Source : Free On-Line Dictionary of Computing

RC4
     
         A {cipher} designed by {RSA Data Security,
        Inc.} which can accept {keys} of arbitrary length, and is
        essentially a {pseudo random number generator} with the output
        of the generator being {XOR}ed with the data stream to produce
        the encrypted data.  For this reason, it is very important
        that the same RC4 key never be used to encrypt two different
        data streams.  The encryption mechanism used to be a trade
        secret, until someone posted source code for an {algorithm}
        onto {Usenet News}, claiming it to be equivalent to RC4.  The
        algorithm is very fast, its security is unknown, but breaking
        it does not seem trivial either.  There is very strong
        evidence that the posted algorithm is indeed equivalent to
        RC4.
     
        The United States government routinely approves RC4 with
        40-bit keys for export.  Keys this small can be easily broken
        by governments, criminals, and amateurs.  The exportable
        version of {Netscape}'s {Secure Socket Layer}, which uses
        RC4-40, was broken by at least two independent groups.
        Breaking it took about eight days; in many universities or
        companies the same computing power is available to any
        computer science student.
     
        See also {Damien Doligez's SSL cracking page
        (http://pauillac.inria.fr/~doligez/ssl/)}, {RC4 Source and
        Information (http://www.cs.hut.fi/crypto/rc4)}, {SSLeay
        (http://www.cs.hut.fi/crypto/software.html#ssleay)}, {Crypto++
        (http://www.cs.hut.fi/crypto/software.html#crypto++)}, {Ssh
        (http://www.cs.hut.fi/crypto/software.html#ssh)}, {A
        collection of articles
        (http://www.cs.hut.fi/crypto/rc4-breaking)}.
     
        (1996-10-28)