.NET License Manager

This small project will help you to manage licensing in your .NET applications. To check for license may be as simple as:

if (!LicenseManager.IsLicenseValid)
  Environment.Exit(1);

Please note that it's still a work in progress (inspired from Stack Overflow Where can I store (and manage) Application license information?) then many things need to be completed (see issues) and code must be reviewed and simplified. All documentation is still a TO DO, please refer to both source code and cited SO post. Any help on this (also for documentation) is more than welcome!

Glossary

Client: the machine where application you want to protect is deployed.

Server: the machine where you create licenses. It may be a public accessible web server or simply your development machine but your customers must not have direct access to its content.

Contact: it's a text string generated at client side which contains some unique identifiers used to uniquely tie one license to a specific machine.

License: it's a text string generated at server side and used client side which contains information about the license (to whom you licensed your application, its expiration and which features are enabled). License is generated from a contact and it's indissolubly tied to a specific machine.

How to Use It

This project cannot be used as-is, to be effective you must include full source code in your repository and change it as described here, read carefully these notes. Project is separated in groups:

• Read about Code Obfuscation.
• Code that must deployed together with your application stays in Radev.Licensing.dll. This assembly also contains code used by server components.
• Code that must be only on your private server (if you wish to have a private license server) stays in Radev.Licensing.Server.dll. Your customers must never have an accessible copy of this assembly because it contains your private key.
• Code that must be directly included in your application, possibly in each single assembly you will check for license (for example if you have a modular architecture, where one assembly implements a set of features, you should include one copy of this file in each assembly). A generic template is LicenseManager.cs but it must be customized for your specific use.

To start using this project you have to carefully follow these steps:

• Create a new public/private key pair and save it as XML. Copy public key in Radev.Licensing\Keys.cs and both public and private keys in Radev.Licensing.Server\Keys.cs.
• Include a copy of LicenseManager.cs in each assembly where you need to check for the license together with a reference to Radev.Licensing.dll.
• Update Feature enum to include all features you want to protect with license. You may simply omit this file (and relative code in LicenseManager) if you just need to know if there is a license but you don't protect single features.
• Create an utility (or something automatic, if you have a public licensing web server) to generate a contact:

ContactWriter.ToFile("path_for_contact", ContactFactory.Create<Contact>());

If you need plain text (for example to send an e-mail) you can use ContactWriter.ToString().

• Create an utility (or something automatic, if you have a public licensing web server) to generate a license from a contact file:

var contact = ContactReader.FromFile("path_for_contact");
var license = LicenseFactory.Create<License>(contact);

// Fill license with any information you need
// ...

LicenseWriter.ToFile("path_for_license", license);

• When you need to check for license (or for a specific feature) simply call LicenseManager methods:

// Mere license presence enables a basic set of features...
if (LicenseManager.IsLicenseValid == false)
    QuitApplicationBecauseInvalidLicense();

// Here we check if an advanced feature is available too...
if (LicenseManager.IsFeatureAvailable(Feature.AdvancedExpensiveFeature)) {
    ExecuteAnAdvancedExpensiveFeature(); 
}

If in doubt about where you license file should be you may read Where to Save License File.

Please note you can assign a validity interval for your license (I strongly suggest to always make it valid from the day it has been created) and also specify which version (again a range) of your software is enabled by that license. For example:

// This license is valid from now to the far future...
license.Validity = new Interval(DateTime.Now, null);

// For any software versione in the 2.xx family
license.MinimumVersion = new Version(2, 0); 
license.MaximumVersion = new Version(2, 65535);

How It Works

To have an adequate protection few assumptions must be satisfied:

• You want individually license each copy of your software and tie a license to the exact machine to which it has been associated: licenses cannot be copied and reused between different machines.
• Licese file may be hidden but its strength does not come from that nor from obfuscation: even if it is accessible and modifiable your application must have a mechanism to realibly validate it.
• Code that check for the license is reasonably protected and it is not easy to patch it to circumvent the license.

One License for Each Client

First of all we need to uniquely identify a specific machine. There are various IDs and settings we may use but we must combine more than one of them because hardware configuration may change and we do not want to invalidate our license for each small change.

I decided to use WMI queries to read configuration values, because they're easy to use and to change, and by default (but it can be changed in Licensing\Token.cs) I read only IDs, because they're less subject to change compared to - for example - memory size. Note that this choice is not mandatory and you may want to change it implementing your own Licensing\Client\HardwareAnalyzer.cs class and changing Licensing\Token.cs accordingly.

Application will compare hardware configuration stored in its license with current live configuration and if they differ more than a specified amount then it will consider license as invalid. For details check License.IsAssociatedWithThisMachine() and License.MaximumNumberOfHardwareChanges. Currently license will check these values:

• Computer manufacturer and model. These values should not ever change over time, unless you also change motherboard, but they're not widely available and they may contain dummy text.
• Motherboard manufacturer, model and serial number. These values should not change over time but sometimes (especially on old systems) serial number is fake.
• BIOS serial number. Also this value may be a fake on very old motherboards.
• CPU ID. This value is not a true unique ID of your CPU but it model and stepping, combined with other IDs, will help you to uniquely identify a specific system.
• OS serial number and installation date. Here you rely on Operating System license protection... if they use your application on two different machines then you assume they also need to use two different OS copies (unless they also want to crack OS). Installation date (absolutely optional) lets you detect false claims like I had to reinstall OS because of broken hard disk.

Note that I didn't use any disk serial number (hard to reliably read) nor MAC addresses (NICs may be turned on and off at run-time).

Note that you may use motherboard and computer manufacturer name to detect virtual machines (Microsoft, Oracle, VMWare and similar). It's not a reliable method but it has the advantage to be extremly simple. For a more reliable check you have to individually check for virtual machines you know, for more details check Wiki page about Virtual Machines.

Secure License File

After we collected all required information into a contact then we encrypt that data with our public key and then encode it as base64 (to make it easy to transfer, for example, via e-mail). Note that here encryption isn't strictly necessary (because everything a cracker needs is available and visible in application code).

Server code will then decode and decrypt contact information, fill license as required and then sign result with our private key. Everything is then encoded again as base64. From this moment our application can detect any change to license content because signature (verified with public key) won't match.

Protect Application Assemblies

Of course you also need to protect application assemblies otherwise they may be tampered. This topic is vast (especially because a cracker has a big surface attack) but you should at least start signing all your assemblies and then - eventually - embed Radev.Licensing.dll into your application assembly as a resource (to make things slightly more complicate for casual crackers), you will load it inside AppDomain.CurrentDomain.AssemblyResolve event handler.

There are more things to consider:

• If application can load untrusted/unsigned plugins then cracker may simply use Reflection to bypass/modify already validated in-memory license data.
• Code may be patched at run-time bypassing license checks (the (in)famous NOP instead of CALL).
• WMI information may be faked.
• An entire system may be cloned using virtual machines.
• Application may be shared using Web Access, Terminal Server or similar technology.

More about this later...