Yoyo Cryptanalysis Against Reduced‐Round L‐Feistel Structure for Recovering the Secret Components

L‐Feistel structure is a new iterative block cipher structure and unifies the Feistel structure and the Lai–Massey structure while maintaining the similarity of encryption and decryption. In this study, we present the first yoyo cryptanalysis against the L‐Feistel structure to evaluate the security under structural attack and give the method to recover the secret round function. We construct the fundamental yoyo distinguisher for the three‐round L‐Feistel structure, which can be used to distinguish the L‐Feistel structure from random permutation and establish the linear equations of the secret round functions. Besides, the fundamental yoyo distinguisher can be extended to more rounds when the invertible linear transformations are given. Then the equivalent structures of the L‐Feistel structure are provided, which helps reduce the guess of the starting point of the secret round functions. Finally, the process of recovering the secret round functions for the three‐round L‐Feistel structure is presented. We believe this study will enrich the application of yoyo cryptanalysis and L‐Feistel structure.