coop/docs/concord-usage.md

Using the Concord backend

crates/concord is a protocol crate: derivations, envelopes, folds and the local state document. It has no GPUI dependency and owns no strings a user reads — the UI layer decides every rendering. This document is the map from a UI action to the calls it makes.

A community is addressed by a community_id (never on the wire) plus three secrets: community_root (read access — holding it is membership), control_root (write access to the Control Plane, held by staff), and per-Channel keys for private channels. Authority is a roster of owner-rooted signed grants, folded independently by every client.

Modules

Module	Owns
derive	Every frozen HKDF derivation and coordinate
stream	The CORD-01 envelope: seal, wrap, open, and the NIP-44 helpers
edition	Chained, versioned editions: parse, hash, fold, floors
roles	Permissions, roles, grants, the banlist, the authority fixpoint
control	The Control Plane: genesis, the fold, the writer, metadata
chat	The Chat Plane: message/reaction/edit/delete builders and the fold
guestbook	Joins, leaves, kicks, snapshots, the member list
invite	Invite bundles, links, the Direct Invite, the Invite List
list	The Community List (a member's own memberships, across devices)
rekey	Key rotations, refounding, compaction, dissolution
pins	Pin Lists, and the key disclosure a keyless reader verifies
store	Local rumor cache, the community state document, relay paging

Read CommunityId as "this community", ChannelId as "this channel", Epoch as "which key generation". Nothing else in the API needs internal state.

Creating a community

use concord::control::{self, CommunityMetadata};
use concord::store::{self, CommunityState, save_state};

let metadata = CommunityMetadata { name: "Room".into(), ..Default::default() };
let minted = control::genesis(&owner_keys, &metadata, now_secs)?;

// minted.identity    — community_id, owner, owner_salt (verify() recomputes it)
// minted.wraps       — the two owner-signed genesis editions, already sealed
// minted.channel_id  — the #general channel
for wrap in &minted.wraps {
    client.send_event(wrap).to(&relays).await?;
}

The owner then needs the folded state, which is also what every member does on join:

use concord::derive::{control_group_key, control_signer_group_key};
use concord::edition::ParsedEdition;

let read = control_group_key(&minted.community_root, &minted.identity.community_id, Epoch(0))?;
let signer = control_signer_group_key(&minted.control_root, &minted.identity.community_id, Epoch(0))?;
let editions: Vec<ParsedEdition> = minted
    .wraps
    .iter()
    .map(|wrap| control::open_edition(wrap, &read, &signer.pk(), true))
    .collect::<Result<_, _>>()?;

let mut state = CommunityState::from_genesis(&minted, &editions, added_at_ms)?;
save_state(database, &state).await?;

Put the community's relay list into state.relays and add those relays to the client explicitly — coop's client is a gossip client with no background refresh.

Joining

An invite link resolves to a bundle:

use concord::invite::{self, BundleState, invite_bundle_key};

let link = invite::parse_link(url)?;      // link_signer, token, bootstrap_relays, naddr

// The crate does no I/O: fetch the naddr from the fragment's relays, then:
let invite = match invite::parse_bundle_event(&event, &link.link_signer, &invite_bundle_key(&link.token))? {
    BundleState::Live(invite) => invite,      // validate() already ran
    BundleState::Revoked => return Ok(None),  // a tombstone at the coordinate
};

A Direct Invite arrives as a NIP-59 gift wrap addressed to the member:

let (inviter, invite) = invite::unwrap_direct_invite(&wrap, &my_keys)?;

Either way the invite carries community_id, owner, owner_salt, community_root, root_epoch, control_pk, the granted channels (ChannelGrant { id, key, epoch, name }) and the relay set. invite.expired(now_ms) is a preview rule: a past expiry still renders, but the join is refused.

Then publish a join so the member list sees the member before any backfill:

use concord::derive::guestbook_group_key;
use concord::guestbook;

let guestbook = guestbook_group_key(&invite.community_root, &invite.community_id, invite.root_epoch)?;
let rumor = guestbook::build_join(my_pk, Some((creator_npub, label)), now_ms);
let (wrap, _) = guestbook::seal_rumor(&rumor, &guestbook, &my_keys)?;
client.send_event(&wrap).to(&relays).await?;

Reading the Control Plane

use concord::control::{self, ControlFold};

let editions: Vec<ParsedEdition> = wraps
    .iter()
    .filter_map(|wrap| control::open_edition(wrap, &read, &control_pk, true).ok())
    .collect();

let control: ControlFold =
    control::fold_control(&owner, &community_id, &editions, &state.floors(), &state.banned);
state.apply_fold(&control);

ControlFold is everything the community UI needs:

Field	Use
community	name, description, icon, banner, message_expiration
channels	the channel list; deleted: true means drop it
roles	role(), roles_of(), effective_permissions(), is_authorized(), is_staff()
banned	the banlist
registries / is_public()	each invite creator's live link signers
pins / pin_content(id, channel)	Pin List content per channel
floors	the committed heads the next fold is judged against
gapped	a chain hole: refetch the Control Plane before trusting what is missing

None on community or a channel means "this client saw no authorized edition", never "the value is gone" — keep what the state already holds rather than walking the community backwards. Feed state.floors() and state.banned into the next fold; they are its memory.

Sending a message

use concord::chat::{self, build_message};
use concord::derive::channel_group_key;

let plane = channel_group_key(&community_root, &channel, epoch)?;   // public channel
let rumor = build_message(my_pk, &channel, epoch, text, None, at_ms, timer);
let (wrap, wrap_key) = chat::seal_rumor(&rumor, &plane, &my_keys, false)?;
client.send_event(&wrap).to(&relays).await?;

• epoch is the channel's current epoch (state.channels carries it). A private channel derives from its own key instead of community_root.
• timer is control.community.message_expiration; pass None when it is off. The builder attaches the NIP-40 tag and seal_rumor mirrors it onto the wrap, so relays drop the ciphertext too.
• ephemeral: true picks kind 21059 for typing indicators. Keep the returned wrap key if the message may be deleted later — a kind-5 delete needs it.
• That send_event is the whole publish path; there is no optimistic echo. Feed the wrap through the same ingest path the subscription uses so send-then-read never waits on a relay round trip.

build_edit, build_reaction and build_delete are the same shape, each a rumor about an existing EventId rather than a mutation.

Reading a channel

use concord::chat::{self, fold, plane_keys};

let planes = plane_keys(&held, &channel)?;            // &[(Epoch, secret)]
let mut rumors = Vec::new();

for wrap in &wraps {
    let Some((epoch, group)) = planes.iter().find(|(_, group)| group.pk() == wrap.pubkey) else {
        continue;
    };
    let Ok((opened, rumor)) = chat::open(wrap, group, &channel, *epoch) else {
        continue;
    };
    store::cache_rumor(database, &channel, &opened).await?;
    rumors.push(rumor);
}

let messages = fold(&rumors, Timestamp::now(), |actor, citation, author| {
    citation_ok(&owner, &community_id, actor, citation, &control.roles.floors)
        && control.roles.can_act_on_member(actor, &owner, author, Permissions::MANAGE_MESSAGES)
});

ChatMessage carries content, at_ms, edited_at, deleted, reactions, reply_to and thread_root already resolved. The fold drops expired rumors; a deleted row is still returned so the timeline keeps its shape.

Relay history pages through the local cache:

let page = store::backfill(client, database, &channel, &held, until, 50).await?;
let cached = store::query_rumors(database, &channel, None, 50).await?;

backfill walks newest-first across every held epoch, caches what it opens, and stops on a short page. query_rumors is the read path when the group keys are gone. Run store::purge_expired(database, &channel, now) on the same cadence as any other local sweep — the timer is cooperative, so the local store is the artifact that has to forget.

ChatAction::TimerNotice { seconds } is a policy notice, not a message: render it as an inline row only when its author passes control.roles.is_authorized(&author, &owner, Permissions::MANAGE_METADATA).

Membership

let states = guestbook::coalesce(&rumors, now_ms, Some(&refounder_pk), |actor, target, citation| {
    citation_ok(&owner, &community_id, actor, citation, &control.roles.floors)
        && control.roles.can_act_on_member(actor, &owner, target, Permissions::KICK)
});
let members = guestbook::complete_memberlist(&states, &observed, &granted, &control.banned, &BTreeMap::new());

• observed is npub → ms for every author this client has seen publish anything usable, which is what makes a member visible before their Join arrives. Only count it forward.
• granted is every npub the roster ranks; they are members with no Guestbook entry at all.
• banned_at is empty today, so a ban is terminal in the fold. Fill it when the banlist head's timestamp is plumbed through.
• Removal is three separate actions, composed by the caller: strip the grant (immediate and cheap), then the kick directive, then — for a ban — the rotation that actually enforces it.

Moderation writes

Every Control Plane write goes through one writer and one edition shape:

let writer = ControlWriter { author: my_pk, read: read.clone(), signer: signer.clone() };
let head = control.floors.get(entity).cloned();

let (wrap, new_head) = writer.set_community_metadata(
    &my_keys, &community_id, &metadata, head.as_ref(), citation, now_secs)?;

citation is the vac the actor acts under — None only for the owner. Build it from the folded Grant that ranks them (AuthorityCitation { entity, version, hash }) and pass the head from the current fold, so the chain cannot silently fork.

Wrappers: set_community_metadata, set_channel_metadata, set_role, set_grant, set_banlist, set_registry, set_pin_list, plus raw publish. A ban is a set_banlist followed by a base rekey; a kick is a set_grant with an empty role_ids followed by guestbook::build_kick.

Pins

use concord::pins;

let entry = pins::build_entry(&opened_message, &plane, &channel)?;
let head_content = control.pin_content(&community_id, &channel).unwrap_or("");
let read = pins::read_list(head_content, |epoch| channel_group_key(&root, &channel, epoch).ok());
let content = pins::publishable(&read, channel_is_private, &plane, epoch)?;
let (wrap, _) = writer.set_pin_list(
    &my_keys, &community_id, &channel, &content, head, citation, now_secs)?;

Reading is verification: read_list decodes either content form (public, or sealed under the channel key at the named epoch), and pins::verify_entry(entry, &channel) returns a VerifiedPin with the proven author, words and time — no history and no old keys needed. read.sealed means the list is sealed under an epoch this client never held: show it as unavailable, and never write from it (publishable refuses). pins::killed_by(&pin, &delete) answers whether a folded kind-5 erases an entry.

Invites

use concord::derive::{invite_bundle_key, TOKEN_LEN};
use concord::invite::{self, InviteEntry, InviteTombstone};

let token: [u8; TOKEN_LEN] = /* 16 bytes from any CSPRNG */;
let bundle_key = invite_bundle_key(&token);
let link_signer = Keys::generate();
let bundle = invite::build_bundle_event(&link_signer, &invite, &bundle_key)?;
let url = invite::build_invite_url(BASE, &link_signer.public_key(), &token, &relays)?;

A link is a coordinate plus a fragment: the naddr fetches the bundle, the token unlocks it, and the fragment names the relays to fetch from. invite::stock_relays() is what a fragment with no relays of its own means.

The link_signer secret is what lets the creator refresh or retire the link, so keep it against the token in the member's own Invite List — a local document encrypted to self, exactly like the Community List:

let mut list = invite::parse_invite_list(&my_keys, &event)?;
list.entries.push(InviteEntry {
    token: HEXLOWER.encode(&token),
    signer_sk: link_signer.secret_key().to_secret_hex(),
    community_id: invite.community_id,
    url,
    label: None,
    created_at: now_ms,
    expires_at: None,
    extra: Default::default(),
});
let event = invite::build_invite_list(&my_keys, &list)?;      // kind 13303

// Retiring is a tombstone, never a deletion: it beats a stale copy terminally.
list.tombstones.push(InviteTombstone {
    token: HEXLOWER.encode(&token),
    community_id: invite.community_id,
    extra: Default::default(),
});

merge_invite_lists merges two devices' copies, is_live(&token_hex) answers whether a link still stands, and fits() is the write gate.

Rekeys, refounding and dissolution

A rotation is authority plus delivery: rekey_authorized(&control.roles, &owner, &me, permission, &removed) gates it, plan_refounding(epoch) mints the new pair, and build_rekey_chunks seals one blob per remaining member:

use concord::derive::epoch_key_commitment;
use concord::rekey::{self, RekeyScope};

let scope = RekeyScope::Channel(channel_id);                    // or RekeyScope::Base
let plan = rekey::plan_refounding(Epoch(epoch + 1))?;

// A base rotation delivers the new control-plane keys beside the root; a channel
// rotation delivers only that channel's fresh key.
let new_key = plan.new_root;
let (control_pk, control_root) = match scope {
    RekeyScope::Base => {
        let pk = plan.signer(&community_id)?.pk().to_bytes();
        (Some(pk), is_staff.then_some(&plan.new_control_root))
    }
    RekeyScope::Channel(_) => (None, None),
};

let blobs = members
    .iter()
    .map(|member| {
        rekey::build_blob(&my_keys, member, scope, plan.epoch, &new_key, control_pk.as_ref(), control_root)
    })
    .collect::<Result<Vec<_>, _>>()?;

let rekey_group = rekey::rekey_group(scope, &community_root, &community_id, plan.epoch)?;
let wraps = rekey::build_rekey_chunks(
    &my_keys,
    &rekey_group,
    scope,
    plan.epoch,
    Epoch(epoch),
    &epoch_key_commitment(Epoch(epoch), &community_root),
    &blobs,
    citation,
    false,
    now_secs,
)?;

On the receiving side, rekey::parse_rekey_chunk(&opened) per wrap, then collect_rotations(&chunks), then am_i_removed(&rotation, &me) — which is None until every chunk is held, because an incomplete set is never a removal. A member finds their delivery with find_my_blobs / open_blob, and adopts the key only if the plaintext binds to the scope and epoch they expect and its prevcommit matches the key they already hold. Two concurrent rotations settle on fork_winner.

Dissolution is owner-only and terminal:

let rumor = rekey::dissolved_tombstone_rumor(owner_pk, &community_id, now_secs);
let wrap = rekey::seal_dissolved(&rumor, &community_id, &my_keys, now_secs)?;

// A receiver seals the community read-only on sight.
if rekey::verify_dissolved(&wrap, &identity) {
    state.dissolved = true;
}

The Community List

A member's own memberships, synced across their devices:

use concord::list;

let material = list::join_material(&invite, staff.then_some(&control_root));
let mut mine = list::parse_list_event(&my_keys, &event)?;
mine = list::merge(mine, list::CommunityList {
    entries: vec![list::CommunityListEntry { community_id, seed: material.clone(), current: material, added_at: now_ms, extra: Default::default() }],
    ..Default::default()
});
let event = list::build_list_event(&my_keys, &mine)?;      // kind 13302, NIP-44 to self

is_live(&id) answers joined-versus-left: a tombstone is terminal until a strictly newer join outruns it. fits() is the write gate — 50 memberships and the NIP-44 size cap, both protocol constants.

GPUI conventions

• Wrap, decrypt, verify, fold and every database or relay call go in cx.background_spawn. A secp256k1 verification per edition is far too expensive for the foreground thread.
• Hold entities foreground: cx.spawn with this.update(cx, |this, cx| …) and the inner cx, keeping the returned Task in a field so it is cancelled with the view.
• In tests, use cx.background_executor().timer(..) for delays, never smol::Timer, or run_until_parked() will find nothing left to run.

Not wired up yet

• No registry and no sync engine. crates/concord has no subscriptions, no init, and no Entity<Community>; the UI owns subscribing, routing a wrap to the plane whose address it carries, and rebuilding a subscription when a plane's address changes (join, channel added, rekey folded).
• Every writer takes &Keys, not a NostrSigner. NIP-46 is one deliberate pass over the builders, not a per-call patch.
• crates/chat/src/lib.rs::handle_notifications treats every kind 1059 event as a NIP-59 gift wrap for the current user. Concord wraps are kind 1059 too, so that handler must route by subscription id before any concord subscription goes live, or every stream wrap lands in the DM trash and raises a toast.
• No plane key can be persisted yet. CommunityState has nowhere to keep a key a rotation delivered and ChannelKeyRef carries no key of its own, so a client can verify a rotation and still lose it on restart — history under a prior root or a prior channel epoch is unreadable until that schema change lands.