That's what security trimming is for. You index everything, including the permissions for each item and provide the searcher's permissions (whether it's an acl

That's what security trimming is for. You index everything, including the permissions for each item and provide the searcher's permissions (whether it's an acl, a claim, or some other form) as part of the query. Anonymous or unauthenticated users get a "public" claim that only returns results available to everyone.