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.

@rfc9421 #rfc9421

RE: https://mitra.social/objects/01974018-9b83-c1ec-a109-24a72a579235

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