Personas prove what goes to relays; the server says what it reads

FEP-521a and FEP-8b32. Every persona has an Ed25519 key of its own (Avatar.SigningKey; migration 014 gives the earlier
ones theirs), named in its actor's assertionMethod as a Multikey, the terms defined in the actor's own context. A
persona's activity going to a relay carries an eddsa-jcs-2022 proof (JSON canonicalised by RFC 8785, Jcs), so what
Activity-Relay forwards reaches Mastodon, which verifies it with its own code. Nothing else carries one: Mitra takes a
proof over the HTTP signature and refuses one by a key it has not read, without reading the actor again. Received: an
actor's own Multikeys are kept, and a forwarded activity whose proof one of them verifies is taken as it came instead of
being read again from its origin.

Discovery: WebFinger for the server's origin links its instance actor (FEP-d556), NodeInfo links it as the application
actor (FEP-2677), and actors name RFC 9421 under implements (FEP-844e).

Checked live: relay 16 (Activity-Relay's forward of alice's post reaches Mastodon), Mitra, GoToSocial and Mastodon
unchanged (165 in all).

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

PrivaPub/Infrastructure/Data/Migrations/_014_personas_have_ed25519_keys.cs

@@ -0,0 +1,18 @@
+using MongoDB.Entities;
+
+using PrivaPub.Federation.Signing;
+using PrivaPub.Models.User;
+
+namespace PrivaPub.Infrastructure.Data.Migrations
+{
+	// every persona signs the proofs its activities carry (FEP-8b32) with an Ed25519 key of its own (FEP-521a): new ones
+	// get theirs when made, the earlier ones once here
+	public class _014_personas_have_ed25519_keys : IMigration
+	{
+		public async Task UpgradeAsync()
+		{
+			foreach (var avatar in await DB.Default.Find<Avatar>().Match(a => a.SigningKey == null).Project(p => p.Include(a => a.ID)).ExecuteAsync())
+				await DB.Default.Update<Avatar>().Match(a => a.ID == avatar.ID && a.SigningKey == null).Modify(a => a.SigningKey, IntegrityProofs.NewSeed()).ExecuteAsync();
+		}
+	}
+}