C++ Interface for SKM Web API
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Introduction to C++ Client API
Martin Svedin
A comprehensive article that describes how the C++ client can be used to access Cryptolens Web API.

SKM Client API for C++

On this page, we have outlined several examples of how to get started with the SKM Client API for C++.

Note, SKM Client API for C++ currently supports activation and deactivation methods. Support for more methods is coming soon.

You can find the API documentation here: https://api.serialkeymanager.com/cpp/.

If you are already familiar with the .NET version of the library, we have summarized key differences in an announcement on our blog.

Table of contents

Example projects

This repository contains some example projects using the library in the examples/ directory. The cmake example project is set up to be compiled against OpenSSL and libcurl, while the VisualStudio project builds against the CryptoAPI and WinHTTP libraries available on Windows. The rest of this section contains instructions for how to build the example projects.


First we need to install libcurl and OpenSSL, including the header files, unless that has already been done.

$ apt-get install libcurl4-openssl-dev libssl-dev
Fedora/Red Hat:
$ yum install libcurl-devel openssl-devel

Next, clone the repository and build the examples

$ git clone https://github.com/Cryptolens/SKM-Client-API-CPP.git
$ cd SKM-Client-API-CPP/examples/cmake
$ mkdir build
$ cd build
$ cmake ..
$ make -j8
$ ./example1

Visual Studio

Getting started with the example project for Visual Studio requires two steps. First we build the library file the example project will statically link against, then we build the example project.

The following steps build the library:

  • Open the solution file vsprojects/Cryptolens.sln in Visual Studio.
  • Set platform and configuration as appropriate, e.g. x64 and Debug
  • Build the project in Visual Studio
  • Now the folder vsprojects/Output/$Platform/$Configuration/ contains the Cryptolens.lib file. With platform and configuration as above, the file is vsprojects/Output/x64/Debug/Cryptolens.lib

Now we can build the example project:

  • Open examples/VisualStudio/VisualStudio.sln
  • Set configuration and platform in the same way as when building the library
  • Build and run the project.

For setting up your own Visual Studio project to use the library, we have a step-by-step guide with pre-compiled binaries here.

Library overview

This section contains an overview of the standard way to implement the library in an application. The first step is to include the appropriate headers:

#include <cryptolens/core.hpp>
#include <cryptolens/Error.hpp>
#include <cryptolens/RequestHandler_XXX.hpp>
#include <cryptolens/SignatureVerifier_YYY.hpp>

We currently support the following RequestHandlers

RequestHandler Description
RequestHandler_curl Uses libcurl
RequestHandler_WinHTTP Uses the WinHTTP library available as part of Windows

and SignatureVerifiers

SignatureVerifier Description
SignatureVerifier_OpenSSL Uses OpenSSL or LibreSSL
SignatureVerifier_CryptoAPI Uses the CryptoAPI available as part of Windows

The next step is to create and set up a handle class responsible for making requests to the Cryptolens Web API.

using Cryptolens = cryptolens::basic_Cryptolens
                     <cryptolens::RequestHandler_curl, cryptolens::SignatureVerifier_OpenSSL>;

Cryptolens cryptolens_handle;
cryptolens::Error e;
cryptolens_handle.signature_verifier.set_modulus_base64(e, "ABCDEFGHI1234");
cryptolens_handle.signature_verifier.set_exponent_base64(e, "ABCD");

Here the strings "ABCDEFGHI1234" and "ABCD" needs to be replaced by your public keys. These can be found when logged in on Cryptolens.io from the menu in the top-right corner ("Hello !") and then Security Settings. The example above corresponds to the following value on the website


Now that the handle class has been set up, we can attempt to activate a license key

cryptolens::optional<cryptolens::LicenseKey> license_key =
    ( // Object used for reporting if an error occured
    , // SKM Access Token
    , // Product id
    , // License Key
    , // Machine Code

if (e) {
  // An error occured. Handle it.
  return 1;

The activate method takes several arguments:

  1. The first argument is used to indicate if an error occured, and if so can provide additional information. For more details see the section Error handling below.
  2. We need an access token with the Activate scope. Access tokens can be created at https://app.cryptolens.io/User/AccessToken/.
  3. The third argument is the product id, this can be found at the page for the corresponding product at https://app.cryptolens.io/Product.
  4. The fourth argument is a string containing the license key string, in most cases this will be input by the user of the application in some application dependent fashion.
  5. The last argument is an optional identifier for the device the application is running on, or something similar.

After the activate call we check if an error occurred by converting e to bool. If an error occured this returns true. If an error occurs, the optional containing the LicenseKey object will be empty, and dereferencing it will result in undefined behaviour.

If no error occurs we can inspect the LicenseKey object for information as follows

std::cout << "License key for product with id: " << license_key->get_product_id() << std::endl;

if (license_key->check()->has_expired(1234567)) {
  std::cout << "Your subscription has expired." << std::endl;
  return 1;

if (license_key->check()-has_feature(1)) { std::cout << "Welcome! Pro version enabled!" << std::endl; }
else                                     { std::cout << "Welcome!" << std::endl; }

Error handling

This section describes in more detail how the library reports when something went wrong. The library uses an exceptionless design and return values use optionals where they can be missing. All functions accept as the first argument a reference to cryptolens::basic_Error which is used to indicate when an error has occured as well as for reporting more detailed error information.

Furthermore, if an error occurs, subsequent calls to the library with that error object as first argument are turned into noops. consider the following code:

cryptolens::Error e;

If an error occurs during the call to f1(), f2() will not be performed. This allows writing several function calls after each other without having to check for errors after every single call.

Offline activation

One way to support activation while offline is to initially make one activation request while connected to the internet and then saving this response. By then reading the saved response and performing the cryptographic checks it is not necessary to make another request to the Web API in the future. Thus we can proceed as above, but save the response to a file

Cryptolens cryptolens_handle;
cryptolens_handle.signature_verifier.set_modulus_base64(e, "ABCDEFGHI1234");
cryptolens_handle.signature_verifier.set_exponent_base64(e, "ABCD");
cryptolens::optional<cryptolens::LicenseKey> license_key =
if (e) { handle_error(e); return 1; }
file << license_key->get_license() << "-" << license_key->get_signature() << std::endl;

and then when offline the license key can be verified via:

std::string s << file;
size_t k = s.find('-');

if (k == string::npos) { return 1; }

std::string license = s.substr(0, k);
std::string signature = s.substr(k+1, string::npos);
cryptolens::Error e;
cryptolens::optional<cryptolens::LicenseKey> license_key =
  cryptolens::LicenseKey::make(e, license, signature);
if (e) { handle_error(e); return 1; }

HTTPS requests outside the library

In some cases it may be needlessly complex to have the Cryptolens library be responsible for initiating the HTTPS request to the Web API, instead it might be easier to have some other part of the application that does the HTTPS request, and then only use this library for making sure that the cryptographic signatures are valid. This can be accomplished by using the handle_activate() function as follows

std::string web_api_response; // Some other part of the code creates and populates this object

SignatureVerifier_XXX signature_verifier;
cryptolens::Error e;
signature_verifier.set_modulus_base64(e, "ABCDEFGHI1234");
signature_verifier.set_exponent_base64(e, "ABCD");

cryptolens::optional<cryptolens::LicenseKey> key =
  cryptolens::handle_activate(e, signature_verifier, web_api_resonse);
if (e) { handle_error(e); return 1; }

This code is also applicable to the case where one wants a completly air-gapped offline activation. In this case the web_api_response string would be prepared and delivered one for example an USB thumb drive, and the application then reads this response from the device and stores it in the web_api_response string.