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/Federation/Rendering/ActivityPubRenderer.cs

@@ -79,9 +79,29 @@ namespace PrivaPub.Federation.Rendering
 				["privacySettings"] = "sm:privacySettings",
 				["wallPosting"] = "sm:wallPosting",
 				["wallPostVisibility"] = "sm:wallPostVisibility",
-				["allowedTo"] = "sm:allowedTo"
+				["allowedTo"] = "sm:allowedTo",
+				// FEP-521a keys and FEP-8b32 proofs, defined here rather than by the data-integrity and multikey contexts,
+				// which strict JSON-LD readers would have to fetch
+				["assertionMethod"] = new JsonObject { ["@id"] = "sec:assertionMethod", ["@type"] = "@id", ["@container"] = "@set" },
+				["Multikey"] = "sec:Multikey",
+				["controller"] = new JsonObject { ["@id"] = "sec:controller", ["@type"] = "@id" },
+				["publicKeyMultibase"] = new JsonObject { ["@id"] = "sec:publicKeyMultibase", ["@type"] = "sec:multibase" },
+				["DataIntegrityProof"] = "sec:DataIntegrityProof",
+				["proof"] = new JsonObject { ["@id"] = "sec:proof", ["@type"] = "@id", ["@container"] = "@graph" },
+				["cryptosuite"] = new JsonObject { ["@id"] = "sec:cryptosuite", ["@type"] = "sec:cryptosuiteString" },
+				["proofValue"] = new JsonObject { ["@id"] = "sec:proofValue", ["@type"] = "sec:multibase" },
+				["proofPurpose"] = new JsonObject { ["@id"] = "sec:proofPurpose", ["@type"] = "@vocab" },
+				["verificationMethod"] = new JsonObject { ["@id"] = "sec:verificationMethod", ["@type"] = "@id" },
+				["implements"] = new JsonObject { ["@id"] = "https://w3id.org/fep/844e/implements", ["@type"] = "@id", ["@container"] = "@set" }
 			});
 
+		// what PrivaPub reads that a sender cannot tell from our documents (FEP-844e): RFC 9421 signatures, with RSA keys
+		static JsonArray Implements() => new(new JsonObject
+		{
+			["href"] = "https://datatracker.ietf.org/doc/html/rfc9421",
+			["name"] = "RFC-9421: HTTP Message Signatures"
+		});
+
 		public static string Html(string markdown) =>
 			string.IsNullOrEmpty(markdown) ? string.Empty : Markdown.ToHtml(markdown, Pipeline).Trim();
 
@@ -150,6 +170,20 @@ namespace PrivaPub.Federation.Rendering
 				document["attributedTo"] = actor.Wardens;
 				document["postingRestrictedToMods"] = actor.PostingRestrictedToModerators;
 			}
+			// what the server reads (FEP-844e): on the application actor itself, on any other as its generator
+			if (actor.Kind == LocalActorKind.Application)
+				document["implements"] = Implements();
+			else
+				document["generator"] = new JsonObject { ["type"] = "Application", ["implements"] = Implements() };
+			// its Ed25519 key (FEP-521a), which signs the proofs its activities carry (FEP-8b32)
+			if (!string.IsNullOrEmpty(actor.SigningKey))
+				document["assertionMethod"] = new JsonArray(new JsonObject
+				{
+					["id"] = actor.AssertionKeyId,
+					["type"] = "Multikey",
+					["controller"] = actor.Uri,
+					["publicKeyMultibase"] = Signing.IntegrityProofs.Multikey(Signing.IntegrityProofs.PublicKey(actor.SigningKey))
+				});
 			if (!string.IsNullOrEmpty(actor.PictureURL))
 				document["icon"] = new JsonObject { ["type"] = "Image", ["url"] = actor.PictureURL };
 			if (!string.IsNullOrEmpty(actor.ThumbnailURL))