Block.cbc_decryptAdd Cipher Block Chaining (CBC) to the given block cipher in decryption mode. This works like Cryptokit.Block.cbc_encrypt, except that input blocks are first decrypted by the block cipher before being xor-ed with the previous input block.
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.