Generating and validating product keys

[JuggaloBrotha]

I have an application where they can use it for free for 30 days then after that they'll need to register it in which I figure using a product key would be something familiar for people, problem is I don't really know where to begin creating a keygenerator that would generate the keys for my app, the other thing would be upon registering in the app it would need to validate the key before accepting it. Anyone know of any articles that would get me started on this topic?

 

[Tom]

Theres probably a better way of doing it but I had a shareware app about 5 years ago that I forced an online registration. To make the keys unique per computer, I manipulated the hard drive serial number to a simple custom encryption. I would retrieve the number, convert each char to its ascii value, subtract 1 from the value and then reverse the entire string. That value would be sent to me during the registration and I would then have a slightly different calculation on that value that would be sent back. The returning value would be decoded and the end result is if it matched the serial number, the registration was successfull.