Obfuscating an ID

Question

I\'m looking for a way to encrypt/obfuscate an integer ID into another integer. More precisely, I need a function int F(int x), so that

• x<->F(x

Answer 1

If xor is acceptable for everything but inferring F(y) given x and F(x) then I think you can do that with a salt. First choose a secret one-way function. For example S(s) = MD5(secret ^ s). Then F(x) = (s, S(s) ^ x) where s is chosen randomly. I wrote that as a tuple but you can combine the two parts into an integer, e.g. F(x) = 10000 * s + S(s) ^ x. The decryption extracts the salt s again and uses F'(F(x)) = S(extract s) ^ (extract S(s)^x). Given x and F(x) you can see s (though it is slightly obfuscated) and you can infer S(s) but for some other user y with a different random salt t the user knowing F(x) can't find S(t).