Add Cipher Block Chaining (CBC) to the given block cipher in encryption mode. Each block of input is xor-ed with the previous output block before being encrypted through the given block cipher. The optional iv argument specifies the string to be xor-ed with the first input block, and defaults to all zeroes. The returned block cipher has the same block size as the underlying block cipher.
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.