Program.cs

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

namespace Aes_Example
{
    class AesExample
    {
        public static void Main()
        {

            // opens and reads target message from a file
            string original = string.Join("", System.IO.File.ReadAllLines(@"C:\git\AESCBC\small.txt"));

            // Create a new instance of the Aes
            // class.  This generates a new key and initialization
            // vector (IV).
            using (Aes myAes = Aes.Create())
            {

                long bestEncrypt = 999999;
                long bestDecrypt = 999999;
                
                // run the cipher 20 times to get a good minimum
                for(int i = 0; i < 20; i++)
                {
                    var watch = new System.Diagnostics.Stopwatch();
                    
                    // time the encryption
                    watch.Start();
                    byte[] encrypted = EncryptStringToBytes_Aes(original, myAes.Key, myAes.IV);
                    watch.Stop();
                    // check if it is the best time
                    if (watch.ElapsedMilliseconds < bestEncrypt)
                    {
                        bestEncrypt = watch.ElapsedMilliseconds;
                    }

                    Console.WriteLine($"encrypt Time: {watch.ElapsedMilliseconds} ms");


                    watch = new System.Diagnostics.Stopwatch();
                    
                    //time the decryption
                    watch.Start();
                    string roundtrip = DecryptStringFromBytes_Aes(encrypted, myAes.Key, myAes.IV);
                    watch.Stop();

                    Console.WriteLine($"encrypt Time: {watch.ElapsedMilliseconds} ms");

                    if (watch.ElapsedMilliseconds < bestDecrypt)
                    {
                        bestDecrypt = watch.ElapsedMilliseconds;
                    }
                }
                Console.WriteLine("best Encrypt TIme {0}", bestEncrypt);
                Console.WriteLine("best Decrypt TIme {0}", bestDecrypt);


            }
        }
        static byte[] EncryptStringToBytes_Aes(string plainText, byte[] Key, byte[] IV)
        {
            // Check arguments.
            if (plainText == null || plainText.Length <= 0)
                throw new ArgumentNullException("plainText");
            if (Key == null || Key.Length <= 0)
                throw new ArgumentNullException("Key");
            if (IV == null || IV.Length <= 0)
                throw new ArgumentNullException("IV");
            byte[] encrypted;

            // Create an Aes object
            // with the specified key and IV.
            using (Aes aesAlg = Aes.Create())
            {
                aesAlg.Key = Key;
                aesAlg.IV = IV;

                // Create an encryptor to perform the stream transform.
                ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);

                // Create the streams used for encryption.
                using (MemoryStream msEncrypt = new MemoryStream())
                {
                    using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                    {
                        using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                        {
                            //Write all data to the stream.
                            swEncrypt.Write(plainText);
                        }
                        encrypted = msEncrypt.ToArray();
                    }
                }
            }

            // Return the encrypted bytes from the memory stream.
            return encrypted;
        }

        static string DecryptStringFromBytes_Aes(byte[] cipherText, byte[] Key, byte[] IV)
        {
            // Check arguments.
            if (cipherText == null || cipherText.Length <= 0)
                throw new ArgumentNullException("cipherText");
            if (Key == null || Key.Length <= 0)
                throw new ArgumentNullException("Key");
            if (IV == null || IV.Length <= 0)
                throw new ArgumentNullException("IV");

            // Declare the string used to hold
            // the decrypted text.
            string plaintext = null;

            // Create an Aes object
            // with the specified key and IV.
            using (Aes aesAlg = Aes.Create())
            {
                aesAlg.Key = Key;
                aesAlg.IV = IV;

                // Create a decryptor to perform the stream transform.
                ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);

                // Create the streams used for decryption.
                using (MemoryStream msDecrypt = new MemoryStream(cipherText))
                {
                    using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                    {
                        using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                        {

                            // Read the decrypted bytes from the decrypting stream
                            // and place them in a string.
                            plaintext = srDecrypt.ReadToEnd();
                        }
                    }
                }
            }
            return plaintext;
        }
    }
}