A simple implementation of the RC4 algorithm, developed by RSA Security, Inc.
Here is the description from RSA's website:

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.