Ex, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Security.Cryptography;
using System.Text;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string encryptedBase64 = "5qG6aB4UZ6bnbbPgFhnC+qp/FJ7ZuZ+fg1cYm+OUM1uP/6PyWfLg0w5bJstmBf2W";
            string key = "745d88b1e8f75d320c2bd9198c08485d";
            var helper = new AESHelper(key, null);
            var result = helper.decrypt(encryptedBase64);
            Console.WriteLine(result);
        }

public class AESHelper
        {
            private AesCryptoServiceProvider _aes;
            private ICryptoTransform _crypto;

public AESHelper(string key, string IV)
            {
                _aes = new AesCryptoServiceProvider();
                _aes.BlockSize = 128;
                _aes.KeySize = 256;
                _aes.Key = ASCIIEncoding.ASCII.GetBytes(key);
                if (!string.IsNullOrEmpty(IV))
                {
                    _aes.IV = ASCIIEncoding.ASCII.GetBytes(IV);
                }
                else
                {
                    _aes.IV = new byte[_aes.BlockSize / 8];
                }

_aes.Padding = PaddingMode.PKCS7;
                _aes.Mode = CipherMode.CBC;
            }

public string encrypt(string message)
            {
                _crypto = _aes.CreateEncryptor(_aes.Key, _aes.IV);
                byte[] encrypted = _crypto.TransformFinalBlock(
                    ASCIIEncoding.ASCII.GetBytes(message), 0, ASCIIEncoding.ASCII.GetBytes(message).Length);
                _crypto.Dispose();
                return System.Convert.ToBase64String(encrypted);
            }

public string decrypt(string message)
            {
                _crypto = _aes.CreateDecryptor(_aes.Key, _aes.IV);
                byte[] decrypted = _crypto.TransformFinalBlock(
                    System.Convert.FromBase64String(message), 0, System.Convert.FromBase64String(message).Length);
                _crypto.Dispose();
                return ASCIIEncoding.ASCII.GetString(decrypted);
            }
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,22 sec, absolute running time: 0,11 sec, cpu time: 0,11 sec, average memory usage: 15 Mb, average nr of threads: 3, absolute service time: 0,38 sec