using System;

using System.IO;

using System.Security.Cryptography;

using System.Text;

public class Program

{

    public static void Main()

    {

        var key = Encoding.ASCII.GetBytes("0123456789abcdef");//Change the key.

        var data = Encrypt("Hello World",key);

        var str = Decrypt(data, key);

        Console.WriteLine(str);

    }

​

    public static byte[] Encrypt(string plaintext, byte[] key)

    {

        using(var desObj = Rijndael.Create())

        {

            desObj.Key = key;

            desObj.Mode = CipherMode.CFB;

            desObj.Padding = PaddingMode.PKCS7;

            using(var ms = new MemoryStream())

            {

                //Append the random IV that was generated to the front of the stream.

                ms.Write(desObj.IV, 0, desObj.IV.Length);

                //Write the bytes to be encrypted.

                using(CryptoStream cs = new CryptoStream(ms, desObj.CreateEncryptor(), CryptoStreamMode.Write))

                {

                    var plainTextBytes = Encoding.UTF8.GetBytes(plaintext);

                    cs.Write(plainTextBytes, 0, plainTextBytes.Length);

                }

                return ms.ToArray();

            }

        }

    }

    public static string Decrypt(byte[] cyphertext, byte[] key)

    {

        using(MemoryStream ms = new MemoryStream(cyphertext))

        using(var desObj = Rijndael.Create())

        {

            desObj.Key = key;

            desObj.Mode = CipherMode.CFB;       

            desObj.Padding = PaddingMode.PKCS7;

            //Read the IV from the front of the stream and assign it to our object.

            var iv = new byte[16];

            var offset = 0;

            while(offset < iv.Length)

            {

                offset += ms.Read(iv, offset, iv.Length - offset);

            }

            desObj.IV = iv;

            //Read the bytes to be decrypted

            using(var cs = new CryptoStream(ms, desObj.CreateDecryptor(), CryptoStreamMode.Read))

            using(var sr = new StreamReader(cs, Encoding.UTF8))

            {

                return sr.ReadToEnd();

            }

        }

    }

}