How do I protect my licensing file?

I have a licensing file for my application. The only way it can be broken right now and the user can have unlimited access is by deleting the licensing file every 30 days and reinstalling the program.

How do I best protect this file? (or information)

My first thought is to hide the file a few folders deep somewhere under windows %AllUsersProfile%. And use obscure names for the folders as to not advertise the location.

Another thought was to write to the registry, but we cannot always write to the HKEY_Local_Machine like I wanted to due to it requiring admin privileges.

Solution

Pretty much any method of hiding or obscuring your license file is crackable. You really need to decide among some scenarios:

I trust my customers, but they might inadvertently break my license terms
My product is highly desirable and thus will be the target of crackers
My product isn't interesting to crackers, but my customers are slimebags and will copy the snot out of it.

In case 1, a simple license file to help your customers with compliance is fine. Probably hiding it or obscuring it isn't necessary. I assume that isn't your case since you posted this question.

In cases 2 and 3, there's little you can do that can't be easily defeated. The tools available to crackers are quite powerful and widely available, along with techniques for using them. Our company (www.wibu.us) has a full-time cryptographer who just watches how people crack software so we can build stronger protection against it.

Probably the most "normal" approach for a DIY solution is to encrypt the license file using some "standard" algorithm like AES 128-bit or triple DES. Then make the key from a hash of several factors, like the MAC address, MB serial number, install date, and perhaps some user-input data ("name" "Phone" etc). However, crypto can get complicated so you want to make sure you know what you're doing with this approach.