In relation to cryptography, what is a trapdoor function?
A trapdoor function is a function that is easy to compute in one direction, yet believed to be difficult to compute in the opposite direction (finding its inverse) without special information, called the “trapdoor”. Trapdoor functions are widely used in cryptography. In mathematical terms, if f is a trapdoor function there exists some secret information y, such that given f(x) and y it is easy to compute x. Consider taking an engine apart. It would not be very easy to put it together again unless of course you had the assembly instructions. These instructions would be the trapdoor that allow you to return the engine to its original state. A mathematical example would be the multiplication of two large prime numbers. Finding and verifying two large primes is easy, as is their multiplication. But factoring the resultant product can be very difficult. Trapdoor functions came to prominence in cryptography in the mid-1970s with the publication of asymmetric encryption techniques by Diffie,
