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 (hs2019 hashed with SHA-256, or SHA-512 as some sign it; a SHA-256= or SHA-512= digest)
• HTTP Message Signatures (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 2.0 and 2.1
• Object integrity proofs (FEP-8b32, eddsa-jcs-2022, JSON canonicalised by RFC 8785) by Ed25519 keys published as Multikeys (FEP-521a), on what personas send to relays and verified on what is forwarded

Tested against

End to end, in a private network on the workstation (tools/pasture/), each peer driven through its own client API and every run starting clean, with signed fetches required (as privapub.thepra.dev runs):

• GoToSocial 0.22.1
• Mastodon 4.7.3
• Misskey 2026.10
• Sharkey 2025.4.7
• Akkoma 3.20.1
• Lemmy 1.0.0-beta.2, and Lemmy 0.19.20 (built with the platform's roots, so it trusts the pasture's CA)
• PeerTube 8.3.1
• Pixelfed 0.14.4
• WordPress 7.1.2 with ActivityPub 9.3.1
• Friendica 2026.05
• Mobilizon 5.2.4
• Gancio 1.28.2
• Funkwhale 2.0.11
• PieFed 1.7.17
• Mbin 1.10.1
• NodeBB 4.16.1
• Smithereen 1.0.3
• Mitra 5.9.1
• snac2 2.95
• WriteFreely 0.17.2
• Owncast 0.3.0
• Ghost 6.67 with its ActivityPub service 1.2.14
• BookWyrm 0.9.3
• Vernissage 1.43.0
• Hubzilla 11.4.1 with its pubcrawl addon
• Forte 26.9.10 (portable identities: actors served through /.well-known/apgateway/did:key:…)
• Castopod 1.15.5 (podcasts as actors, their episodes with the sound)
• Ktistec 3.13.0
• Activity-Relay 2.0.9 and aode-relay 0.3.129, as relays PrivaPub reads from
• in the town only (a seeded community checked server by server): Hollo 0.9.19, Iceshrimp.NET 2026.1.2-beta, Pleroma 2.10.2

Checked both ways, where the peer has the feature:

• follows, locked accounts included;
• every visibility, content warnings, replies and their notifications;
• likes, boosts and emoji reactions with their undos;
• direct messages;
• polls;
• quotes;
• images with alt text;
• edits with history and deletes;
• communities and circles;
• blocks and unfollows.

docs/INTEROP.md has each peer's evidence and what is still expected to fail. The platforms not yet in the pasture are covered by unit tests written in their documents' shape.

Supported FEPs

• FEP-67ff: FEDERATION.md
• FEP-f1d5: NodeInfo in Fediverse Software
• FEP-2c59: Discovery of a WebFinger address from an ActivityPub actor: every actor names its handle, and a peer's webfinger on another domain than its actor's host (Mastodon's LOCAL_DOMAIN) is the handle shown once that domain's WebFinger points back to the actor; asked again on a rename
• FEP-d556: Server-Level Actor Discovery Using WebFinger (WebFinger for the server's origin links its instance actor as …#Service) and FEP-2677: Identifying the Application Actor (/.well-known/nodeinfo links it as …#Application)
• FEP-844e: Capability discovery: the instance actor's implements, and every other actor's generator, name RFC 9421 (verified with RSA keys)
• FEP-1b12: Group federation (communities; see "Groups")
• FEP-044f: Consent-respecting quote posts (QuoteAuthorization at /parrot-licences/{id}; checked with Mastodon both ways)
• FEP-9967: Polls
• FEP-c0e0: Emoji reactions (EmojiReact, and Misskey's Like with content)
• FEP-5feb: Search indexing consent (indexable)
• FEP-521a: Representing actor's public keys and FEP-8b32: Object Integrity Proofs (see "Integrity proofs")
• FEP-171b: Conversation Containers, received: a thread owner's Add of someone's Create, Update or Delete to the thread's context is taken as that activity passed on by the owner (on its FEP-8b32 proof, or as its origin has it)
• FEP-8fcf: Followers collection synchronization across servers (sent and honoured; see "Followers synchronisation")
• FEP-5624: Per-object reply control policies: read (see "Replies its author approves")

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-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}
Quote permissions (QuoteAuthorization)	/peasants/{name}/parrot-licences/{id}
Direct-message context	/peasants/{name}/whispers/{id}
A persona's wall (FEP-400e)	/peasants/{name}/graffiti
Followers on the asking server (FEP-8fcf)	/peasants/{name}/groupies/roll-call
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 unless the persona turns it on, and discoverable is true unless it turns that off (the deploy's own persona, @thepra, is undiscoverable).
• 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. It also answers at the server's root (/) for a request that asks for ActivityPub, as Lemmy's site actor does: PieFed sends a community's announces to the inbox of the Application at a peer's root, and to /inbox when there is none.
• The reporter is /peasants/privapub_reports (type Service, "Reports from "), one for the whole server with its own key. It sends the reports Lemmy, PieFed and Mbin take in Lemmy's shape (see Reports below) and does nothing else: nobody follows or mentions it, the Mastodon API has no account for it, and it has no page.

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 and to the author's own server, whether anyone there follows it or not (Lemmy keeps its user's post pending until then). 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. Each member's copy also names that member in cc, so servers that keep only posts naming one of their accounts (Mastodon, GoToSocial) keep it; it names no other member. The posts are served only to a signed request from a member, or from the instance actor of a member's server, and that copy names the member (or the members on that server); anyone else gets 404. A Mastodon member's replies reach only the people they mention. Its owner lets a request in with an Accept of the Follow, and declines one or takes a member from elsewhere out with a Reject of it, as Mastodon removes a follower.
• 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. They are taken from a group someone here follows, and also when they relay a vote on one of our posts in a thread of that group (Lemmy sends a vote to the community alone). A post keeps the group its audience names, however it arrived.
• A post in a remote group followed here that the group's own server delivers itself, as its author's Create with the group as its audience (Mbin sends a magazine's threads that way, with no announce), is kept as if announced: the group's server speaks for posts in it. The same from another server is not.
• A remote community's moderation reaches the posts it holds. A removal (Announce{Delete}) is believed at once from a community on the post's own server, which speaks for it (PieFed keeps serving a thread its moderator removed), and from a community elsewhere once the post's origin answers it gone. A lock sent as it is (Lock, Mbin) is taken from the post's own server only. A lock (Announce{Lock}, or commentsEnabled: false on the post) refuses replies, ours included, until Undo{Lock}. A ban of a persona (Announce{Block} with the community as target) shows as blocked_by on the community and refuses the persona's posts and replies there until the Undo.

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; once a follow holds, the account's latest public posts are read from its outbox's first page (twenty at most, once a day), so its profile is not empty; homes get only what arrives from then on
Accept{Join}, Reject{Join}	from the event's server: a persona's participation in the event is accepted or refused
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 persona is addressed by its actor id or by its profile page, as Mbin addresses its private messages); 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 of its own 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, though a new post its server publishes under that id after the deletion (Gancio numbers events from the last one kept) is kept
Flag	becomes a report for this server's moderators
Move	an account moving: believed as Mastodon believes it, when the account sends it about itself and the new account, read again from its server, names it in alsoKnownAs. The old account then shows where it went (moved), and, as Mastodon does it (owner decision 2026-10-05), the personas following it follow the new one instead (a Follow to its server, an Undo to the old), in the same lists; a mute or a block of the old account carries over
Add/Remove on the actor's featured	the account pins or unpins one of its own posts (fetched from its server when not held); inside a community's announce, the community features a post made in it. Its profile shows them first (pinned=true). The collection itself is read with the account's counts, at most once a day. On the account's wall (sm:wall), see "Walls". Any other target (a community's moderators) is dropped; one on the account's own server that PrivaPub does not know has its document read again first, at most hourly
Block of a persona	the follows between them end, the blocker's posts and notifications are hidden from the persona, nothing of the persona's is addressed to the blocker, and the relationship says blocked_by; Undo{Block} lifts it

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, Join and Leave of a remote event, and the replies to a persona's posts passed on to its followers. 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.
• Threads (owner decision 2026-10-06). A persona's public or unlisted post outside any group names its replies (…/scribbles/{id}/replies) and its conversation, context (FEP-7888): the root post's …/scribbles/{id}/context, which a reply inherits from its parent, ours or another server's. Both list only the public and unlisted posts PrivaPub holds, the context every one under the root (at most 500); followers-only, circle, direct and local-only posts name neither, and their collections answer 404. Mastodon completes a thread from them.
• Followers synchronisation (FEP-8fcf, owner decision 2026-10-06). A persona's delivery addressed to its followers carries a signed Collection-Synchronization header: its followers, its roll-call (…/groupies/roll-call) and the digest (XOR of each id's SHA-256) of its accepted followers on the receiving server only. The roll-call answers a signed request with the persona's followers on the signer's server and nobody else's; unsigned, 401. A server whose view differs mends itself from it, Mastodon both ways (checked live). Mastodon's Undo{Follow} for a follow it never knew of has one id per account (…#follows//undo); when it comes again after a new follow, it ends that follow too. Honoured the other way: when a delivery's header digests the sender's followers here otherwise than the personas following it, PrivaPub reads the list it names (same origin, signed by the instance actor). A follow the list leaves out ends, but only when the list is the one the digest describes; a request it lists is taken as accepted; a persona it lists that follows nothing there sends Undo{Follow}, as Mastodon does.
• Walls (FEP-400e, Smithereen's; owner decision 2026-10-06). A persona's actor names its wall (…/graffiti, sm:wall) and Smithereen's privacySettings: its followers may write on it (wallPosting), everyone may read it (wallPostVisibility). A public post that is not a reply, sent with the wall as its target by an account following the persona, is hosted: the persona is notified (as a mention), it reaches the persona's home and its followers' here, and the persona's followers' servers and the author's are told with Add{Note} (the wall as the target). Anything else written there is dropped. The persona may delete it (DELETE /api/v1/statuses/:id), which sends Remove{Note} with the wall as a plain id; Smithereen deletes the post then, as it does when an owner deletes a post on its wall. The wall lists the persona's public posts that start a thread and what was written on it, newest first. An account elsewhere that tells its followers here, with Add{Note} on its own wall, of a post written there has it shown to them as on its wall (privapub.wall on the status); a Remove there takes it away.
• Pinned posts are the actor's featured collection (/trophies); featuredTags is /tattoos. A pin or an unpin is told to the post's audience as Add or Remove on featured, as Mastodon tells it.
• 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, never by the reporting account and never naming it.
  • To the reported account's server: from the instance actor, with the account and the posts as object and the persona's words in content.
  • To a community on a server whose NodeInfo names Lemmy, PieFed or Mbin (owner decision 2026-10-06, the second place PrivaPub decides by a server's software; PieFed and Mbin once the pasture showed each keeps such a report with its reason): one Flag per reported post or comment that was made in that community (its own audience, else its thread's), from the reporter, to the community, the post alone as object, the words (or, with none, the category) in summary and content, sent to the community's inbox. Lemmy refuses a Flag from an Application, with no to or with no reason; PieFed drops one from an Application; Mbin reads the reason from summary alone. Such a server gets no instance actor's Flag: an account alone, or a post outside its communities, is not reported there, since Lemmy takes neither (PieFed would take an account alone from the reporter; that is not sent yet).
• Direct messages go out as a Note addressed to their recipients, except a message to one account elsewhere that writes to us as ChatMessages (Pleroma's type), or whose server takes nothing else: Lemmy before 1.0 and Mbin, as their NodeInfo names them (owner decision 2026-10-05, the first place PrivaPub decides by a server's software). That message goes out as a ChatMessage, to the account alone, without a mention in its text.
• Replies are passed on (inbox forwarding; owner decision 2026-10-05). A public or unlisted reply from another server to a persona's public, unlisted or followers-only post goes on to the persona's followers as its author's server sent and signed it, the way Mastodon forwards it: to every follower's server but the replier's own. Its Update and Delete follow the same way. Nothing is passed on for a local-only post, a group's post or a reply that is not public or unlisted. A server that receives it checks it as it checks any forwarded activity: Mastodon, Misskey and Sharkey keep it only when its author LD-signed it (Mastodon does; Misskey 2026.10 and Akkoma do not), others read the reply from its origin.
• Events (owner decision 2026-10-05). A persona joins another server's event with a Join and leaves it with a Leave, both sent to the event's organiser only (…/grunts/join-<id>, …/grunts/leave-<id>). The organiser's server answers Accept (Mobilizon at once for an open event) or Reject, which the persona sees as privapub.event.participation: pending, accepted or rejected. An event that takes participants by invitation or on another site (Mobilizon's joinMode invite or external) is refused before anything is sent (422). Who takes part is public on the event's server.

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. A post's earlier versions in formerRepresentations (Pleroma, Akkoma) are its edit history, read as the post itself is: a post first met after its edits has them, and an edit that carries them brings the versions missed in between.

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.
• Quoting our accounts. Our public and unlisted posts state interactionPolicy.canQuote. By default anyone may quote, automatically (as on Mastodon); an account can choose followers only or nobody, and so can each post.
  • A QuoteRequest is answered with Accept{result} naming a stamp at /peasants/{name}/parrot-licences/{id}, or with Reject.
  • A stamp is a QuoteAuthorization naming both posts. A revoked stamp answers 410 and is announced with Delete{stamp}.
  • Followers-only posts and DMs can never be quoted.

Interaction policies (GoToSocial's canReply, canLike, canAnnounce, with the older always and approvalRequired):

• Read on every remote post; one left out means anyone, automatically. A persona is let in at once when the rule names the public, the persona itself, the author's followers while it follows the author, or the accounts the author follows while the author follows it.
• Shut out: the client API answers 422 to a reply, favourite or boost the rule does not allow at all.
• Asked first: a ReplyRequest, LikeRequest or AnnounceRequest (the interaction as its instrument) goes to the author alone, and the interaction waits (privapub.approval: pending). The author's Accept brings an authorization (result), which must be on the author's origin, attributed to them and naming the interaction and the post; only then does the reply go out with replyAuthorization, the boost with announceAuthorization and the like with likeAuthorization. A Reject leaves it ours alone (rejected), and takes a like or a boost back.
• As a third party, a reply that a post's policy does not let in at once is kept only with an authorization that verifies the same way. A rule naming a collection we cannot list (followers, following) lets the reply in.
• Shown to clients as GoToSocial's interaction_policy (can_favourite, can_reply, can_reblog).
• Our own posts state only canQuote: anyone may reply to, like and boost them.

Replies its author approves (FEP-5624's canReply, as PeerTube sets it on a video whose comments are moderated; a null one says nothing): a persona it names, or that the post mentions, may reply, and the reply waits (privapub.approval: pending), its Create going to the author alone. The author's ApproveReply, signed by the author and naming the post answered, lets it out to its audience with replyApproval; a RejectReply leaves it ours alone (rejected). An empty canReply refuses every reply (422).

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.

Relays

PrivaPub reads from the relays Federation:Relays names (by their actor's address, or their inbox's), none by default. Its instance actor follows Public at each, as Mastodon subscribes (Federation/Relays/Relays.cs, a minute after start and every six hours: an unanswered or refused subscription is asked again a day later, and a relay no longer named gets the Undo). What an accepted relay passes on comes to the federated timeline, never to anyone's home: a public post it forwards as its author sent it (Activity-Relay), read again from its origin like any forwarded post, and a post it 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: PrivaPub's carry a proof, so they reach Mastodon through both kinds of relay (checked live).

Server descriptions and the crawler

PrivaPub keeps statistics about servers, never about accounts (see /stargazing on the server).

• Describing a server: a server it exchanges activities with is described at most once a week, from:

  • /.well-known/nodeinfo and the NodeInfo document it links;
  • /api/v2/instance, falling back to /api/v1/instance.

  These requests are unsigned, because they are not ActivityPub documents.

• The crawler is on at privapub.thepra.dev. It identifies as PrivaPub-Stargazer/<version> (+https://privapub.thepra.dev/stargazing).

  • What it reads: /robots.txt, then the documents above and /api/v1/instance/peers, and nothing else: no accounts, posts or directories.
  • How often: one server a minute, each at most weekly.
  • Opting out: in robots.txt, the crawler obeys the group PrivaPub-Stargazer, then PrivaPub, then *. A robots.txt that answers with a server error or times out also keeps it out, as do this server's domain blocks.

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.
• Another account of the same server. An object attributed to another account of the actor's own server (Mobilizon's organiser creates, edits and deletes the group's event) is that server's to vouch for: a creation or an edit is taken as the server has the object, a deletion once it answers 404 or 410. Attributed to an account elsewhere, it is refused (400).
• Forwarded activities (ActivityPub's inbox forwarding: a thread's server passing on the replies in it, signed with its own key) are answered 202 and believed only as far as their origin vouches for them: a created or edited post is read again from its author's server, a deletion of a public or unlisted post is applied once that server answers 404 or 410, and anything else is dropped. LD signatures are not verified. A reply forwarded this way is kept when someone here follows the author of the post it answers, as Mastodon does.
• Follow requests still unanswered are sent again after 15 minutes, an hour, 6 hours, a day, two and four days: a server can take a Follow and lose its answer (Lemmy 1.0 sends nothing it queued for a server before it started sending there), and one that holds the follow answers it again. Each is the same follow under a new id (…/follow-<id>-again-<n>), since Lemmy ignores an id it has seen; an answer naming any of them counts.
• 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.
• Threads. A reply's missing parents are fetched, up to 10 levels. When someone here opens a public remote thread, its replies are read from their servers, at most once an hour: the thread's context collection (FEP-7888) if it has one, else its replies (PeerTube's comments) and theirs, two levels down; 5 pages and 100 posts at most. Only public and unlisted replies are kept, each fetched from its own origin.
• Reading our documents (SecureMode). privapub.thepra.dev answers ActivityPub GETs only when they are signed, like Mastodon's authorized fetch; the server's own actors (the instance actor /peasants/privapub and the reporter /peasants/privapub_reports) are the exception, since their keys are needed first. A browser asking for HTML is redirected to the public page instead.
• Posts that are not public (followers-only, direct, circle) are served to a signed request from someone they were for, or from the instance actor of a server where someone they were for lives, and to nobody else (404). Once deleted they answer those same readers 410. A direct message's context, /peasants/{name}/whispers/{id}, is an OrderedCollection of the conversation's posts for its participants.
• 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, a community's announces included, until the post they carry is deleted (then 410); a persona's boost id sends a browser to the boosted post. 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. Neither do Update, Delete, EmojiReact, QuoteRequest and its answers, Flag, Ignore or poll votes. All of them are always sent with their object embedded. Every boost and every favourite is an activity of its own (announce-{boost}, like-{favourite}), so one given again after its Undo is new to the server it reaches.
• Hashtags. A post's Hashtag links go to /tags/{tag}, a public page of this server's public posts with that tag.
• Delivery. An activity goes once to each server, to its shared inbox when the server has one, whoever there it follows, names or answers (a circle post excepted: each member's copy names that member). 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: unreachable, timing out, or a gateway answering 502, 503 or 504. A 500 is the server failing on that one activity, retried on its own while its other deliveries go on. A server can also take a Follow (202) and drop it afterwards, as Pleroma does when it cannot yet fetch our actor, so a follow request still unanswered is sent again, same activity, when the persona follows once more, at most once an hour.

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. Video and audio are streamed through it with byte ranges passed on, so a PeerTube file is never fetched whole for one viewer.
• 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.
• 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).