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.

Answers


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

  1. I trust my customers, but they might inadvertently break my license terms
  2. My product is highly desirable and thus will be the target of crackers
  3. 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.


Create a license file during the setup. And consider the absence of a license file as an invalid license file. Uninstalling, deleting the file and then reinstalling is much more annoying than just deleting a file.

And I hate it if files with an obscure random name appear somewhere on my system, since I wonder if I got infected with a virus, or if it's just some badly behaved software.

And don't try too hard. It's no use to make it harder than downloading a crack or license-reset tool from the next warez site. And a cracker will find your license file very quickly with tools like FileMon.

One idea that might actually work(except against cracks that patch your binary): Fix the expiration date on download and embed the license file in the setup. That way they actually need to download a new version whenever their license expires. But of course your users might find that unacceptable or might not fit your distribution model...


Force the application to go to a remote server over port 80 to check a hash which was set at the first install, perhaps against the MAC address (not an absolute guarantee but good enough). If they try to install again it is at least tied to the MAC address and you can stop the install.

EDIT:

If your customer base is small and you have the resources to support them you can perform the same behavior without internet access except that the customer needs to come to you to get a license file. They generate a key via their system, again tied to the MAC address, then send you the key which you generate the license file from. This is dependent of course on the number of outgoing downloads a day.


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