We are implementing the final version of RFC9421 (HTTP Signatures) in Mastodon, and would like to test this with other ActivityPub implementations.
Do you know of any AP implementations supporting both incoming (verification) and outgoing (signing) RFC9421 signatures, and if possible with support for the double-knocking mechanism as described in https://swicg.github.io/activitypub-http-signature/ (section 3.5)?