CDR: any good idea?

[Kenix]

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