Ciphertext Only Attack on LFSR Based Encryption Scheme

Abstract

Here a security attack on LFSR based stream cipher systems is described. The attack depends on some weakness of the memory less Boolean combining function used in the system and breaks the key using only the ciphertext. Siegenthaler used a correlation measure to define certain statistical test for finding out feasible keys. This can also be interpreted as testing the independence of two binary bit sequences. Here we extend the statistical test eonsidering the joint conditional probability of more than one binary random variables. We diacuaa algorithms with an example of non correlation immune function in the esse of three LFSRs and a multiplexer as a nonlinear combining function. Next we extend the method for correlation immune functions and illustrate that with suitable example. The motivation here is to present an applicetion of statisticsl method in cryptanalysis. Theoreticsl questions have been posed to motivate future research in this direction.