The client SDKs: native C++, pure .NET, industry-standard cryptography

// Every start: the purchased key once bought, the trial key until then.
std::string purchased_key = read_file("purchased-key.txt");
bool trial = purchased_key.empty();
auto c = create_client(trial ? TRIAL_KEY : purchased_key);

// the stored license; with none (the first run), validation activates: the trial starts
auto params = c->create_license_validation_params();
// ... params->set_license(stored) when license.dat holds one

auto result = c->validate_license(params);

// save the license whenever the SDK issues a new or renewed one
auto updated = result->get_updated_license();
if (updated)
    std::ofstream("license.dat") << updated->serialize();

if (result->is_license_valid())
{
    auto& current = updated ? updated : stored;

    if (current->is_trial())
    {
        long long days_left = (current->get_expiration_date() - std::time(nullptr)) / 86400;
        std::cout << "Trial: " << days_left << " day(s) left. To buy, run: quickstart_cpp <license-key>\n";
    }
    else
        std::cout << "Licensed.\n";
}