Monday, August 6th, 2007

Atompub & OpenID

One of the thinmgs I would like to do is to interact with Atompub (aka Atom Publishing Protocol) stores in Javascript through the browser. Since this effectively the browser itself interacting with the Atompub server, browser-like authentication methods would be nice. But services like Atompub don’t work nicely with the kinds of authentication methods that normal websites use. One of these is OpenID, which is particularly browser-focused.

From the perspective of a client, OpenID basically works like this:

  • You need to login. You tell the original server what your OpenID URL is, somehow.
  • The original server does some redirects, maybe some popups, etc.
  • Your OpenID server (attached to your OpenID URL) authenticates you in some fashion, and then tells the original server.
  • The original server probably sets a signed cookie so that in subsequent requests you stay logged in. You cannot do this little redirection dance for every request, since it’s actually quite intrusive.

So what happens when I have an XMLHttpRequest that needs to be authenticated? Neither the XMLHttpRequest nor Javascript generally can do the authentication. Only the browser can, with the user’s interaction.

One thought I have is a 401 Unauthorized response, with a header like:

WWW-Authenticate: Cookie location="http://original.server/login.html"

Which means I need to open up http://original.server/login.html and have the user log in, and the final result is that a cookie will be set. XMLHttpRequest sends cookies automatically I believe, so once the browser has the cookie then all the Javascript requests get the same cookie and hence authentication.

One problem, though, is that you have to wait around for a while for the login to succede, then continue on your way. A typical situation is that you have to return to the original page you were requesting, and people often do something like /login?redirect_to=original_url. In this case we might want something like /login?opener_call=reattempt_request, where when the login process is over we call window.opener.reattempt_request() in Javascript.

Maybe it would make sense for that location variable to be a URI Template, with some predefined variables, like opener, back, etc.

For general backward compatibility, would it be reasonable to send 307 Temporary Redirect plus WWW-Authenticate, and let XMLHttpRequests or other service clients sort it out, while normal browser requests do the normal login redirect?

Update: Another question/thought: is it okay to send multiple WWW-Authenticate headers, to give the client options for how it wants to do authentication? It seems vaguely okay, according to RFC 2616 14.47.

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This is the personal site of Ian Bicking. The opinions expressed here are my own.