Remove hardcodes from Mutual Authenticate

Support nonces that are not only 8 bytes in
Mutual Authenticate. Use the witness length
to determine the nonce size, thus existing
systems using 8 bytes will continue to use 8
bytes. However, with AES 256, the nonces could
be a single block size of 16 bytes or greater.
Merge pull request #280 from shootingatshadow/aes-support

Remove hardcodes from Mutual Authenticate