Federated Authentication

[ixxie]

I've been pondering a kind of federated identify model, which could be of interest in enabling users to have a shared identity in Fediverse services. I want to know if this is an established concept and if not, whether this is a sane idea. If it is, perhaps it could be documented in Lucia at some point.

Say we you two servers (A and B) with more or less "normal" authentication patterns, like those described by Lucia. Now a user has an identity in both servers, and wants to link them. They go to server A and initiate a procedure for affiliating with their identity in B. A calls an endpoint in B, and B then asks the user to confirm. Once confirmed, API tokens are exchanged between the servers, allowing the user to access data from B in A and A in B. Let's say that the user identities in A and B are no coauthenticated.

The following question come to mind:

1. Soundness - is this sound and secure? Are there significant challenges to implementing something like this?
2. Reversibility - would there be potential issues in allowing users to undo this linking, decoupling the identity going forward?
3. Signup - could we allow a user of A to sign up to server B through their identity in A while still maintaining reversibility, or event allowing the for closing the account in service A?
4. Scaling - could this scale to N servers / services? Since the number of potential links would grow quadratically, probably a sane implementation would need to somehow avoid linking all servers to one-another naively.