I need to be able to create serial numbers for my application and I'm not quite sure how this is normally done. I've Googled for this and all I get is how to beat the system and get CD Keys, but I need an ALGORITHM to create the CD Key with one application and then my target (main) application must use the same algorithm to verify that the serial number is valid (mathematically). The ideal situation would be where the user would call me to generate a serial number for their application which would expire after a certain period of time. If any one has any suggestions I would greatly appreciate it.