Serial number validation code

One common method to checking serial numbers is to use a hash algorithm. A hash algorithm is a one-way function, this means you calculate a sort of checksum, but it is not possible to calculate the serialnumber if the hash value is known.

I am sure you will the sourcecode to a hash algorithm can be found at the Internet.

Thank you tovergaard. I'm sorry for not accepting your answer. If I use some sort of hash function (& check the calculated value against some unique predefined value) then how do I guarantee that I can find at least N valid serial numbers? What's more, to use this technique I need an algorithm to generate valid serial numbers with acceptable growth rate - straightforward iteration over all values and checking their hash value just won't do. I beleive the solution to this is to use some encryption technique based on prime numbers or something of the kind. Unfortunatly I have no time to get into all this theory, what I want is the C code which does this job more or less with a bit of explanation to get me started. Thank you.