Strengthening NLS Against Crossword Puzzle Attack

Abstract

NLS is a stream cipher proposal submitted to eSTREAM project. In SAC 2006 Cho and Pieprzyk presented a linear distinguishing attack called Crossword Puzzle attack on NLS where they have shown that the bias of the distinguisher is around O(2^− 30). In this work we have proposed a new function modular Slash which is nonlinear in nature and strongly resistant against Linear Cryptanalysis. Replacing the modular addition in the nonlinear filter (NLF) of NLS we have shown that the Crossword puzzle attack presented by Cho and Pieprzyk can be prevented. In the modified NLS the bias of the linear distinguisher reduces to around O(2^− 60). Also we have shown that the implementation cost of modular Slash, in terms of hardware and time delay, is less than modular addition. The proposed function could be an interesting alternative to modular addition, due to its better cryptographic properties and lesser implementation cost.