The concept of the creation of a trapdoor in NP-complete or NP-hard problems I am reading the book An Introduction to Mathematical Cryptography. In its chapter 7, there is the following statement:

In real world scenarios, cryptosystems based on NP-hard or NP-complete problems tend to rely on particular subclass of problems either to achieve efficiency or to allow the creation of a trapdoor.

Questions:
Is is possible to explain what does it mean the phrase &quot;to allow the creation of a trapdoor&quot;?
Thanks for any help
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.