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Arcfour is a stream cipher symmetric key algorithm,
believed to be fully interoperable with the RC4 algoritm.
RC4 was developed in 1987 by Ronald Rivest and kept as
a trade secret by RSA Data Security. The name "RC4" is
trademarked by RSA Data Security, Inc. So anyone who writes
his own code has to call it something else. ARCFOUR has been
widely accepted as the name of the alleged RC4.
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Arcfour uses a variable length key from 1 to 256 bytes to
initialize a 256-byte state table. The state table is used
for subsequent generation of pseudo-random bytes and then to
generate a pseudo-random stream which is XORed with the
plaintext to give the ciphertext. Each element in the state
table is swapped at least once.
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This algorithm can be operated with several different key
sizes. If the key is 128 bits in length then this algorithm
is believed to be secure. If the key length is significantly
shorter, specifically 40 bits, then there are known attacts
that have been successfully applied. For this algorithm to be
operated in a cryptographically sound manner it is believed
that a key length of 128 bits or more should be used.
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