RFC 9421 signatures are verified, not refused

WordPress's ActivityPub plugin (and Ghost and Fedify) sign with RFC 9421
first and fall back to draft-cavage only after a refusal, so each first
delivery cost two requests and a 401 in our statistics. Now a request
carrying Signature-Input is verified as an HTTP message signature: its
covered components (the method and our own public target, the body's
Content-Digest), its created and expires, with the actor's RSA key under
PKCS#1 v1.5 or PSS. Deliveries and signed fetches both take it; the
ledger names the scheme (rfc9421:rsa-v1_5-sha256). What PrivaPub sends
stays draft-cavage, which every server reads. Ed25519 waits for FEP-521a
keys.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

FEDERATION.md

@@ -8,6 +8,8 @@ PrivaPub is an ActivityPub server written in C#. This document follows
 - [ActivityPub](https://www.w3.org/TR/activitypub/) (server-to-server)
 - [WebFinger](https://webfinger.net/)
 - [HTTP Signatures](https://datatracker.ietf.org/doc/html/draft-cavage-http-signatures), `rsa-sha256` / `hs2019` with RSA keys
+- [HTTP Message Signatures](https://www.rfc-editor.org/rfc/rfc9421) (RFC 9421) and Content-Digest (RFC 9530), verified on
+  deliveries and signed fetches: `rsa-v1_5-sha256` and `rsa-pss-sha512` with RSA keys
 - [NodeInfo](https://nodeinfo.diaspora.software/) 2.0 and 2.1
 
 ## Tested against
@@ -251,4 +253,5 @@ Posts with a location (shown to nearby users of this server) never leave the ser
 - Collections expose counts, not members: `/groupies` and `/stalking` give the same totals as the Mastodon API's
   follower and following counts, and the outbox's `totalItems` is the persona's post count, though only public posts
   are listed. `/api/v1/instance/peers` is empty: which servers this one talks to is not published.
-- Only `rsa-sha256`-style keys are verified. RFC 9421 signatures are planned.
+- Only RSA keys are verified, under draft-cavage or RFC 9421; Ed25519 (FEP-521a) is planned. What PrivaPub sends is
+  signed with draft-cavage only, which every server reads.