source: TFP-WebServer/WebServer/src/OpenID.cs@ 442

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.IO;
using System.Net;
using System.Net.Security;
using System.Reflection;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Text.RegularExpressions;
using HttpListenerRequest = SpaceWizards.HttpListener.HttpListenerRequest;

namespace Webserver {
        public static class OpenID {
                private const string STEAM_LOGIN = "https://steamcommunity.com/openid/login";

                private static readonly Regex steamIdUrlMatcher =
                        new Regex (@"^https?:\/\/steamcommunity\.com\/openid\/id\/([0-9]{17,18})");

                private static readonly X509Certificate2 caCert =
                        new X509Certificate2 ($"{Path.GetDirectoryName (Assembly.GetExecutingAssembly ().Location)}/steam-rootca.cer");

                private static readonly X509Certificate2 caIntermediateCert =
                        new X509Certificate2 ($"{Path.GetDirectoryName (Assembly.GetExecutingAssembly ().Location)}/steam-intermediate.cer");

                private const bool verboseSsl = false;
                public static bool debugOpenId;

                static OpenID () {
                        for (int i = 0; i < Environment.GetCommandLineArgs ().Length; i++) {
                                if (Environment.GetCommandLineArgs () [i].EqualsCaseInsensitive ("-debugopenid")) {
                                        debugOpenId = true;
                                }
                        }

                        ServicePointManager.ServerCertificateValidationCallback = (_srvPoint, _certificate, _chain, _errors) => {
                                if (_errors == SslPolicyErrors.None) {
                                        if (verboseSsl) {
                                                Log.Out ("[OpenID] Steam certificate: No error (1)");
                                        }

                                        return true;
                                }

                                X509Chain privateChain = new X509Chain ();
                                privateChain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;

                                privateChain.ChainPolicy.ExtraStore.Add (caCert);
                                privateChain.ChainPolicy.ExtraStore.Add (caIntermediateCert);

                                if (privateChain.Build (new X509Certificate2 (_certificate))) {
                                        // No errors, immediately return
                                        privateChain.Reset ();
                                        if (verboseSsl) {
                                                Log.Out ("Steam certificate: No error (2)");
                                        }

                                        return true;
                                }

                                if (privateChain.ChainStatus.Length == 0) {
                                        // No errors, immediately return
                                        privateChain.Reset ();
                                        if (verboseSsl) {
                                                Log.Out ("Steam certificate: No error (3)");
                                        }

                                        return true;
                                }

                                // Iterate all chain elements
                                foreach (X509ChainElement chainEl in privateChain.ChainElements) {
                                        if (verboseSsl) {
                                                Log.Out ($"Validating cert: {chainEl.Certificate.Subject}");
                                        }

                                        // Iterate all status flags of the current cert
                                        foreach (X509ChainStatus chainStatus in chainEl.ChainElementStatus) {
                                                if (verboseSsl) {
                                                        Log.Out ($"   Status: {chainStatus.Status}");
                                                }

                                                if (chainStatus.Status == X509ChainStatusFlags.NoError) {
                                                        // This status is not an error, skip
                                                        continue;
                                                }

                                                if (chainStatus.Status == X509ChainStatusFlags.UntrustedRoot && chainEl.Certificate.Equals (caCert)) {
                                                        // This status is about the cert being an untrusted root certificate but the certificate is one of those we added, ignore
                                                        continue;
                                                }

                                                // This status is an error, print information
                                                Log.Warning ($"[OpenID] Steam certificate error: {chainEl.Certificate.Subject} ### Error: {chainStatus.Status}");
                                                privateChain.Reset ();
                                                return false;
                                        }
                                }

                                foreach (X509ChainStatus chainStatus in privateChain.ChainStatus) {
                                        if (chainStatus.Status != X509ChainStatusFlags.NoError &&
                                            chainStatus.Status != X509ChainStatusFlags.UntrustedRoot) {
                                                Log.Warning ($"[OpenID] Steam certificate error: {chainStatus.Status}");
                                                privateChain.Reset ();
                                                return false;
                                        }
                                }

                                // We didn't find any errors, chain is valid
                                privateChain.Reset ();
                                if (verboseSsl) {
                                        Log.Out ("[OpenID] Steam certificate: No error (4)");
                                }

                                return true;
                        };
                }

                public static string GetOpenIdLoginUrl (string _returnHost, string _returnUrl) {
                        Dictionary<string, string> queryParams = new Dictionary<string, string> {
                                { "openid.ns", "http://specs.openid.net/auth/2.0" },
                                { "openid.mode", "checkid_setup" },
                                { "openid.return_to", _returnUrl },
                                { "openid.realm", _returnHost },
                                { "openid.identity", "http://specs.openid.net/auth/2.0/identifier_select" },
                                { "openid.claimed_id", "http://specs.openid.net/auth/2.0/identifier_select" }
                        };

                        return $"{STEAM_LOGIN}?{buildUrlParams (queryParams)}";
                }

                public static ulong Validate (HttpListenerRequest _req) {
                        string mode = getValue (_req, "openid.mode");
                        if (mode == "cancel") {
                                Log.Warning ("[OpenID] Steam OpenID login canceled");
                                return 0;
                        }

                        if (mode == "error") {
                                Log.Warning ($"[OpenID] Steam OpenID login error: {getValue (_req, "openid.error")}");
                                if (debugOpenId) {
                                        PrintOpenIdResponse (_req);
                                }

                                return 0;
                        }

                        string steamIdString = getValue (_req, "openid.claimed_id");
                        ulong steamId;
                        Match steamIdMatch = steamIdUrlMatcher.Match (steamIdString);
                        if (steamIdMatch.Success) {
                                steamId = ulong.Parse (steamIdMatch.Groups [1].Value);
                        } else {
                                Log.Warning ("[OpenID] Steam OpenID login result did not give a valid SteamID");
                                if (debugOpenId) {
                                        PrintOpenIdResponse (_req);
                                }

                                return 0;
                        }

                        Dictionary<string, string> queryParams = new Dictionary<string, string> {
                                { "openid.ns", "http://specs.openid.net/auth/2.0" },
                                { "openid.assoc_handle", getValue (_req, "openid.assoc_handle") },
                                { "openid.signed", getValue (_req, "openid.signed") },
                                { "openid.sig", getValue (_req, "openid.sig") },
                                { "openid.identity", "http://specs.openid.net/auth/2.0/identifier_select" },
                                { "openid.claimed_id", "http://specs.openid.net/auth/2.0/identifier_select" }
                        };

                        string[] signeds = getValue (_req, "openid.signed").Split (',');
                        foreach (string s in signeds) {
                                string name = $"openid.{s}";
                                queryParams [name] = getValue (_req, name);
                        }

                        queryParams.Add ("openid.mode", "check_authentication");

                        byte[] postData = Encoding.ASCII.GetBytes (buildUrlParams (queryParams));
                        HttpWebRequest request = (HttpWebRequest) WebRequest.Create (STEAM_LOGIN);
                        request.Method = "POST";
                        request.ContentType = "application/x-www-form-urlencoded";
                        request.ContentLength = postData.Length;
                        request.Headers.Add (HttpRequestHeader.AcceptLanguage, "en");
                        using (Stream st = request.GetRequestStream ()) {
                                st.Write (postData, 0, postData.Length);
                        }

                        HttpWebResponse response = (HttpWebResponse) request.GetResponse ();
                        string responseString;
                        using (Stream st = response.GetResponseStream ()) {
                                using (StreamReader str = new StreamReader (st)) {
                                        responseString = str.ReadToEnd ();
                                }
                        }

                        if (responseString.ContainsCaseInsensitive ("is_valid:true")) {
                                return steamId;
                        }

                        Log.Warning ($"[OpenID] Steam OpenID login failed: {responseString}");
                        return 0;
                }

                private static string buildUrlParams (Dictionary<string, string> _queryParams) {
                        string[] paramsArr = new string[_queryParams.Count];
                        int i = 0;
                        foreach ((string argName, string argValue) in _queryParams) {
                                paramsArr [i++] = $"{argName}={Uri.EscapeDataString (argValue)}";
                        }

                        return string.Join ("&", paramsArr);
                }

                private static string getValue (HttpListenerRequest _req, string _name) {
                        NameValueCollection nvc = _req.QueryString;
                        if (nvc [_name] == null) {
                                throw new MissingMemberException ($"[OpenID] OpenID parameter \"{_name}\" missing");
                        }

                        return nvc [_name];
                }

                private static void PrintOpenIdResponse (HttpListenerRequest _req) {
                        NameValueCollection nvc = _req.QueryString;
                        for (int i = 0; i < nvc.Count; i++) {
                                Log.Out ($"   {nvc.GetKey (i)} = {nvc [i]}");
                        }
                }
        }
}