Looking for plugin that does “anyone with the link” type of access

I use WordPress for a family blog. On the one hand, I don't want my family pictures totally publicly accessible for spiders, bots, and passers-by; OTOH, neither do I want to require friends and family to have to create yet another login just to see my kid pictures. I want be able to hand out a link (via email/WhatsApp/Facebook) and users just have to click to see them. A solution is something like Google Docs/Drive's "anyone with the link" type of access control, where there is a "key" GET parameter in the URL, which then sets a cookie that allows certain viewing permissions, and keys can be revoked or newly issued as need be.

What I'm using right now is this Maintenance Mode, which isn't designed for this purpose, but works pretty well. However it only does allow/deny on a whole-site level (as one would expect, given its purpose). It would be neat to be able to hand out such keys that permit users to view only particular posts, or perhaps only certain post categories or tags. Does anyone know of a plugin that does such a thing? Ideally one that is open source.