Mitra v4.4.0 supports verification of RFC-9421 HTTP signatures. It is known to be compatible with @fedifyFedify: an ActivityPub server framework, but I haven't tested it with Mastodon or Streams yet.
Outgoing requests are still signed according to the draft-cavage
. I don't plan to implement double-knocking, but something like FEP-844e could be supported in the future, though this is not urgent.
My implementation of a parser is available in APx library.
RE: https://mitra.social/objects/01974018-9b83-c1ec-a109-24a72a579235