Federated authentication in Sharepoint 2013: getting rtFa and FedAuth cookies

In fact, only FedAuth cookie is mandatory when it comes to SharePoint Online/Office 365 authentication.

According to Remote Authentication in SharePoint Online Using Claims-Based Authentication:

The FedAuth cookies enable federated authorization, and the rtFA cookie enables signing out the user from all SharePoint sites, even if the sign-out process starts from a non-SharePoint site.

So, it is enough to provide SPOIDCRL HTTP header in order to perform authentication in SharePoint Online/Office 365, for example:

var request = (HttpWebRequest)WebRequest.Create(endpointUri);
var credentials = new SharePointOnlineCredentials(userName,securePassword);
var authCookie = credentials.GetAuthenticationCookie(webUri);
request.Headers.Add(HttpRequestHeader.Cookie, authCookie);

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The following examples demonstrates how to perform active authentication in SharePointOnline/Office 365 by providing FedAuth cookie.

Example 1: Retrieve FormDigest via SharePoint 2013 REST API (uisng MsOnlineClaimsHelper class)

public static string GetFormDigest(Uri webUri, string userName, string password)
{
   var claimsHelper = new MsOnlineClaimsHelper(webUri, userName, password);
   var endpointUri = new Uri(webUri,"/_api/contextinfo");
   var request = (HttpWebRequest)WebRequest.Create(endpointUri);
   request.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
   request.Method = WebRequestMethods.Http.Post;
   request.Accept = "application/json;odata=verbose";
   request.ContentType = "application/json;odata=verbose";
   request.ContentLength = 0;

   var fedAuthCookie = claimsHelper.CookieContainer.GetCookieHeader(webUri); //FedAuth are getting here
   request.Headers.Add(HttpRequestHeader.Cookie, fedAuthCookie); //only FedAuth cookie are provided here
   //request.CookieContainer = claimsHelper.CookieContainer;
   using (var response = (HttpWebResponse) request.GetResponse())
   {
        using (var streamReader = new StreamReader(response.GetResponseStream()))
        {
                var content = streamReader.ReadToEnd();
                var t = JToken.Parse(content);
                return t["d"]["GetContextWebInformation"]["FormDigestValue"].ToString();
        }     
    }
}

Example 2: Retrieve FormDigest via SharePoint 2013 REST API (using SharePointOnlineCredentials class)

public static string GetFormDigest(Uri webUri, string userName, string password)
{
   var endpointUri = new Uri(webUri, "/_api/contextinfo");
   var request = (HttpWebRequest)WebRequest.Create(endpointUri);
   request.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
   request.Method = WebRequestMethods.Http.Post;
   request.Accept = "application/json;odata=verbose";
   request.ContentType = "application/json;odata=verbose";
   request.ContentLength = 0;

   var securePassword = new SecureString();
   foreach (char c in password)
   {
       securePassword.AppendChar(c);
   }
   request.Credentials = new SharePointOnlineCredentials(userName,securePassword);

   using (var response = (HttpWebResponse)request.GetResponse())
   {
       using (var streamReader = new StreamReader(response.GetResponseStream()))
       {
           var content = streamReader.ReadToEnd();
           var t = JToken.Parse(content);
           return t["d"]["GetContextWebInformation"]["FormDigestValue"].ToString();
        }
   }
}

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Update

The modified version of the example for downloading a file:

public static async Task<byte[]> DownloadFile(Uri webUri,string userName,string password, string relativeFileUrl, CancellationToken ct, TimeSpan? timeout = null)
{
        try
        {

            var securePassword = new SecureString();
            foreach (var c in password)
            {
                securePassword.AppendChar(c);
            }
            var credentials = new SharePointOnlineCredentials(userName, securePassword);
            var authCookie = credentials.GetAuthenticationCookie(webUri);
            var fedAuthString = authCookie.TrimStart("SPOIDCRL=".ToCharArray());
            var cookieContainer = new CookieContainer();
            cookieContainer.Add(webUri, new Cookie("SPOIDCRL", fedAuthString));


            HttpClientHandler handler = new HttpClientHandler();
            handler.CookieContainer = cookieContainer;

            HttpClient _client = new HttpClient(handler);
            if (timeout.HasValue)
                _client.Timeout = timeout.Value;
            ct.ThrowIfCancellationRequested();

            var fileUrl = new Uri(webUri, relativeFileUrl);
            var resp = await _client.GetAsync(fileUrl);
            var result = await resp.Content.ReadAsByteArrayAsync();
            if (!resp.IsSuccessStatusCode)
                return null;
            return result;
        }
        catch (Exception) { return null; }
 }