Block.cfb_decryptAdd Cipher Feedback Block (CFB) to the given block cipher in decryption mode. See Cryptokit.Block.cfb_encrypt.
transform src spos dst dpos encrypts or decrypts one block of data. The input data is read from byte array src at positions spos, ..., spos + blocksize - 1, and the output data is stored in byte array dst at positions dpos, ..., dpos + blocksize - 1.