what is a trapdoor function?
It is a mathematical operation which is trivially easy to calculate, but hideously difficult to invert (i.e.. like a trapdoor – it’s easy to go one way, but difficult to come back the other). Putting this mathematically: f() is our trapdoor function, it is easy to calculate. Let’s assume that f-1() is the inverse, such that: x = f(y) y = f-1(x) f-1() will be hideously difficult to calculate. “There are a number of trapdoor functions available. Cryptographic hashes (MD5, SHA-1 etc.) are examples of these. “However, there are classes of functions which are trapdoor functions which have specific properties, in that they are reversible, if you know certain things. This is more commonly known as public key cryptography. “Let me illustrate this mathematically: “Our encryption function is called e(), our decryption function is called d(). x = e(y, p1) y = d(x, p2) x = ciphertext (encrypted text) y = plaintext (unencrypted text) p1 = public key p2 = private key “So, IF AND ONLY IF I possess ke
