You might wonder that if this is the
strongest cipher., then there is
nothing to study anymore.
But there is still a problem.
By the way I would like to emphasize an important principle
in the theory of cipher. That is:
The security of a cipher must not depend on keeping secret the algorithm.
The security depends only on keeping secret the key.
(The security of a cipher must be discusses on the assumption that
the code breakers know the algorithm.)
OK. Let's go back to the problem that we are facing.
The problem is that the sender and
receiver have to agree with
the same key before using the cipher.
The sender must send the key to the
receiver, but the enemy will see it.
So the cipher is not safe anymore.
This is the problem, and it is called Key
Distribution Problem.
