hs2019 signatures hashed with SHA-512, and SHA-512 digests

A draft-cavage hs2019 signature leaves the hash to the key: with an RSA key it is tried with SHA-256, as nearly everyone
signs, then with SHA-512, as some do; rsa-sha256 stays SHA-256 only. A Digest of SHA-512= is checked as SHA-256= is.

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

FEDERATION.md

@@ -8,6 +8,7 @@ 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
+  (`hs2019` hashed with SHA-256, or SHA-512 as some sign it; a `SHA-256=` or `SHA-512=` digest)
 - [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