using System;

using System.Collections.Generic;

using System.IO;

using System.Linq;

using System.Security.Cryptography;

using System.Text;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

          //string text = "  Hello World  ";

            string key = "44 52 d7 16 87 b6 bc 2c 93 89 c3 34 9f dc 17 fb 3d fb ba 62 24 af fb 76 76 e1 33 79 26 cd d6 02";

​

            //string encryptedText = Encryption.encryptData(text, key);

        string decryptedText = Encryption.decryptData(@"1211ad43f08caead497049804667ad202aa7822a6574e2be1caaf965b754417f4fc76a932de8add2725b52bc60757aee102cae4e674b283ef462251636d872e8a98e6b45bbe07d5f7a48ddd37e777ec3d6f6a60414fb93e7351a2eef42457f33f6402600e06ac78a012bb5060f712b48", key);

        Console.WriteLine(decryptedText);

    }

}

​

public class Encryption

    {

        public static string generateRandomHex(int byteLength)

        {

            int stringLength = byteLength * 2;

​

            string alphabet = "abcdef0123456789";

            string s = "";

​

            Random rnd = new Random();

​

            for (int i = 0; i < stringLength; i++)

            {

                int r = rnd.Next(0, alphabet.Length);

​

                s += alphabet[r];

            }

​

            //prevent NULL byte block.

            s = s.Replace("00", "11");

​

            return s;

        }

​

        private static byte[] dataFromHexString(string hexString)

        {

            hexString = hexString.Trim();

            hexString = hexString.Replace(" ", "");

​

            byte[] data = Enumerable.Range(0, hexString.Length / 2)

                                    .Select(x => Convert.ToByte(hexString.Substring(x * 2, 2), 16))

                                    .ToArray();

​

            return data;

        }

​

        private static string dataToHexString(byte[] data)

        {

            string hexString = String.Concat(Array.ConvertAll(data, x => x.ToString("X2")));

​

            return hexString;

        }

​

        public static string encryptData(string text, string hexKey)

        {

            checkKey(hexKey);

​

            //generate random IV (16 bytes)

            string hexIV = generateRandomHex(16);

            //convert plainText to hex string.

            byte[] bytesData = System.Text.Encoding.UTF8.GetBytes(text);

            string hexStr = dataToHexString(bytesData);

​

            string cipherHexStr = __encryptData(hexStr, hexKey, hexIV);

​

            string hmacHexKey = generateRandomHex(16);

            string hmacHexStr = Encryption.__computeHMAC(hexIV, cipherHexStr, hexKey, hmacHexKey);

​

            string encryptedHexStr = hexIV + hmacHexKey + hmacHexStr + cipherHexStr;

            return encryptedHexStr;

        }

​

        public static string decryptData(string hexStr, string hexKey)

        {

            checkKey(hexKey);

            string plainText = null;

​

            if (hexStr.Length > 128)

            {

                string hexIV = hexStr.Substring(0, 32);

                string hmacHexKey = hexStr.Substring(32, 32);

                string hmacHexStr = hexStr.Substring(64, 64);

                string cipherHexStr = hexStr.Substring(128);

​

                string computedHmacHexStr = Encryption.__computeHMAC(hexIV, cipherHexStr, hexKey, hmacHexKey);

​

                if (computedHmacHexStr.ToLower() == hmacHexStr.ToLower())

                {

                    string decryptedStr = __decryptData(cipherHexStr, hexKey, hexIV);

​

                    byte[] data = dataFromHexString(decryptedStr);

                    plainText = System.Text.Encoding.UTF8.GetString(data);

                }

            }

​

            return plainText;

        }

​

        private static void checkKey(string hexKey)

        {

            hexKey = hexKey.Trim();

            hexKey = hexKey.Replace(" ", "");

            hexKey = hexKey.ToLower();

​

            if(hexKey.Length != 64)

            {

                throw new Exception("key length is not 256 bit (64 hex characters)");

            }

​

            int i;

            for(i=0;i<hexKey.Length;i+=2)

            {

                if(hexKey[i] == '0' && hexKey[i+1] == '0')

                {

                    throw new Exception("key cannot contain zero byte block");

                }

            }

        }

​

        private static string __computeHMAC(string hexIV, string cipherHexStr, string hexKey, string hmacHexKey)

        {

            hexKey = hexKey.Trim();

            hexKey = hexKey.Replace(" ", "");

            hexKey = hexKey.ToLower();

​

            hmacHexKey = hmacHexKey.ToLower();

​

            string hexString = hexIV + cipherHexStr + hexKey;

            hexString = hexString.ToLower();

​

            byte[] data = System.Text.Encoding.UTF8.GetBytes(hexString);

            byte[] hmacKey = System.Text.Encoding.UTF8.GetBytes(hmacHexKey);

​

            HMACSHA256 hmac = new HMACSHA256(hmacKey);

            byte[] hashbytes = hmac.ComputeHash(data);

            string hashHexStr = Encryption.dataToHexString(hashbytes);

​

            return hashHexStr;

        }

​

        public static string __encryptData(string hexString, string hexKey, string hexIV)

        {

            byte[] data = dataFromHexString(hexString);

            byte[] key = dataFromHexString(hexKey);

            byte[] iv = dataFromHexString(hexIV);

​

            var aes = Aes.Create();

            aes.Mode = CipherMode.CBC;

            aes.Padding = PaddingMode.PKCS7;

            var encryptor = aes.CreateEncryptor(key, iv);

            MemoryStream memoryStream = new MemoryStream();

            CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);

            cryptoStream.Write(data, 0, data.Length);

            cryptoStream.FlushFinalBlock();

​

            byte[] encryptData = memoryStream.ToArray();

​

            string encryptHexData = dataToHexString(encryptData);

            return encryptHexData;

        }

​

        private static string __decryptData(string hexString, string hexKey, string hexIV)

        {

            byte[] data = dataFromHexString(hexString);

            byte[] key = dataFromHexString(hexKey);

            byte[] iv = dataFromHexString(hexIV);

            var aes = Aes.Create();

            aes.Mode = CipherMode.CBC;

            aes.Padding = PaddingMode.PKCS7;

            var decryptor = aes.CreateDecryptor(key, iv);

​

            MemoryStream memoryStream = new MemoryStream();

            CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Write);

            cryptoStream.Write(data, 0, data.Length);

            cryptoStream.FlushFinalBlock();

​

            byte[] decryptData = memoryStream.ToArray();

​

            string decryptHexData = dataToHexString(decryptData);

            return decryptHexData;

        }

    }