Analysis of SNOW 3G⌖ resynchronization mechanism

The stream cipher SNOW 3G designed in 2006 by ETSI/SAGE is a base algorithm for the second set of 3GPP confidentiality and integrity algorithms. This paper is the first attempt of cryptanalysis of this algorithm in the public literature. We look at SNOW 3G in which two modular additions are replaced by xors, which is called SNOW 3G^⌖. We show that the feedback from the FSM to the LFSR is very important, since we can break a version without such a feedback using a pair of known IVs with practical complexities (2⁵⁷ time and 2³³ keystream). We then extend this technique into a differential chosen IV attack on SNOW 3G^⌖ and show how to break 16 out of 33 rounds with the feedback.