(cfb-decrypt cfb encrypted-string) -> string
Given a CFB and a string previously encrypted by that CFB's counterpart, returns the plaintext. If the CFBs do not match or are out of synchronization, garbage is returned.
Example:
>> (cfb-decrypt d-cfb ciphertext) :: "Testing
If performed again:
>> (cfb-decrypt d-cfb ciphertext) :: "??edRޏgRYމ?(O}"