Structural Cryptanalysis of SASAS

In this paper we consider the security ofblo ck ciphers which contain alternate layers of invertible S-boxes and affine mappings (there are many popular cryptosystems which use this structure, including the winner of the AES competition, Rijndael). We show that a five layer scheme with 128 bit plaintexts and 8 bit S-boxes is surprisingly weak even when all the S-boxes and affine mappings are key dependent (and thus completely unknown to the attacker). We tested the attack with an actual implementation, which required just 216 chosen plaintexts and a few seconds on a single PC to find the 217 bits of information in all the unknown elements of the scheme.