Crossword Puzzle Attack on NLS

NLS is one of the stream ciphers submitted to the eSTREAM project. We present a distinguishing attack on NLS by Crossword Puzzle (CP) attack method which is newly introduced in this paper. We build the distinguisher by using linear approximations of both the non-linear feedback shift register (NFSR) and the nonlinear filter function (NLF). Since the bias of the distinguisher depends on the Konst value, which is a key-dependent word, we present the graph showing how the bias of distinguisher vary with Konst. In result, we estimate the average bias to be around O(2). Therefore, we claim that NLS is distinguishable from truly random cipher after observing O(2) keystream words on the average. The experiments also show that our distinguishing attack is successful on 90.3% of Konst among 2 possible values.