ECE 575  Data Security and Cryptography

Overview:

RC4 is the most widely used stream cipher in these days, especial for the software applications. This is because the structure of RC4 is very simple and can be implement in software efficiently. Although RC4 has a huge internal state which includes a permutation of {0, 1, …., N-1}, it has the light-weight key scheduling and the output generation processes. It was seemed as a safe cryptography algorithm until 1994. In September, 1994, the algorithm was posted by someone on the Internet and is available for public analysis now.

RC4 Stream Cipher:

There are so many stream ciphers in these days, and most of them are implemented by using linear feedback shift registers. These stream ciphers are easy to implement in hardware. However, maybe not in the software. In 1987, Ron Rivest developed the RC4 stream cipher. It is suitable for software applications, and is used widely nowadays. For example, RC4 is used in SSL ( Secure Sockets Layer) protocol. It is also be used in WEP (Wired Equivalent Privacy) and many network applications.

There are mainly two steps in RC4 implementation. One is the key-scheduling algorithm (KSA) and the other is the pseudo-random generation part (PRGA). KSA turns a random key into an initial permutation S, and PRGA uses the permutation to generate the pseudo-random output sequence. It loops the following four operations:

                       (1) Increase i as a counter.

                       (2) Increase j pseudo-randomly.

                       (3) Exchange the values of S which are pointed to i and j.

                       (4) Generate the S pointed to S[ i ]+S[ j ].

KSA initializes S to the identity permutation. It has a round operation N times, just like PRGA. Finally, it updates the j by adding S [ i ] and the next word of the key.

RC4 Analysis:

    Pros: