using System;

using System.IO;

using System.Security.Cryptography;

using System.Text;

public class Program

{

    public static void Main()

    {

        string str = string.Empty;

        str = DecryptString("BQO5l5Kj9MdErXx6Q6AGOw==", "c4scadek3y654321");

        Console.WriteLine(str);

    }

    public static string DecryptString(string EncryptedString, string Key)

    {

      byte[] buffer = Convert.FromBase64String(EncryptedString);

      Aes aes = Aes.Create();

      ((SymmetricAlgorithm) aes).KeySize = 128;

      ((SymmetricAlgorithm) aes).BlockSize = 128;

      ((SymmetricAlgorithm) aes).IV = Encoding.UTF8.GetBytes("1tdyjCbY1Ix49842");

      ((SymmetricAlgorithm) aes).Mode = CipherMode.CBC;

      ((SymmetricAlgorithm) aes).Key = Encoding.UTF8.GetBytes(Key);

      using (MemoryStream memoryStream = new MemoryStream(buffer))

      {

        using (CryptoStream cryptoStream = new CryptoStream((Stream) memoryStream, ((SymmetricAlgorithm) aes).CreateDecryptor(), CryptoStreamMode.Read))

        {

          byte[] numArray = new byte[checked (buffer.Length - 1 + 1)];

          cryptoStream.Read(numArray, 0, numArray.Length);

          return Encoding.UTF8.GetString(numArray);

        }

      }

    }

}