This is the proposed FEP-61cf: The OpenWebAuth Protocol. OpenWebAuth is the “single sign-on” mechanism used by Hubzilla, (streams) and other related projects. It allows a browser-based user to log in to services across the Fediverse using a single identity. Once logged in, they can be recognised by other OpenWebAuth-compatible services, …
The author wrote this FEP by reverse engineering the Hubzilla implementation. The point of proposing it is to find and answer questions like these.