The design of Adjustable Generalized Feistel structure

Abstract

The Feistel-like structure is widely adopted in designing block ciphers due to advantages in implementation. This paper focuses on a class of Feistel-like structure where the sizes of branch and round function may differ. We introduce a new primitive termed the Adjustable Generalized Feistel Structure. We establish a necessary and sufficient condition for its invertibility. To rule out trivial attacks, we further propose necessary conditions for the security of an Adjustable Generalized Feistel structure. Additionally, we investigate the classification of the Adjustable Generalized Feistel structure from the perspective of affine equivalence and present two normalized forms. Compared with prior work on the unified structure at CRYPTO 2024—which primarily addressed the regular case where the sizes of branch and round function are equal—our model allows the size of the round function to be greater than or equal to the size of the branch. For practical applications, the Adjustable Generalized Feistel structure is particularly well-suited for designing wide-block block ciphers, especially when the round functions are themselves block-cipher-based.

Keywords

Feistel-like structure / adjustable design / affine equivalence / wide-block block cipher

RIGHTS & PERMISSIONS

Higher Education Press 2026

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