Differential Cryptanalysis of the BSPN Block Cipher Structure

BSPN (byte-oriented SPN ) is a general block cipher struc­ ture presented at SAC’96 by Youssef et al. It was designed as a more ef­ ficient version of the bit-oriented SPN structure published earlier in 1996 by Heys and Tavares in the Journal of Cryptology. BSPN is a flexible SPN structure in which only the linear transformation layer is exactly specified, while s-boxes, key-scheduling details, and number of rounds are intentionally left unspecified. Because BSPN can be implemented very efficiently in hardware, several researchers have recommended the 64-bit version as a lightweight cipher for use in wireless sensor networks (WSNs). Youssef et al. perform preliminary analysis on BSPN (using typical block sizes and numbers of rounds) and claim it is resistant to differential and linear cryptanalysis. However, in this paper we show that even if BSPN (similarly parameterized) is instantiated with strong AESlike s-boxes, there exist high probability differentials that allow BSPN to be broken using differential cryptanalysis. In particular, up to 9 rounds of BSPN with a 64-bit block size can be attacked, and up to 18 rounds with a 128-bit block size can be attacked.