estimated around 112 to 120 bits in practice, due to Meet-in-the-M middle and related attacks that exploit the triple-layer structure.

Security context: DES-X provides a practical strengthening of DES without changing the block size or altering the DES algorithm itself. It is generally cited as offering about 2^112 to 2^120 work-factor security against generic attacks, which is higher than DES alone but still far below modern standards. DES-X is largely of historical interest, as it predates and is superseded by Triple DES and, more recently, AES.

Implementation notes: Like DES, DES-X operates on 64-bit blocks and uses the same Feistel structure in the middle DES step, with additional XORs before and after. The two extra XOR layers add only modest computational overhead.

See also: Data Encryption Standard, Triple DES, Advanced Encryption Standard.