Over the last few days I've been rewriting this blog in ASP.NET MVC. As it gets closer to a state where I can upload it, I found myself needing to implement security for the administration section (adding, editing posts, etc.). I don't want yet another username/password to remember, and I don't want to IP-restrict it because that's not very flexible (and I don't know how static my IP is!), so what are my options?

OpenID

OpenID is nothing new, it's been around since late 2005. I've been aware of what it did and how it worked, but never really played with it. I did, however, get the impression it might solve my problem. Especially having seen that you can use your Google account as an OpenID!

What's OpenID? Why is it Cool?

OpenID is a standard for authentication, allowing you to use the same identitiy/login for multiple services. It is not the same as using the same username/password at multiple websites (that's a very bad idea). Let's see an example.

I want to be able to login to my blog to edit posts. I don't want another username/password. As Google now works as an identity provider, my blog can redirect me to Google and let them authenticate me. Google will then return me to my blog saying "Yes, this is definitely Danny Tuppeny". This means I don't need any user tables, login forms, or anything else on my blog!

This might sounds complicated, but as with most things, there's a nice .NET library called dotnetopenid to hide the complexity. Let's see some code!

On the first request, dotnetopenid will return a null response. After logging in at the identity providers website, the user will be redirected back (to the same page by default, but this can be changed) with a token on the query string. This will cause dotnetopenid to return a response. The basic code looks like this:

var openId = new OpenIdRelyingParty();

if (openId.Response == null)
{
	// No response means this is the first page load
}
else
{
	// This means we're been redirected back after authentication
	if (openId.Response.Status == AuthenticationStatus.Authenticated)
		// User was logged in (as someone!)
}

On the first page load, we would usually ask the user for their OpenID Identifier/URL, however since in my case it's always going to be Google, I'm going to hard-code this as a single value.

dotnetopenid supports adding claim requests so that you can request (or even demand) specific pieces of information. In my case I only care about authenticating me, I don't need to request my name or email address. As such, I'm just going to fire a simple request off without any claim requests.

openId.CreateRequest("https://www.google.com/accounts/o8/id").RedirectToProvider();

In the else block we need to check the response. We want to make sure that the status is Authenticated and the ClaimedIdentifier matches the known identifier for my own login.

// We got a response - check it's valid
if (openId.Response.Status == AuthenticationStatus.Authenticated && openId.Response.ClaimedIdentifier.ToString() == "http://google.com/blah/blah/blah")
{
	Session["Admin"] = true;
	return Redirect("/posts/edit");
}
else
	return Content("Go away, you're not me.");

The ClaimedIdentifier will be unique to each Google account. You can run the code once and examine the returned value to find out your own, and then you can check against it.

If we put all this together into a controller action, it'll look something like this:

public ActionResult Login()
{
	var openId = new OpenIdRelyingParty();

	// If we have no response, start
	if (openId.Response == null)
	{
		// Create a request and redirect the user
		openId.CreateRequest(Settings.AdminOpenIDIdentifier).RedirectToProvider();

		return null;
	}
	else
	{
		// We got a response - check it's valid and that it's me
		if (openId.Response.Status == AuthenticationStatus.Authenticated && openId.Response.ClaimedIdentifier.ToString() == Settings.AdminClaimedIdentifier)
		{
			Session["Admin"] = true;
			return Redirect("/posts/edit");
		}
		else
			return Content("Go away, you're not me.");
	}
}

That's really all there is to it. Now when I hit the Login action I'll be redirected to Google's login page. After logging in, I end up back at /posts/edit on my blog with the correct session variable set. Of course, you could instead call the built-in ASP.NET authentication methods, or look up a user from your database based on their ClaimedIdentifier. There are a lot of ways you can extend this, and I'll cover using OpenID for blog comments in a future article!

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