Is rend-spec's 'service-authentication' a thing?

Hi Nick, on #15004 (closed) I added hidden server descriptor parsing to Stem. donncha has been a fantastic help, providing test data and python crypto examples. Interestingly when we came to the rend-spec's service-authentication lines he wasn't able to find them in the tor codebase.

Were they never implemented? They're very strange fields, prefixing encrypted introduction-points with a plaintext field. Stem presently has (untested) support for them but if they're not actually a thing we should drop them from the spec.