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

FEDERATION.md

@@ -11,6 +11,10 @@ PrivaPub is an ActivityPub server written in C#. This document follows
 - [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
+- Object integrity proofs ([FEP-8b32](https://codeberg.org/fediverse/fep/src/branch/main/fep/8b32/fep-8b32.md),
+  `eddsa-jcs-2022`, JSON canonicalised by [RFC 8785](https://www.rfc-editor.org/rfc/rfc8785)) by Ed25519 keys published
+  as Multikeys ([FEP-521a](https://codeberg.org/fediverse/fep/src/branch/main/fep/521a/fep-521a.md)), on what personas
+  send to relays and verified on what is forwarded
 
 ## Tested against
 
@@ -64,6 +68,12 @@ covered by unit tests written in their documents' shape.
 - [FEP-67ff: FEDERATION.md](https://codeberg.org/fediverse/fep/src/branch/main/fep/67ff/fep-67ff.md)
 - [FEP-f1d5: NodeInfo in Fediverse Software](https://codeberg.org/fediverse/fep/src/branch/main/fep/f1d5/fep-f1d5.md)
 - [FEP-2c59: Discovery of a WebFinger address from an ActivityPub actor](https://codeberg.org/fediverse/fep/src/branch/main/fep/2c59/fep-2c59.md)
+- [FEP-d556: Server-Level Actor Discovery Using WebFinger](https://codeberg.org/fediverse/fep/src/branch/main/fep/d556/fep-d556.md)
+  (WebFinger for the server's origin links its instance actor as `…#Service`) and
+  [FEP-2677: Identifying the Application Actor](https://codeberg.org/fediverse/fep/src/branch/main/fep/2677/fep-2677.md)
+  (`/.well-known/nodeinfo` links it as `…#Application`)
+- [FEP-844e: Capability discovery](https://codeberg.org/fediverse/fep/src/branch/main/fep/844e/fep-844e.md): the instance
+  actor's `implements`, and every other actor's `generator`, name RFC 9421 (verified with RSA keys)
 - [FEP-1b12: Group federation](https://codeberg.org/fediverse/fep/src/branch/main/fep/1b12/fep-1b12.md) (communities; see "Groups")
 - [FEP-044f: Consent-respecting quote posts](https://codeberg.org/fediverse/fep/src/branch/main/fep/044f/fep-044f.md)
   (`QuoteAuthorization` at `/parrot-licences/{id}`; checked with Mastodon both ways)
@@ -71,12 +81,15 @@ covered by unit tests written in their documents' shape.
 - [FEP-c0e0: Emoji reactions](https://codeberg.org/fediverse/fep/src/branch/main/fep/c0e0/fep-c0e0.md) (`EmojiReact`, and Misskey's
   `Like` with content)
 - [FEP-5feb: Search indexing consent](https://codeberg.org/fediverse/fep/src/branch/main/fep/5feb/fep-5feb.md) (`indexable`)
+- [FEP-521a: Representing actor's public keys](https://codeberg.org/fediverse/fep/src/branch/main/fep/521a/fep-521a.md) and
+  [FEP-8b32: Object Integrity Proofs](https://codeberg.org/fediverse/fep/src/branch/main/fep/8b32/fep-8b32.md) (see
+  "Integrity proofs")
 - [FEP-8fcf: Followers collection synchronization across servers](https://codeberg.org/fediverse/fep/src/branch/main/fep/8fcf/fep-8fcf.md)
   (sent and honoured; see "Followers synchronisation")
 
 Planned (see `docs/ROADMAP.md`, phases P7 and P8, and the per-platform notes in `docs/INTEROP.md`): FEP-9098 (custom
 emoji), FEP-7888 and FEP-f228 (threads), FEP-7628 (Move), FEP-8967 (link
-attachments), FEP-521a and FEP-8b32 (keys and integrity proofs), FEP-ae0c (relays).
+attachments), FEP-ae0c (relays).
 
 ## Actors
 
@@ -297,8 +310,8 @@ forwards as its author sent it (Activity-Relay), read again from its origin like
 announces (aode-relay), kept as its author's and never as the relay's boost. Nothing else is taken from a relay. A
 persona's public post outside any group, its edit and its deletion also go to the relays that accepted us (owner decision
 2026-10-06), as Mastodon sends them; nothing less public, and no boost. Mastodon takes a post Activity-Relay forwards
-only with an LD signature or an FEP-8b32 proof, which PrivaPub does not yet add, so it reaches Mastodon through relays
-that announce (aode-relay).
+only with an LD signature or an FEP-8b32 proof: PrivaPub's carry a proof, so they reach Mastodon through both kinds of
+relay (checked live).
 
 ## Server descriptions and the crawler
 
@@ -379,5 +392,15 @@ 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 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.
+- HTTP signatures are verified with RSA keys only, under draft-cavage or RFC 9421. What PrivaPub sends is signed with
+  draft-cavage only, which every server reads.
+- **Integrity proofs** (FEP-8b32, FEP-521a). Every persona has an Ed25519 key of its own (never shared between personas),
+  named in its actor's `assertionMethod` as a `Multikey` (`…#ed25519-key`, `publicKeyMultibase`); the terms are defined
+  in the actor's own context, so no context document has to be fetched. A persona's activity going to a relay carries a
+  `DataIntegrityProof` (`eddsa-jcs-2022`, `proofPurpose` `assertionMethod`) over the activity as delivered; Mastodon
+  4.7 verifies it (checked against its own verifier), so what the relay forwards reaches it. Nothing else carries one:
+  Mitra takes a proof over the HTTP signature, and refuses one by a key it has not read yet without reading the actor
+  again, which every server that knew a persona before it had its key would do. What a persona passes on of others'
+  carries theirs or none. Received: an actor's own Multikeys (at most five, under its id, controlled by it) are kept;
+  a forwarded activity whose proof one of them verifies is taken as it came, instead of being read again from its origin
+  (an unknown key of the actor's has the actor read again first).