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Question 87. What is RC4?

RC4 is a stream cipher designed by Rivest for RSA Data Security. It is a variable key-size stream cipher with byte-oriented operations. The algorithm is based on the use of a random permutation and 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. While the algorithm is confidential and proprietary to RSA Data Security, Inc., it has been scrutinized under conditions of non-disclosure by independent analysts and it is considered secure. The RC4 stream cipher has a special status by which export from the U.S. can often be facilitated (see Question 75).