Plaintext Structures Lead to More Rounds: Improved Yoyo Attack on Type-I Generalized Feistel Networks with Secret Round Functions

Yoyo attack is a cryptanalysis method for block ciphers or cipher structures with secret round functions. This method constructs “yoyo” cycles to collect connected plaintexts or ciphertexts for some equations, and recover the secret round functions by solve these equations. In this paper, we present an improved yoyo cryptanalysis framework for type-I Generalized Feistel Networks (GFNs) by utilizing three plaintext structures: Differential-Optimized Structure (Plaintext-Structure-I), Deterministic Pairing Structure (Plaintext-Structure-II), and Iterative Filtering Structure (Plaintext-Structure-III). Based on these structures, we demonstrate the round function recovery attacks on 11-round and 12-round type-I GFNs. The proposed attack achieves a data complexity of O(2^n) chosen plaintexts and a time complexity of O(2^2.73n), where n is the branch size. Compared with existing attacks on this structure, our approach advances the attack frontier by two rounds. The structural optimization techniques developed in this work may provide new tools for analyzing other ciphers or structures.