using System;

using System.Security.Cryptography;

​

public class Program

{

    public static void Main()

    {

        using (var rijndael = Rijndael.Create())

        {

            rijndael.KeySize = 256;

            rijndael.GenerateKey();

            var keyBase64 = Convert.ToBase64String(rijndael.Key);

            Console.WriteLine(keyBase64);

        }

    }

}