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    STUDIA INFORMATICA - Issue no. 1 / 2010  
         
  Article:   SOME COMBINATORIAL ASPECTS OF THE KSAm-LIKE ALGORITHMS SUITABLE FOR RC4 STREAM CIPHER.

Authors:  FLORIAN MIRCEA BOIAN. 		 
       
         
  Abstract:  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 Key-Scheduling 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.

Key words and phrases: KSA, KSAm, RC4, sign of permutation, randomness of state table.
  
         
     
         
         
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