source: binary-improvements/MapRendering/Web/OpenID.cs@ 261

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.IO;
using System.Net;
using System.Net.Security;
using System.Text;
using System.Text.RegularExpressions;

namespace AllocsFixes.NetConnections.Servers.Web
{
        public static class OpenID {
                private const string STEAM_LOGIN = "https://steamcommunity.com/openid/login";
                private static Regex steamIdUrlMatcher = new Regex (@"^http:\/\/steamcommunity\.com\/openid\/id\/([0-9]{17,18})");

                static OpenID () {
                        ServicePointManager.ServerCertificateValidationCallback = (srvPoint, certificate, chain, errors) => {
                                if (errors == SslPolicyErrors.None)
                                        return true;

                                Log.Out ("Steam certificate error: {0}", errors);

                                return true;
                        };

                }

                public static string GetOpenIdLoginUrl (string _returnHost, string _returnUrl) {
                        Dictionary<string, string> queryParams = new Dictionary<string, string> ();

                        queryParams.Add ("openid.ns", "http://specs.openid.net/auth/2.0");
                        queryParams.Add ("openid.mode", "checkid_setup");
                        queryParams.Add ("openid.return_to", _returnUrl);
                        queryParams.Add ("openid.realm", _returnHost);
                        queryParams.Add ("openid.identity", "http://specs.openid.net/auth/2.0/identifier_select");
                        queryParams.Add ("openid.claimed_id", "http://specs.openid.net/auth/2.0/identifier_select");

                        return STEAM_LOGIN + '?' + buildUrlParams (queryParams);
                }

                public static ulong Validate (HttpListenerRequest _req) {
                        if (getValue (_req, "openid.mode") == "cancel") {
                                Log.Warning ("Steam OpenID login canceled");
                                return 0;
                        }
                        string steamIdString = getValue (_req, "openid.claimed_id");
                        ulong steamId = 0;
                        Match steamIdMatch = steamIdUrlMatcher.Match (steamIdString);
                        if (steamIdMatch.Success) {
                                steamId = ulong.Parse (steamIdMatch.Groups [1].Value);
                        } else {
                                Log.Warning ("Steam OpenID login result did not give a valid SteamID");
                                return 0;
                        }

                        Dictionary<string, string> queryParams = new Dictionary<string, string> ();

                        queryParams.Add ("openid.ns", "http://specs.openid.net/auth/2.0");

                        queryParams.Add ("openid.assoc_handle", getValue (_req, "openid.assoc_handle"));
                        queryParams.Add ("openid.signed", getValue (_req, "openid.signed"));
                        queryParams.Add ("openid.sig", getValue (_req, "openid.sig"));
                        queryParams.Add ("openid.identity", "http://specs.openid.net/auth/2.0/identifier_select");
                        queryParams.Add ("openid.claimed_id", "http://specs.openid.net/auth/2.0/identifier_select");

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

                        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 = null;
                        using (Stream st = response.GetResponseStream ()) {
                                using (StreamReader str = new StreamReader (st)) {
                                        responseString = str.ReadToEnd ();
                                }
                        }

                        if (responseString.ToLower ().Contains ("is_valid:true")) {
                                return steamId;
                        } else {
                                Log.Warning ("Steam OpenID login failed: {0}", responseString);
                                return 0;
                        }
                }

                private static string buildUrlParams (Dictionary<string, string> _queryParams) {
                        string[] paramsArr = new string[_queryParams.Count];
                        int i = 0;
                        foreach (KeyValuePair<string, string> kvp in _queryParams) {
                                paramsArr [i++] = kvp.Key + "=" + Uri.EscapeDataString (kvp.Value);
                        }
                        return string.Join ("&", paramsArr);
                }

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

        }
}