A trapdoor function is easy to compute in one direction but computationally infeasible to reverse without special information. This asymmetry supports public key cryptography: multiplying two large primes is quick, while recovering the original primes from their product is impractical. section 6.2
Why the asymmetry matters
Public key systems use mathematical problems that are easy to perform but difficult to undo. RSA relies on factoring the product of two large primes; elliptic-curve systems rely on the elliptic-curve discrete logarithm problem. Related public and private keys make the private information the protected part of the arrangement. section 6.2
