@dave @renchap I've just implemented RFC 9421 in Fedify with both verification and signing support, including the double-knocking mechanism. I was actually looking for other ActivityPub implementations to test interoperability with.

I'd love to collaborate on testing between Fedify and Mastodon. Our implementation also prioritizes RFC 9421 first and falls back to draft cavage if needed, similar to the approach described in the ActivityPub and HTTP Signatures spec.

Would you be interested in coordinating some interoperability tests? Feel free to reach out if this sounds helpful for both our implementations!