Category: Software Licensing Best Practices

How to protect SDKs with Software Licensing in .NET

Software Development Kits (SDKs) are a great way to give your users the ability to build on top of the functionality offered by your library/package. From a licensing perspective, desktop apps and SDKs are quite similar, which we will go through in this article. We will first take a look at the applicable licensing models and then skim through some example code. You can jump directly to the tutorial here.

Licensing Models

SDK licensing is special since the developer of the SDK (the customer) is not its end user. Instead, it’s their customers that will be the end users. In this article we focus on “node-locked” and “pay per install” licensing models (you can read about all applicable licensing models here).

Node-locked is equivalent to “pay per machine”, which essentially means that each time a new machine activates the license, this is recorded so that it can be taken into account when you charge the developers (your customers). Each user will be able to re-install the app that uses the SDK any number of times, without affecting the counter.

Pay per install is similar to “pay per machine”, with the only difference being that fingerprints of the end user machines are not recorded. Instead, a counter is used that increment whenever the SDK is first launched. With this model you get a bit less control of end user instances, but since the fingerprints (aka machines codes) are not tracked, the subscription cost for Cryptolens will reduce significantly (since you are only paying per license key).

In both of the models above, you could create multiple plans for your customers that depend on the actual usage of the SDK. Eg. 1-10 could be a testing tier, 10-10,000 could be another pricing tier, and so on.


From a developer standpoint (eg. your customer), the license key will have to be specified to unlock functionality of your SDK. You could potentially have different pricing tiers depending on the methods that your customers will use. Below is an example of class initialisation that requires a license key to work.

var math = new MathMethods("FULXY-NADQW-ZAMPX-PQHUT");


To see all the code, please take a look at the entire tutorial.


If you have algorithms in your SDK that you want to be 100% secure from reverse-engineering, we would recommend to create an API endpoint for them hosted in the cloud. Most of the cloud providers support “server less” functions, eg Azure Functions and AWS Lambdas. These are quite simple to setup. Your server less functions would require a license key and potentially a machine code to return a successful response. On the client side, you could use libraries such as RestSharp to access your API endpoint. We will cover this in a future article.

Computing and verifying VAT in .NET (for EU businesses)

When you sell your software as an EU business, you need to take into account the VAT, which depends on whether you sell to a private individual or a company, and their country of residence. Moreover, you need to ensure that the VAT id that they have provided is correct.

In order to solve these two problems, we have published a library for .NET, available as a NuGet package (with source code on GitHub).


The library has two methods, CalculateVAT and IsValidVAT, which are quite simple to use. We explain their purpose below:

  • CalculateVATThis method asks for the country of residence of the individual or the company, and their VAT id (if applicable). Based on this information, it will calculate the necessary tax that should be applied to the order. Note, we assume you sell products or services that are covered by the standard VAT (i.e. some categories such as books have a lower tax in some countries).
  • IsValidVATThis method is responsible for VAT id verification. We use the European Commission’s API for that. Note, this API is not up 24/7 and can be unresponsive some times. You can view all the times it is down (given the country of residence) here.

3 steps how to protect your software application before release

Let’s assume that you have developed a software application (eg. app) that you are about to sell. Then, there are three things you need to consider:

  • Licensing – this is used to keep track of the type of features that end users have bought. A simple example of this when your user has to type a license key to unlock more functionality. When selecting these kinds of systems, it’s important that the system both supports offline mode and is cloud-based. The advantage of cloud-based systems is that they are more scalable and secure (eg. you have full control of all end users).
  • Obfuscation – this is used to make your program binaries (eg. exe and dll files) harder to disassemble. This is especially important for .NET apps, since existing tools make this very simple. A word of warning though: none of the available systems are 100% safe, and even the well-respected systems are being cracked within days of software release.
  • Web API – imagine your algorithm is so important that you don’t want to risk it being leaked. Since obfuscators are never 100% safe (mainly because in the end, the code will be executed on the client machine), the only secure way is to never run this code on client machines that you don’t control. Instead, you can create a Web API method that you host yourself and then allow your program to consume it. In this case, the algorithm is safe at the cost of constant internet access requirement.

To sum up, the first system to consider is licensing, since this will remove the administrative burden of keeping track of the type of rights your customers have to the software. As a bonus, many cloud-based licensing systems support integration with payment processors. In the end of the day, the goal is to ensure payments and license verification are automated, so that you can focus on developing the features that really matter to your customers.

For more information, please see this page.

Secure your account using 2-step verification

Now you can secure your account even further using two-factor authentication (2FA). In addition to your password, you will have to enter a short 6-digit code each time you log in, which helps to keep your account protected.

Enable 2-step verification

You can access this page directly here.

  1. Go to the Account Settings page
  2. Click on “Configure 2-step verification”
  3. Check “Enable Two Step Authentication” checkbox
  4. Scan the QR code with Google Authenticator (note: there are other alternatives such as Authy if you already have oen of them installed). If you want to get Google Authenticator, you can get it for

Open Source

At SKM, open-mindedness and transparency are at the core of everything we are doing. Therefore, we’ve open-sourced the core parts of 2-step verification, freely available on GitHub. You can learn more about our other open-source projects at

Fixed downtime Dec 18th due to invalid certificate

Today we had a major downtime because the new TLS certificate was not upgraded properly. This caused most of the versions of SKM Client API (aka SKGL Extension) not being able to validate license keys.

I’m very sorry for all the problems that this caused you. Downtimes occur because of various issues; at SKM we are constantly working on making sure to reduce them and their impact.

I’my happy to tell that this issue is now fixed!


Lead Developer