Add Output Feedback Block (OFB) to the given block cipher. The integer argument n is the number of bytes processed at a time; it must lie between 1 and the block size of the underlying cipher, included. The returned block cipher has block size n. It is usable both for encryption and decryption.
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.