Feistel

Feistel refers to a symmetric encryption structure developed by Horst Feistel at IBM in the 1970s. This structure is fundamental to the design of many block ciphers, most notably the Data Encryption Standard (DES). The core idea of a Feistel cipher is to divide a block of plaintext into two halves, typically left and right. The encryption process then iteratively applies a round function to one half, XORs the result with the other half, and then swaps the two halves. This process is repeated for a fixed number of rounds.

The key advantage of the Feistel structure is that the same algorithm can be used for both encryption and decryption. To decrypt, the ciphertext is processed in the same manner as encryption, but the subkeys used in each round are applied in reverse order. This symmetry significantly simplifies the implementation of block ciphers. The strength of a Feistel cipher depends heavily on the properties of the round function and the key scheduling algorithm, which generates the subkeys used in each round. If these components are well-designed, a Feistel cipher can be highly resistant to cryptanalysis. While DES, a prominent Feistel cipher, has been superseded due to its small key size, the Feistel structure itself remains a widely used and influential design principle in modern cryptography.