There actually is, kind of.

The IndieWeb[0] community has come up with a protocol called AutoAuth[1] that solves this exact problem.

It's only a rough draft at the moment and it is built upon the IndieAuth[2] protocol (which, in turn, is based on OAuth 2), but I think the way it works is highly fascinating.

Here[3] is a an overview of how it works.

[0]: https://indieweb.org/ [1]: https://indieweb.org/AutoAuth [2]: https://indieauth.spec.indieweb.org/ [3]: https://www.svenknebel.de/temp/autoauth.html

I've seen many sites that generate feed URLs with secret token appended as a query parameter. This token only gives an access to the feed, so this as safe as it gets, and requires no special support on reader app part.