I’ve been digging around in the Solid repo to see how MIT/TBL have been solving this. The answer, again, is “generally handing it off to already-existing identities.” There’s a little bit, though: a signup app and, perhaps most relevant, a list of identity providers in JSON format. It’s not (yet?) any kind of machine-readable standard, and as of this writing its only entry, Databox.me, serves an nginx default page on its http port and a demo on its https, but it’s a start.
Server capability discovery mentions Turtle and JSON-LD being on the roadmap, so that’s promising.
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