Quantum rectangle attack and its application on Deoxys-BC

In recent years, it has become a popular trend to propose quantum versions of classical attacks. The rectangle attack as a differential attack is widely used in symmetric cryptanalysis and applied on many block ciphers. To improve its efficiency, we propose a new quantum rectangle attack firstly. In rectangle attack, it counts the number of valid quartets for each guessed subkeys and filters out subkey candidates according to the counter. To speed up this procedure, we propose a quantum key counting algorithm based on parallel amplitude estimation algorithm and amplitude amplification algorithm. Then, we complete with the remaining key bits and search the right full key by nested Grover search. Besides, we give a strategy to find a more suitable distinguisher to make the complexity lower. Finally, to evaluate post-quantum security of the tweakable block cipher Deoxys-BC, we perform automatic search for good distinguishers of Deoxys-BC according to the strategy, and then apply our attack on 9/10-round Deoxys-BC-256 and 12/13/14-round Deoxys-BC-384. The results show that our attack has some improvements than classical attacks and Grover search.