MODIFICATION OF RC4 ALGORITHEM TO INCREASE ITS SECURITY BY USING MATHEMATICAL OPERATIONS

RC4 algorithm is one of the most widely used stream ciphers. It is fast, simple and suitable for software and hardware. It is used in many applications, but it has a weakness in the distribution of key stream bytes, the first few key stream bytes of PRNG are biased or related to some secret key bytes and thus the analysis of key stream bytes makes it possible to attack RC4, and there is a correlation between the key stream bytes that make it weak and breakable by single and double byte bias attack. This work shows a new algorithm proposed by using initial state factorial to solve the correlation issue between public known outputs of the internal state and making this algorithm is robust against attack by using an additional state table with the same length of the state to contain the factorial of initial state elements. Also, shows the single byte bias attack on RC4 by using the newly designed algorithm. The results showed that the proposed algorithm is robust against attack and could retrieve the first 32 bytes of the plain text by using the proposed algorithm of single byte bias attack with a probability of 100%. Additionally, the developed algorithm is robust against many attacks such as distinguishing attack.