I will put here an interesting question or an open problem from time to time, so stay tuned.

For the starter here is one:

1. Can you construct an non-trivial example of a block-cipher *E(x,k)* with
the following property:

a. The size of the block is equal to the size of
the key |*K*| = |*X*|.

b.There exists plaintext *P* such that
*E(P,k)* is a permutation.

Why this is of interest? Try to guess.