SOME COMBINATORIAL ASPECTS OF THE KSAm-LIKE ALGORITHMS SUITABLE FOR RC4 STREAM CIPHER

RC4 remains one of the most widely used stream cipher. In order to face the main critical weaknesses, a number of proposals for modifying RC4 algorithm have been advanced. In this paper we analyze some combinatorial aspects regarding the randomness of a variant of the KeyScheduling Algorithm (KSA), called KSAm, proposed by Crainicu and Boian in [2] as a better protection against Initialization Vectors (IVs) weakness of Wired Equivalent Privacy (WEP) cryptosystems. Based on a model presented by Mironov in [19], we calculate the sign of the entries’ permutation of the internal state table S after KSAm, which provides a negligible advantage of guessing a particular bit. Then, we analyze the probability of the event where a particular initial value follows a linear forward movement through the vector S, with possible undesirable consequences in predicting the value during that movement.