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 2^96.74 encryptions, 2^53.6 chosen plaintexts and 2^56.6 64 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.

[2021-09-05] Printed Article /doc/2020.11.0554/v2