Feistellike

Feistellike is a term used in cryptography to describe a class of round-based block cipher constructions that resemble the Feistel network designed by Horst Feistel in the 1970s. In a Feistellike design, a data block is divided into two halves. A round function is applied to one half and its output is XORed into the other half, followed by a swap of the halves. This pattern is repeated for multiple rounds. Variations exist in how the round function is fed with key material, whether the halves are swapped every round, and how final output is produced.

Security and design notes: The security of a Feistellike cipher depends on the nonlinearity of the round function, the diffusion across rounds, and the quality of the key schedule. Some Feistellike ciphers include a final permutation or whitening step. The term emphasizes structural similarity rather than a fixed specification, so different implementations may have different performance and security profiles.

Origins and examples: The term acknowledges the influence of the original DES approach, and some historical ciphers such as DES and Blowfish are commonly described as Feistel-like due to their core structure. Over time many modern ciphers have adopted different architectural paradigms, such as substitution-permutation networks, but the Feistellike concept remains a reference point in discussions of round functions and block cipher design.

See also: Feistel network, Blowfish, DES, substitution-permutation network.