RC4 is a stream cipher designed by Rivest for RSA Data Security (now RSA Security). It is a variable key-size stream cipher with byte-oriented operations. The algorithm is based on the use of a random permutation. Analysis shows that the period of the cipher is overwhelmingly likely to be greater than 10100. Eight to sixteen machine operations are required per output byte, and the cipher can be expected to run very quickly in software. Independent analysts have scrutinized the algorithm and it is considered secure.
RC4 is used for file encryption in products such as RSA SecurPC (see Question 5.2.4). It is also used for secure communications, as in the encryption of traffic to and from secure web sites using the SSL protocol (see Question 5.1.2).