you did google for "checksum algorithm", didn't you?
The basic idea of a checksum (as you may already know) is to create a (sufficiently) unique numerical value for a certain string of characters. That way, you can send a short checksum with a large file, and easily verify if the large file (=string) is correct.
I *think* a CRC does pretty much the same, but I'm no expert...
I've found <a href="http://www.flounder.com/checksum.htm">this page</a>, which may or may not be what you need. No promises.
//added info
By the way, shouldn't this be in tilted programming?
