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.