PROG challange: fetish corroborator

write an algorithm that enables two persons over a communication channel to find out which fetishes from a preset list they happen to share.

it should done in such a way that no extra information is leaked, see http://en.wikipedia.org/wiki/Yao%27s_Millionaires%27_Problem to get started.

Bonus: Extra points are awarded to written in Lisp or Scheme.

Deadline: The deadline is 03/07/2015 so get cracking!

>preset list
1. Assign each possible fetish a large prime (and make sure they're primes, by magic), share that list
2. multiply the fetishes
3. compare size by announcing rough estimates of the products
4. if one party thinks they're being fooled, exit or go to 3.
5. exchange numbers
6. compute gcd
voila. Not super versed in crypto so this might have some weaknesses.