CDR: any good idea?

hi all, suppose we have two numbers, one is serial added(about 8~10 digits), another is a random number(about 7~12 digits), then how can i generate a 5 digits number depends on both of them? there shouldn't have any key so i can verify the 5-digit number later just use the pervious two numbers, i know the security is completely depends on the method that how to generate this 5-digit number. any good method you all can hint me? any good random number generator? TIA. Kenix