SocialPub/FEDERATION.md

Federation

PrivaPub is an ActivityPub server written in C#. This document follows FEP-67ff and describes how it federates.

Supported federation protocols and standards

• ActivityPub (server-to-server)
• WebFinger
• HTTP Signatures, rsa-sha256 / hs2019 with RSA keys
• NodeInfo 2.0 and 2.1

Tested against

• GoToSocial 0.22.1, end to end. It runs in a private network on the workstation (tools/pasture/) and is driven through its own client API. Checked both ways:
  • follows, including to a locked account;
  • public posts with a content warning;
  • replies with notifications;
  • likes and boosts;
  • direct messages;
  • edits and deletes;
  • unfollow.
• Mastodon, Misskey, Lemmy and PeerTube, by unit tests only. The tests feed the parser documents written in each server's shape. No live exchange with any of them has run yet.

Supported FEPs

• FEP-67ff: FEDERATION.md
• FEP-f1d5: NodeInfo in Fediverse Software
• FEP-2c59: Discovery of a WebFinger address from an ActivityPub actor
• FEP-1b12: Group federation (communities; see "Groups")

Planned (see docs/ROADMAP.md, phases P5 to P8, and the per-platform notes in docs/INTEROP.md): FEP-044f (quotes), FEP-9967 (polls), FEP-c0e0 (emoji reactions), FEP-9098 (custom emoji), FEP-7888 and FEP-f228 (threads), FEP-7628 (Move), FEP-8fcf (followers synchronisation), FEP-5feb (indexable), FEP-8967 (link attachments), FEP-521a and FEP-8b32 (keys and integrity proofs), FEP-ae0c (relays).

Actors

Every account is an avatar: one private login can own several, and they are deliberately unlinkable. Nothing in an actor document, a collection, NodeInfo or a delivery relates two avatars of the same login.

Thing	Address
Actor (Person, Group, Application)	/peasants/{name} (/users/{name} redirects)
Inbox	/peasants/{name}/mouth
Outbox	/peasants/{name}/anus
Shared inbox	/human-centipede
Followers / following	/peasants/{name}/groupies, /peasants/{name}/stalking
Objects	/peasants/{name}/scribbles/{id}
Activities	/peasants/{name}/grunts/{id}
Direct-message context	/peasants/{name}/whispers/{id}
Profile and post pages	/@{name}, /@{name}/{id}

The names are the project's own and are stable; resolve actors through WebFinger, not by guessing a path.

• The key is {actor}#main-key, RSA 2048, served as SPKI PEM with owner set to the actor.
• published on an actor is a whole day, chosen at random up to two weeks before the account was made, so two personas made on the same day do not share a date. indexable is false.
• The instance actor is /peasants/privapub (type Application). It signs every fetch PrivaPub makes, so no avatar's key is used to read another server's content.

Groups

A group is either a community or a circle.

• A community follows FEP-1b12. Posts addressed to it (in to, cc or audience) are accepted according to its posting policy (followers, anyone, or moderators only) and the group Announces the whole activity, with audience set, to its followers. A new post is also announced as an object so Mastodon shows it. Updates and deletes of community content are announced too. A top-level post is a Page with a name. Members are counted at /flock; moderators are listed at /wardens, which the actor's attributedTo points to, with postingRestrictedToMods as Lemmy expects. A mention of a community posts into it.
• A circle is private. Its actor is not discoverable and every follow is a request. Its posts are addressed to the circle and its members collection, delivered to each member's own inbox and never announced. They are served only to a signed request from a member, or from the instance actor of a member's server; anyone else gets 404. A Mastodon member's replies reach only the people they mention.
• Announces from remote groups (Lemmy communities) are followed through to the activity: the object is fetched from its own origin, never taken from the announce.

Activities

Received:

Activity	Effect
Follow	follows an avatar or community; Accept is sent unless the community approves members by hand
Accept{Follow}, Reject{Follow}	completes or ends a follow an avatar requested
Undo{Follow, Like, Announce}	reverses it
Create{Note, Article, Page, Question, Video, Audio, Event, ChatMessage, …}	stored when a local avatar follows the author, is addressed or mentioned, when it replies to a local post, or when it is addressed to a community the author follows; a public parent is fetched to complete the thread
Update{Note}	replaces the content; the previous version is kept
Update{Person}	refetches the actor
Like	counted and notified, on posts the liker could see
Dislike	counted as a downvote (Lemmy, PieFed, Mbin, Friendica); Undo takes it back
EmojiReact, Like with an emoji content	an emoji reaction (FEP-c0e0; Pleroma, Akkoma, Iceshrimp.NET, Misskey, Sharkey), Unicode or a custom emoji from its tag; a Like whose content is ❤ stays a favourite; Undo takes it back
Join	answered with Ignore: PrivaPub hosts no events yet (FEP-8a8e)
Announce	counted and notified for local posts; shown to followers of the announcer, with the original refetched from its origin
Delete	deletes the object, or the actor and its follows; a deleted object id is remembered for 90 days, so a late Create cannot bring it back
Flag	becomes a report for this server's moderators

Sent: Follow, Undo{Follow}, Create{Note}, Create{Question} and poll votes, EmojiReact and its Undo, Update{Note}, Update{Person}, Delete{Tombstone}, Accept{Follow}, Reject{Follow}, Like, Announce and their Undo, Flag. A deleted post answers 410 with a Tombstone.

• Attachments are Documents with mediaType, name (alt text), blurhash, focalPoint, width and height. Uploaded files have all metadata removed.
• Pinned posts are the actor's featured collection (/trophies); featuredTags is /tattoos.
• Blocks are sent. A blocked remote account receives Block from the blocking account (and Reject{Follow} if it followed); an unblock sends Undo{Block}.
• Reports are sent as Flag by the instance actor, never by the reporting account.

A Create's Note carries Mastodon's content, contentMap, summary and sensitive, plus Mention and Hashtag tags.

Polls (FEP-9967) go out as a Question with oneOf or anyOf, each option's count in replies.totalItems, endTime, votersCount, and closed once it has ended. Incoming votes are Create{Note{name, inReplyTo}} with no content, one per choice, counted once per voter. Counts are refreshed with an Update{Question} at most every three minutes. Our own votes on other servers' polls are sent the same way to the poll's author only, without published. An incoming Update without a newer updated only refreshes counts and details; it is never recorded as an edit.

Quotes (FEP-044f, with the older keys):

• Received. A quote is read from quote, quoteUrl, quoteUri, _misskey_quote or a FEP-e232 Link tag.
  • With a quoteAuthorization, the stamp is fetched and must be a QuoteAuthorization on the quoted author's origin, attributed to them, naming exactly the two posts.
  • A FEP-044f quote without a stamp is pending until an Update brings one.
  • A quote made with the older keys only, by software that asks nobody, is shown when the quoted post is public.
  • A Delete of a stamp revokes the quote.
• Sent. Our accounts quote with every key, plus the RE: fallback.
  • When the quoted post states an interactionPolicy.canQuote that allows us, quote is included and a QuoteRequest (with the quoting post as instrument) goes to the quoted author. The quote stays pending until their Accept brings a stamp we can verify; the post is then updated with quoteAuthorization.
  • Posts that state no policy are quoted the older way, without quote.
  • Quoting posts are also delivered to the quoted author.
• Not yet. Our own posts cannot be quoted by others yet: we do not issue stamps.

Custom emoji (Emoji tags) are read on posts, display names, bios and profile fields, at most 64 per object. Profiles keep their header, profile fields, manuallyApprovesFollowers, published, movedTo, indexable, memorial and avatar and header descriptions. A post's title becomes name and is also the first, bold line of content, because Mastodon does not show name. A content warning without its own text uses the title, or "Content warning".

Received summary is a content warning only on a Note or Question, or when sensitive is true. On an Article, Page, Event, Video or Audio it is an excerpt or description, kept as such and never hidden.

Visibility is expressed in to/cc the way Mastodon does it: public, unlisted, followers-only and direct. Inbound followers-only posts are recognised by the author's own followers collection.

Link previews

For a public post that links to a page, PrivaPub builds a preview card. It uses, in order:

1. a Link attachment's own preview (FEP-8967) or the post's own image, title and summary;
2. otherwise, the linked page, read once by the server between 0 and 60 seconds after the post arrives (never when someone reads it), with an Accept: text/html request that reads at most 512 KB, through the same address checks as every other fetch.

The page's OpenGraph and Twitter tags give the title, description and image; the result is cached per address for 7 days and shared by every account on the server. Images are served to clients only through PrivaPub's media proxy. Federation:FetchLinkPreviews=false turns page fetching off.

Local-only posts

Posts with a location (shown to nearby users of this server) never leave the server, in any form.

Security rules a peer will notice

• Signatures. Inbox POSTs must be signed over (request-target), host, digest and date (or (created)). The date may be at most one hour old and fifteen minutes ahead. A bad signature gets 401, malformed input 400, an accepted activity 202, too many requests 429. Activities are processed after the 202.
• Origins. An actor document is accepted only from the address it names as its id. A key only if its actor lists it with owner set to the actor and on the actor's origin. An activity's id, and any object it creates, updates or deletes, must be on its actor's origin. An embedded object from another origin is fetched from that origin.
• Fetching. All fetches are signed by the instance actor. They go only to public addresses, follow at most three redirects and read at most 1 MB.
• HTML. Received HTML is sanitised to Mastodon's allowlist.
• Keys we cannot fetch for now. When a sender's key cannot be fetched because its server timed out or answered 5xx, the inbox answers 503 with Retry-After: 300 rather than 401.
• Activity ids. Create and Announce ids dereference. Follow, Like, Block and the Accept, Reject and Undo that answer them do not, because serving them would reveal who follows, likes and blocks whom; they are always sent with their object embedded.
• Delivery. Failed deliveries are retried with Mastodon's backoff (16 attempts). A host that keeps failing is paused, starting at an hour and growing to a week.

Known limitations

• Our own posts carry no custom emoji: emoji are received and shown, not offered.
• Remote media is fetched through this server's proxy when a local client displays it.
• Collections expose counts, not members.
• Only rsa-sha256-style keys are verified. RFC 9421 signatures are planned.