An alternative possibility would be to prescribe the format of IndieAuth access codes, as part of the standard. For instance, we could prefix the usual arbitrary implementation-specific access code blob with the expected
me value, making it easy for token endpoints to discover the correct authorization endpoint.
code=https://00dani.me/$C5r1cuqJk1fUTGrWX4DPHz44jxpgHF or something like that. Then, of course, pure OAuth 2.0 clients would pass through that extra piece of information with no trouble whatsoever, since it's embedded in an existing standard parameter.
It's certainly a messy approach, though, and one might question whether OAuth client compatibility is worth adding this complexity to IndieAuth. Additionally, making a change like this now would introduce potential incompatibility: a token endpoint that knows it can pull information out of the access code might still receive an access code from an authorization endpoint that doesn't embed information in the prescribed format, for instance.
Still, prescribing a format for access codes might not be quite as unreasonable as it seems: after all, client IDs are also treated as opaque in pure OAuth 2.0, whereas in IndieAuth they have a prescribed and meaningful format.
tl;dr The more I think about it, the more I think this parameter enables a use case that isn't really necessary. The
me parameter in the code exchange step specifically allows for a token endpoint to be detached from both the Micropub endpoint and the authorization endpoint.
Full details below.
The different use cases that are all supported right now:
This is the simplest case in terms of architecture, but the most amount of work for a developer. In this case, someone writes all three parts of the system. Since they are part of the same system, the mechanism by which the token endpoint validates authorization codes does not need to be standardized, it's all internal.
Both my website and the Wordpress IndieAuth plugin fall under this case.
In this case, someone is building a CMS that includes a Micropub endpoint as well as a token endpoint. However, they want to speed up their development, so they use an authorization endpoint service such as indieauth.com.
The client sends the auth code to the token endpoint, and since the token endpoint is part of the CMS, it already knows the only place it can go to validate the auth code is the authorization endpoint service that it's configured to use. Therefore there is no need for the
me parameter, which normally tells the token endpoint where to go to verify the auth code.
Specifically this case is where a service provides both an authorization endpoint and token endpoint. This is the quickest path to building a Micropub endpoint, since all you need to do is build out the Micropub endpoint itself, and when any requests come in with a token, the endpoint goes and checks whether the token is valid by testing it against the token endpoint service.
This is a very common case with peoples' individual websites, as it offloads the development and maintenance of the security bits to a service. I provide these as a service at indieauth.com and tokens.indieauth.com.
The interesting thing though is that when a single service provides both, there is also no need for the
me parameter at the code exchange step, since the token endpoint already knows where it needs to verify the authorization code since the code was issued by the same system.
The only case where the
me is needed is when the authorization endpoint and token endpoint are both used as services and they are separate services. Imagine a standalone token endpoint service: the job of this service is to verify authorization codes and issue access tokens, and later verify access tokens. In this situation, a request comes in with an unknown authorization code and it needs to verify it. Since it was not part of the system that issued the code, it needs to know how to verify it. Right now, this is enabled because this request also includes the
me parameter, so the token endpoint goes and looks up the user's authorization endpoint and verifies the code there.
The thing I'm realizing though is that this is really quite an edge case, and one that I don't think is actually very important. Typically someone who is building a Micropub endpoint themselves will first start by using an authorization/token endpoint service, and there is no benefit to them if those are two separate services. In fact it's probably easier if they are just part of the same system since it's less moving parts to think about at this stage.
Later, that person can decide they want to take over issuing tokens, but still don't want to build out the UI of an authorization service. At this point, they fall under the second use case above. They build out a token endpoint into their software, and since they're using the authorization endpoint service they know where to verify authorization codes.
On the other end of the spectrum, you have people who build the whole thing out themselves, like my website and the Wordpress plugin. In these cases the
me is completely irrelevant in the code exchange step.
The particular situation that the
me enables is using a separate service for the authorization and token endpoints, and I can't think of a case where that is actually important.
What we really need is federated authentication, but that doesn't exist yet.
This sounds like a great use case for IndieAuth. w3.org/TR/indieauth
IndieAuth is an OAuth 2.0 extension, which avoids the centralized problems with existing OAuth solutions by using DNS for "registration" of client IDs and user IDs. Every user account is identified by a URL (for Gitea this could be your Gitea user page), and client IDs are also URLs (would be the Gitea instance home page in this case.)
To log in to your Gitea instance, I would enter my own Gitea profile URL. Your instance would then do discovery on my URL to find where to send me to authorize the login on my own OAuth server (my Gitea server), which would then send me back to your Gitea where it would be able to verify the authorization code against my Gitea instance.
I'd be happy to walk through this in more detail if you're interested!