Quantum attacks on generalized Feistel networks based on the strong–weak separability

Generalized Feistel networks are important components of symmetric ciphers, and detailed security evaluations in the quantum setting remain to be explored. In this paper, based on the strong–weak separability of certain branch output function, we present polynomial-time quantum distinguishers for 4F-function and 2F-function structures in quantum chosen-plaintext attack setting for the first time, and then quantum key-recovery attacks are achieved through Grover-meet-Simon algorithm, respectively. Under the condition of the semi-strong separability, firstly, we give a quantum distinguisher on 8-round 4F-function structure, from which we carry out a 12-round quantum key-recovery attack to guess 10 n -bit subkey, whose time complexities gain a factor of 2^5n . When attacking r>12 rounds, we can recover 4(r - 12)n + 10n -bit subkey in time 2^4(r - 12)n + 10n/2 . Secondly, we show a quantum distinguisher on 5-round 2F-function structure, and a 7-round quantum key-recovery attack is performed on it, which can recover 3 n -bit subkey in time 2^1.5n . When r>7 , 2(r - 7)n + 3n -bit subkey can be recovered with time complexities by a factor of 2^2(r - 7)n + 3n/2 . Furthermore, based on the weak separability, a 6-round quantum distinguisher for 2F-function structure is constructed, and an 8-round quantum key-recovery attack is achieved, and the time complexity is 2^2(r - 8)n + 3n/2 when r>8 . The results show that the time complexity of each attack scheme we proposed is much better than that of Grover’s brute force search.