using System;

using System.Linq;

using System.IO;

using System.Text;

using System.Security.Cryptography;

public class Program

{

    public static void Main()

    {

        string encryptionKey = "EI8y2aJ76CqI5hMsLhWScBSnqwFB8lurheDJ1VYls2c=";

        string hmacKey = "JeX0KT2gv9Mn5aONY0HeiYkMfI6nuC+PLbflH+FJCVo=";

​

        String encodedString = "NJpfa8kCD6FDp+KeYFrnC+Cyf7vwNazOYg001m3k0Or7eeJh0HOeULOxVONMie7tNTo/61f+q8Z0aq784io1LLobHTOgOLU6L0KyekRqDms=";

        var data = Convert.FromBase64String(encodedString);

        var hmac = data.Take(32).ToArray();

        var cipherAndIV = data.Skip(32).ToArray();

        var iv = cipherAndIV.Take(16).ToArray();

        var cipher = cipherAndIV.Skip(16).ToArray();

​

        var mac = new HMACSHA256(Convert.FromBase64String(hmacKey));

        var computedMac = mac.ComputeHash(cipherAndIV);

        bool err = false;

        Console.WriteLine(Convert.ToBase64String(hmac));

        Console.WriteLine(Convert.ToBase64String(computedMac));

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

        {

            if (computedMac[i] != hmac[i])

            {

                err = true;

            }

        }

        if (err)

        {

            Console.WriteLine("Hash values differ! Signed file has been tampered with!");

        }

        else

        {

            Console.WriteLine("Hash values agree -- no tampering occurred.");

        }

​

        var aesAlg = Aes.Create("aes-256-cbc");

        aesAlg.Key = Convert.FromBase64String(encryptionKey);

        aesAlg.IV = iv;

        aesAlg.Padding = PaddingMode.PKCS7;

        aesAlg.Mode = CipherMode.CBC;

        Console.WriteLine("KeySize=" + aesAlg.KeySize);

        Console.WriteLine("Blocksize=" + aesAlg.BlockSize);     

​

        String decryptedData = null;

        ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);

​

        using(MemoryStream ms = new MemoryStream(cipher))

        {

            using (CryptoStream csDecrypt = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))

            {

                using (StreamReader sw = new StreamReader(csDecrypt))

                {

                    sw.ReadToEnd();

                    sw.Close();

                }

​

                csDecrypt.Close();;

                decryptedData = Encoding.UTF8.GetString(ms.ToArray());

            }

        }

        Console.WriteLine("IV=" + Convert.ToBase64String(aesAlg.IV) + " Length: " + aesAlg.IV.Length);

        Console.WriteLine("MAC=" + Convert.ToBase64String(hmac) + " Length: " + hmac.Length);

        Console.WriteLine("Cipher=" + Convert.ToBase64String(cipher) + " Length: " + cipher.Length);

        Console.WriteLine("CipherAndIv=" + Convert.ToBase64String(cipherAndIV) + " Length: " + cipherAndIV.Length);

        Console.WriteLine(decryptedData);

    }

}