CAST-128

CAST-128 is an encryption algorithm described in RFC 2144, an informational memo by C. Adams of Entrust Technologies published in May 1997. The memo presents it as an existing algorithm that can meet the Internet community need for an unencumbered cipher with a range of key sizes for many cryptographic applications and protocols, and describes it as a DES-like substitution-permutation network that appears to resist differential, linear and related-key cryptanalysis. It belongs to the class of Feistel ciphers: a 64-bit plaintext block is split into 32-bit halves and passed through 16 rounds that use three alternating types of round function, with a pair of subkeys per round. The main description takes a 128-bit key and a key schedule that relies on eight substitution boxes, four for the round function and four for the key schedule. Decryption is the same procedure with the round subkeys used in reverse order.