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/Models/User/Avatar.cs

@@ -14,6 +14,7 @@ namespace PrivaPub.Models.User
 		public Dictionary<string, string> SharedPersonalContacts { get; set; } = new();
 		public string PrivateKey { get; set; }
 		public string PublicKey { get; set; }
+		public string SigningKey { get; set; }//the seed of its Ed25519 key (FEP-521a), which signs its activities' proofs (FEP-8b32)
 		public AvatarAccountState AccountState { get; set; } = AvatarAccountState.Normal;
 		public AvatarSettings Settings { get; set; } = new();
 		public Dictionary<string, string> Fields { get; set; } = new();
@@ -76,6 +77,7 @@ namespace PrivaPub.Models.User
 		public string InboxURL { get; set; }
 		public string OutboxURL { get; set; }
 		public string FeaturedURL { get; set; }//featured: the posts it pins
+		public List<AssertionKey> AssertionKeys { get; set; } = new();//its Ed25519 keys (FEP-521a), which prove what it sends (FEP-8b32)
 		public string WallURL { get; set; }//sm:wall (FEP-400e): where others write to it, Smithereen's walls
 		public string MovedToURL { get; set; }
 		public List<string> AlsoKnownAs { get; set; } = new();//alsoKnownAs: the accounts it says it also is
@@ -158,4 +160,10 @@ namespace PrivaPub.Models.User
 		Banned,
 		Deleted
 	}
+
+	public class AssertionKey
+	{
+		public string Id { get; set; }
+		public string PublicKey { get; set; }//the raw Ed25519 key, base64
+	}
 }