Privatics team

3rd year

Cryptography

Generalized Feistel Networks

Thierry P. Berger

Marine Minier

XLIM Research Institute (Limoges)

With substitution-permutation networks, Feistel networks are one of the main family of iterated block ciphers. Popularized by DES, the Feistel cipher has been generalized many times since by increasing the number of blocks the plaintext is subdivided. One can then wonder how many times this construction must be iterated for the blocks to be mixed up enough. More precisely, we look at the notion of diffusion delay which is the minimum number of iterations to do in order for all output blocks to depend on all input blocks. We exhibit a matrix representation that encompasses all existing generalized Feistel networks in relation with full diffusion delay. From here, it was possible to create a new class of such schemes well-suited for cryptographic applications.