Program.cs

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

namespace Encrypt_Decrypt
{
    class Program
    {
        private static byte[] _salt = Encoding.ASCII.GetBytes("VgZuNXSr9wKXOMuvTVbHiewRHUezlJ");

        public static string EncryptStringAES(string plainText, string sharedSecret)
        {
            if (string.IsNullOrEmpty(plainText))
                throw new ArgumentNullException("plainText");
            if (string.IsNullOrEmpty(sharedSecret))
                throw new ArgumentNullException("sharedSecret");

            string outStr = null;
            RijndaelManaged aesAlg = null;

            try
            {

                Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedSecret, _salt);


                aesAlg = new RijndaelManaged();
                aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8);

                ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);

                using (MemoryStream msEncrypt = new MemoryStream())
                {
                    msEncrypt.Write(BitConverter.GetBytes(aesAlg.IV.Length), 0, sizeof(int));
                    msEncrypt.Write(aesAlg.IV, 0, aesAlg.IV.Length);
                    using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                    {
                        using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                        {

                            swEncrypt.Write(plainText);
                        }
                    }
                    outStr = Convert.ToBase64String(msEncrypt.ToArray());
                }
            }
            finally
            {

                if (aesAlg != null)
                    aesAlg.Clear();
            }
            return outStr;
        }

        public static string DecryptStringAES(string cipherText, string sharedSecret)
        {
            if (string.IsNullOrEmpty(cipherText))
                throw new ArgumentNullException("cipherText");
            if (string.IsNullOrEmpty(sharedSecret))
                throw new ArgumentNullException("sharedSecret");

            RijndaelManaged aesAlg = null;

            string plaintext = null;

            try
            {

                Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedSecret, _salt);


                byte[] bytes = Convert.FromBase64String(cipherText);
                using (MemoryStream msDecrypt = new MemoryStream(bytes))
                {

                    aesAlg = new RijndaelManaged();
                    aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8);
                    aesAlg.IV = ReadByteArray(msDecrypt);
                    ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
                    using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                    {
                        using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                            plaintext = srDecrypt.ReadToEnd();
                    }
                }
            }
            finally
            {
                if (aesAlg != null)
                    aesAlg.Clear();
            }

            return plaintext;
        }

        private static byte[] ReadByteArray(Stream s)
        {
            byte[] rawLength = new byte[sizeof(int)];
            if (s.Read(rawLength, 0, rawLength.Length) != rawLength.Length)
            {
                throw new SystemException("Stream did not contain properly formatted byte array");
            }

            byte[] buffer = new byte[BitConverter.ToInt32(rawLength, 0)];
            if (s.Read(buffer, 0, buffer.Length) != buffer.Length)
            {
                throw new SystemException("Did not read byte array properly");
            }

            return buffer;
        }
        static void Main(string[] args)
        {
            string input, encrypted, decrypted, key = "";
            Console.WriteLine("Bemenet: ");
            input = Console.ReadLine();
            Console.WriteLine("Kulcs: ");
            key = Console.ReadLine();
            encrypted = EncryptStringAES(input, key);
            Console.WriteLine(encrypted);

            decrypted = DecryptStringAES(encrypted, key);
            Console.WriteLine(decrypted);
            Console.ReadLine();
            Main(args);
        }
    }
}