New Approach towards Generalizing Feistel Networks and Its Provable Security

This paper proposes a new approach to generalizing Feistel networks, which unifies the classical (balanced) Feistel network and the Lai–Massey structure. We call the new structure extended Feistel (E-Feistel) network. To justify its soundness, we investigate its indistinguishability using Patarin’s H-coefficient technique. As a result, it is proved that the 4-round key-alternating E-Feistel (KAEF) cipher with adequately derived keys and identical round functions is secure up to 2n/2 queries, i.e., birthday-bound security. In addition, when adjacent round keys are independent and independent round functions are used, the 6-round KAEF is secure up to beyond-birthday-bound 22n/3 queries. Our results indicate that the E-Feistel structure is secure and reliable and can be adopted in designing practical block ciphers.