For the protocol, we have a value x, which is just 1 bit,
and I've been writing the message as a result of encrypting x with some key.
But if we're using a block cipher, well, that assumes that the input is 128 bits.
The key is also 128 bits, and AES does operate with different size keys,
but the minimum key size is 128 bits.
The ciphertext out is 128 bits.
So in this case, we only have 1 bit.
So the question is, what should we do about this?
I'll give you 3 choices.
We could use the Cipher Feedback Mode and set s to 1,
that allows us to encrypt 1 bit at a time.
We could use the ECB mode.
That's just using AES passing in the input, and we could pad this input x
by adding 127 random bits to the end of it,
or we could use ECB mode, padding the input with 127 0 bits added after x.
For the protocol that I've described, which one of these would be the best choice?
