A simple program to encrypt and decrypt a file using AES-GCM and Bouncy Castle

AES-GCM-File.cs

using System;
using System.IO;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Modes;
using Org.BouncyCastle.Crypto.Parameters;


namespace TestConsole
{
    class Program
    {
        static void Main()
        {
            // most of the code is from https://stackoverflow.com/a/26434031/4213397
            // FUCKED UP security; Use RNGCryptoServiceProvider
            Random rnd = new Random();
            var key = new byte[16];
            var nonce = new byte[12];
            rnd.NextBytes(key);
            rnd.NextBytes(nonce);

            using (Stream  sr = File.OpenRead("words.txt"))
            {
                using (Stream sw = File.OpenWrite("words.aes"))
                {
                    var cipher = new GcmBlockCipher(new AesEngine());
                    cipher.Init(true,new AeadParameters(new KeyParameter(key), 128, nonce));
                    byte[] input = new byte[8192];
                    byte[] output = new byte[cipher.GetBlockSize() +  cipher.GetOutputSize (input.Length)];
                    int bytesRead;
                    do
                    {
                        bytesRead = sr.Read(input, 0, input.Length);
                        if (bytesRead > 0)
                        {
                            int count = cipher.ProcessBytes(input, 0, bytesRead, output, 0);
                            sw.Write(output,0,count);
                        }
                    } while (bytesRead > 0);

                    int lastSize = cipher.DoFinal(output, 0);
                    sw.Write(output,0,lastSize);
                }
            }

            using (Stream sr = File.OpenRead("words.aes"))
            {
                using (Stream sw = File.OpenWrite("words.final.txt"))
                {
                    var cipher = new GcmBlockCipher(new AesEngine());
                    cipher.Init(false,new AeadParameters(new KeyParameter(key), 128, nonce));
                    byte[] input = new byte[8192];
                    byte[] output = new byte[cipher.GetBlockSize() +  cipher.GetOutputSize (input.Length)];
                    int bytesRead;
                    do
                    {
                        bytesRead = sr.Read(input, 0, input.Length);
                        if (bytesRead > 0)
                        {
                            int count = cipher.ProcessBytes(input, 0, bytesRead, output, 0);
                            sw.Write(output,0,count);
                        }
                    } while (bytesRead > 0);

                    int lastSize = cipher.DoFinal(output, 0);
                    sw.Write(output,0,lastSize);
                }
            }
        }
    }
}