You are designing a new encryption system that works in the following way:
For server-client communication you need a key k, composed of m sections, each of length l, and the key consists only of lowercase characters in the set {a, b, c, d, e, f}. The server has a key k1 and the client has a key k2 where:
k1 = f(k). f is a function that receives a key and replace some random letters by ? indicating that those characters can be any lowercase letter of the set described before.
k2 = f(g(k)). g is a function that takes a key and produces a random permutation of its m sections. And f is the function defined above.
For example: let m = 3, l = 2
f('abacbc') = '?ba??c'