using System;

using System.Security.Cryptography;

using System.Text;

public class Program

{

    public static void Main()

    {

        //var base64Secret = CreatePrivateKey();

        var base64Secret = "+bCzRidhVxwx5TKpkz14nf5cL+BMH+ahZIOCy4bPVzdXWhysq+tZfOtrHsw9vdg5vKes/lVwzHIyquvmz9taDg==";

​

        var licenseKey = CreateLicense(base64Secret);

​

        var isValid = ValidateLicense(licenseKey, base64Secret);

        Console.WriteLine($"Is Licsense Valid = {isValid}");

    }

    static string CreatePrivateKey()

    {

        // Step 01: Create your private hashkey and store it on the database

        byte[] hashKey = GenerateRandomCryptographicBytes(64);

​

        // Convert the key to base64 so you can easily store it on the database.

        // This should be kept private and never leaves your control.

        var base64Secret = Convert.ToBase64String(hashKey);

        Console.WriteLine($"Private Key = {base64Secret}");

​

        return base64Secret;

    }

    static string CreateLicense(string secretKey)

    {

        // Generate a license key of 10 chars (split into groups of 5)

        var licenseKey = Guid.NewGuid().ToString().ToUpper().Replace("-", "").Substring(0, 15);

​

        Console.WriteLine($"licenseKey = {licenseKey}");

​

        // Generate a Hmac license using secretkey

        var storedHmacOnDB = CalculateHmac(licenseKey, secretKey).ToUpper();

        var HMACTruncated = storedHmacOnDB.Substring(0, 15);

​

        Console.WriteLine($"HMAC = {HMACTruncated}");

​

        var licenseAndHMAC = InsertHyphen($"{licenseKey}{HMACTruncated}");

​

        Console.WriteLine($"Final User Licsense = {licenseAndHMAC}");

​

        return licenseAndHMAC;

    }

    static bool ValidateLicense(string licenseKey, string secretKey)

    {

        var tmp = licenseKey.Split('-');

​

        var license = $"{tmp[0]}{tmp[1]}{tmp[2]}";

        var licenseHmac = $"{tmp[3]}{tmp[4]}{tmp[5]}";

​

        string calculatedHmac = CalculateHmac(license, secretKey);

        var HMACTruncated = calculatedHmac.ToUpper().Substring(0, 15);

​

        bool isValid = licenseHmac.Equals(HMACTruncated, StringComparison.OrdinalIgnoreCase);

​

        return isValid;

    }

    static string InsertHyphen(string input, int everyNthChar = 5)

    {

        var sb = new StringBuilder();

        for (int i = 0; i < input.Length; i++)

        {

            sb.Append(input[i]);

            if ((i + 1) % everyNthChar == 0 && i != input.Length - 1)

            {

                sb.Append("-");

            }

        }

        return sb.ToString();

    }

    static byte[] GenerateRandomCryptographicBytes(int keyLength)

    {

        byte[] key = new byte[64];

        using (RandomNumberGenerator rng = RandomNumberGenerator.Create())

        {

            rng.GetBytes(key);

        }

        return key;

    }

    static string CalculateHmac(string data, string hashKeyBase64)

    {

        var byteArray = Convert.FromBase64String(hashKeyBase64);

        return CalculateHmac(data, byteArray);

    }

​

    static string CalculateHmac(string data, byte[] hashKey)

    {

        var hmac = new HMACMD5(hashKey);

        byte[] hashBytes = hmac.ComputeHash(Encoding.UTF8.GetBytes(data));

        return BitConverter.ToString(hashBytes).Replace("-", "").ToLower();

        }

    }