21轮CRAFT算法不可能差分分析

Impossible differential cryptanalysis of 21-round CRAFT

CRAFT是FSE 2019年提出的一种轻量级可调分组密码,适用于硬件实现面积小且资源受限设备保护信息的安全。该算法使用128 bit密钥和64 bit调柄值加密64 bit明文,对其进行安全性评估,可以为日后使用提供理论依据。通过研究CRAFT的结构特点和密钥编排方案的冗余性,利用预计算表、等效密钥和轮密钥线性关系等技术,选取一条充分利用密钥冗余性的13轮不可能差分链,在其前后分别接3轮和5轮,提出了对21轮CRAFT的不可能差分分析。攻击的时间、数据和存储复杂度为296.74次加密,253.6个选择明文和256.664-比特块。此攻击是对缩减轮CRAFT算法在单密钥和单调柄值情形下时间复杂度最低的分析。该方法依赖于调柄值调度算法的线性相关,有助于更进一步理解CRAFT的设计。

CRAFT is a lightweight tweakable block cipher proposed in FSE 2019,which is suitable for the resource-constrained equipment with the small hardware area to protect information security.It adopts the128 bit key and 64 bit tweak to encrypt a 64 bit plaintext.The evaluation of its safety can provide a theoretical basis for future use.The paper researched the structural properties of CRAFT and the redundancy of its key schedule,and chose a 13-round impossible differential of CRAFT,which made full use of the linear relations of round subkeys.Appended three rounds and five rounds at its top and bottom,this paper proposed a 21-round impossible differential,which applied some techniques such as the precomputation tables,the differential enumeration and the key-dependent property.The time,data and memory complexities of the attack were 296.74 encryptions,253.6 chosen plaintexts and 256.664 bit blocks,respectively.This impossible differential attack on reduced-round CRAFT was the lowest time complexity attack under the single-key and single-tweak model.This method relied on the linear correlation of the handle value tweakey schedule,which was helpful to further understand the design of CRAFT.