There is no reliable licensing algorithm. Really. Not even one. For the most popular, most expensive proprietary software you can buy, you can also find "key generators" and hacked versions that don't require licensing.
Instead of worrying about making it "unbreakable", just do something simple. A popular mechanism is to, at purchase, ask for the user's name, and then give him a license key that's derived from a cryptographic hash (e.g. MD5 sum) of the user's name, or some variation on it. Then, in the software you ask for their name again, plus the registration key (that MD5-derived thing); you check to see that they match, which activates the software.
Can this be hacked? Absolutely. Once someone figures out how you're generating the license keys, they can generate their own. But if you keep a database of the "official" license keys you've generated so far, at least you'll be able to identify the fraudsters later on (perhaps when they try to download "premium" content or something).
But don't worry so much about stopping the hackers from cracking your code. It's going to happen, but they're such a tiny part of the market that it won't significantly affect your overall sales.
