Utils.cs

using System;
using System.IO;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Security.Cryptography;
using System.Text;
using Google.Apis.Util.Store;

namespace GoogleAssistantWindows
{
    public class Utils
    {
        public static string GetDataStoreFolder()
        {
            return new FileDataStore(Const.Folder).FolderPath + "\\";
        }

        public static bool HasTokenFile() => GetTokenFile() != null;

        public static string GetTokenFile()
        {
            // Can't find a nicer way in the OAuth2 Lib to see if we've ever authorised, so look for the datastore file 
            return Directory.EnumerateFiles(GetDataStoreFolder()).FirstOrDefault(file => file.Contains($"-{Const.User}"));
        }

        public static void GetImage(string url, string imageFile)
        {
            using (WebClient webClient = new WebClient())
            {
                webClient.DownloadFile(new Uri(url), imageFile);
            }
        }

        // ref http://stackoverflow.com/a/3978040
        public static int GetRandomUnusedPort()
        {
            var listener = new TcpListener(IPAddress.Loopback, 0);
            listener.Start();
            var port = ((IPEndPoint)listener.LocalEndpoint).Port;
            listener.Stop();
            return port;
        }

        /// <summary>
        /// Returns URI-safe data with a given input length.
        /// </summary>
        /// <param name="length">Input length (nb. output will be longer)</param>
        /// <returns></returns>
        public static string RandomDataBase64Url(uint length)
        {
            RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
            byte[] bytes = new byte[length];
            rng.GetBytes(bytes);
            return Base64UrlEncodeNoPadding(bytes);
        }

        /// <summary>
        /// Returns the SHA256 hash of the input string.
        /// </summary>
        /// <param name="inputStirng"></param>
        /// <returns></returns>
        public static byte[] Sha256(string inputStirng)
        {
            byte[] bytes = Encoding.ASCII.GetBytes(inputStirng);
            SHA256Managed sha256 = new SHA256Managed();
            return sha256.ComputeHash(bytes);
        }

        /// <summary>
        /// Base64url no-padding encodes the given input buffer.
        /// </summary>
        /// <param name="buffer"></param>
        /// <returns></returns>
        public static string Base64UrlEncodeNoPadding(byte[] buffer)
        {
            string base64 = Convert.ToBase64String(buffer);

            // Converts base64 to base64url.
            base64 = base64.Replace("+", "-");
            base64 = base64.Replace("/", "_");
            // Strips padding.
            base64 = base64.Replace("=", "");

            return base64;
        }        
    }
}