EC Cryptography Tutorials - Herong's Tutorial Examples - v1.00, by Dr. Herong Yang
What Is Trapdoor Function
This section describes what is Trapdoor Function - An operation that is much easier to perform than its reverse operation.
Let's pause for a moment on the discussion of DLP and take a look at the definition of Trapdoor Function.
What Is Trapdoor Function? A Trapdoor Function is an operation that is much easier to perform than its reverse operation.
Multiplication of two large prime numbers is a trapdoor function because:
1. Performing the multiplication of two given large prime numbers is easy. For example, p=... and q=... are two prime number, f(p,q)=p*q=... can be easily calculated.
2. If f(p,q)=p*q=... is given, find p and q is called factoring an integer into prime numbers, and is difficult.
Of course, when p and q are very small, factoring p*q is not that difficult. So f(p,q)=p*q is a trapdoor function, only when p and q and large prime numbers.
By the way, f(p,q)=p*q, as a trapdoor function has been widely used in cryptography to generate public and private keys.
Trapdoor concept is also used build mouse traps (source: caramembuatkue.info). Easy to pass in one direction, and difficult to pass in the reverse direction.
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