Improve Oblivious Transfer for arbitrary message length

[th4s]

Some bandwidth can be saved if we assume that the random block which the receiver holds after the Random OT setup is already an AES key. Then the only thing which the Sender needs to do is to take one of his 2 random blocks based on the derandomization bit and encrypt the message with it. This saves the need to send the actual AES key over the wire.

[sinui0]

#186