Now we'll have a couple of quizzes to see that you understand Cipher Feedback Mode.
The first question is, how does one decrypt a message encrypted
using Cyper Feedback Mode?
Here are the choices.
The first choice is we have to go through the blocks backwards,
XORing out the cipher text from the results of encrypting the X values,
and we can compute the X values in reverse order like this.
The other choice is we do the decryption going forward,
XORing out the cipher text with encrypted Xi values and updating them
as we did in the encryption mode.
A third choice is the same as the second choice except instead of using
the encryption function, we're using the decryption function,
which is the inverse of the encryption function.
