

My MDC cipher (which uses any one-way hash function as a CFB-mode stream
cipher, the current implementation uses MD5) uses a key of up to 2048 bits
(that is, you can use a 1-bit key if you want and copy it over the entire
2048-bit range, or you can use the entire 2048 bits).  Runtime is
independant of key size, the system runs slightly slower than MD5 itself.
I presume RC2 and RC4 use a similar system (or possibly they just hash an
arbitrary-length key down to n bits, maybe 128, using something like MD5).