Improved Fast Correlation Attacks on the Sosemanuk Stream Cipher

Abstract

In this paper, we present a new algorithm for fast correlation attacks on stream ciphers with improved cryptanalysis results on the Sosemanuk stream cipher, one of the 7 finalists in the eSTREAM project in 2008. The new algorithm exploits the direct sum construction of covering codes in decoding phase which approximates the random vectors to a nearest codeword in a linear code. The new strategy provides large flexibility for the adversary and could reduce the time/memory/data complexities significantly. As a case study, we carefully revisit Sosemanuk and demonstrate a state recovery attack with a time complexity of 2^134.8, which is 2²⁰ times faster than achievable before by the same kind of attack and is the fastest one among all known attacks so far. Our result indicates an inefficiency in longer keys than 135 bits and depicts that the security margin of Sosemanuk is around 2⁸ for the 128-bit security for the first time.