The concept of the creation of a trapdoor in NP-complete or NP-hard problems
A &quot;trapdoor&quot; function is a function which is hard to compute, but will become very easy (or a lot easier) to compute if you have some specific knowledge.
The classical example is solving y = x^3 (modulo n) where n is the product of two very large primes; it is the basis of RSA encryption and practically impossible to solve given x and n, unless you are given the two primes p, q such that n = pq.