@BeAware I have not gone over the details, but to the best of my knowledge Authorized Fetch is just the name Mastodon uses for a similar mechanism to what I'm doing in FedBOX. In their case I think they don't allow any C2S get request on collections if they're not authorized, in mine I just filter out non Public activities/objects.
But, I'm pretty sure that my work on having the collections filtered based on recipients predates Mastodon's by quite some time. :P