0be31803f2
`docs/subgroups.md` was written as ten phases of reasoning kept in the order they were argued, and phase 4 spent forty lines on why the child's ceremony was *not* held in the parent's Marmot room -- explicitly so the decision would not be re-litigated without its price attached. That section is now a shopping list that has been carried out, so it keeps its argument and gains a pointer forward, and the three costs it enumerated are checked off one by one in a new phase 10. The parts of the note that state the old arrangement as present-tense fact are updated rather than annotated: the three-ceremonies table now reads one room, three ceremonies, two quorums, and says the thing that needs saying twice -- an MLS message reaches the whole tree, so a ceremony in the parent's room has to name who it is with. Phase 10 itself is written the way the others are, around what fails silently: - `DkgSession.chatRoomId` stopped identifying a ceremony, and the place that matters is `completedKey`'s last fallback, which every member welcomed after a group's own ceremony lands on; - `signingPath` had to admit a Marmot room, which widens the one function whose contract is that a path never comes off a proposal; - the p-tags had to stay on both transports, which is the opposite of what `FrostSigningManager` correctly does. The two sections that argued the old collision -- "The collision this buys" and "Why a subgroup cannot be the whole group was withdrawn" -- keep their reasoning and gain the end of it: `(room, parent)` stopped telling two subgroups of one parent apart, so the lookup moved to `(room, parent, admins)`, and the permanent half of the refusal disappeared with the derived room. The limitation and the appendix entry are struck through rather than deleted, since what they were weighing is why the phase exists. `docs/shared-key-ceremony.md` no longer says a ceremony runs over a NIP-17 chat. The participant set is the proposal's p-tags on both transports, and that distinction is the whole reason it is stated that way rather than as "the group". `docs/mls-skipped-keys.md` keeps `proposeRitual` in its table of reliable triggers and now says what changed about it: it reached that table on gift wraps, where the bug does not apply, and a subgroup's ceremony now rides group events. It is the entry with the worst consequence -- a ChillDKG cannot finish until every participant takes part, so one lost round-1 message stalls it permanently for everybody rather than costing one member a line of chat. That is the thing the quartz fix in that note is now load-bearing for. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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The shared key ceremony
A group creates a t-of-n FROST key by running ChillDKG over its own chat. No
trusted dealer, no single device ever holding the whole key.
Implemented in ChillDkgRitualManager, on fr.acinq.bitcoin.crypto.dkg.chill.ChillDKG.
Shape
The participant set is who the proposal p-tags, the member who opens the
ceremony coordinates it, and every protocol message travels on a pipeline chat
messages already use — a gift-wrapped rumor in a NIP-17 room, an MLS application
message in a Marmot one — so there is no second transport to operate.
A group's own ceremony runs in the NIP-17 room its members share, and there the participant set and the room's membership are the same thing. A subgroup's runs in its parent's Marmot room over a subset of it, which is why the set is the p-tags rather than the room: see subgroups.md.
The coordinator is a participant too, and ChillDKG treats it as untrusted: it relays and aggregates but cannot learn secrets or bias the key. Being the room's creator buys it no authority, only work.
| kind | from | carries |
|---|---|---|
| 30310 | coordinator | proposal — "let's make a t-of-n key" |
| 30311 | participant | host public key |
| 30312 | participant | pmsg1, this device's contribution |
| 30313 | coordinator | cmsg1, everyone's contributions combined |
| 30314 | participant | CertEq signature confirming the combined result |
| 30315 | coordinator | cmsg2, the success certificate |
| 30316 | anyone | failure — abort and blame |
Every step is a pure function of inputs the device has already stored, so there is
no long-lived in-memory session to lose. Each inbound message is persisted and
then the ritual is asked whether it can move; if the app dies mid-round it resumes
on the next message. DkgSession deliberately stores inputs — the randomness
and the received messages — rather than protocol state, which is what makes that
work.
A ceremony cannot finish until every member takes part. That is unusual for a chat feature and drives most of the UI: the progress ladder names who it is waiting on rather than showing a count, because "2 of 3" does not tell anyone whose door to knock on.
Nothing publishes without approval
The ritual is driven by arriving messages, which originally meant a relay
delivering an event to a phone in someone's pocket was enough to enrol its owner
in a group's permanent signing quorum. acceptProposal published the host key on
arrival; rounds 1 and 2 followed automatically.
It now publishes nothing on this device's behalf until its owner agrees, at three separate gates:
| step | publishes | why it is its own decision |
|---|---|---|
HOST_KEY |
the host public key | joins the ceremony and fixes n. Joining then going quiet holds it open for everyone |
ROUND_1 |
the key contribution | the member's secret material starts shaping a key they must help sign with |
ROUND_2 |
the CertEq signature | a real check: it is what stops a coordinator substituting a key the members never contributed to |
The coordinator's two aggregations are not gated. They relay other members' already-published messages and disclose nothing of the coordinator's own, so an approval there would stall the whole group on one person's attention without protecting anybody.
The member who opens a ceremony is auto-approved for the host key alone — starting one is already the act of agreeing to be in it — and is still asked for rounds 1 and 2.
Each gate returns rather than throwing. The ritual is not failing, it is waiting on a person; everything received stays stored and it resumes on approval.
pendingApprovalmust mirror the gates inadvanceexactly. If they drift, the screen offers an approval that does nothing, or offers none while the ritual sits still. Neither produces an error.
Approvals are recorded as three nullable timestamps on DkgSession, plus
approvalRequestedThrough so the chat line asking for each is written once.
Faults are values, not exceptions
ChillDKG reports a faulty participant in the ChilldkgFault field of each result,
because a faulty participant is a normal outcome of a DKG rather than a bug.
This ritual has one response to all of them — the key is unusable, the session
dies, the group is told — so raiseIfFaulty turns them into an exception carrying
the culprit and lets them join advance's single failure path. The gain is the
failure text: "ChillDKG round 2 failed: a participant is faulty (participant 3)"
rather than whatever e.message happened to hold. On a failed DKG, which
participant to blame is the only actionable thing there is.
The transcript
Every protocol message becomes a line in the group's chat naming the member whose device sent it, plus the three that bracket them: started, complete, abandoned.
These rows are not anybody's words: no giftWrapPayloadId, no event behind
them, nothing sent to say them. Each device writes its own from messages it
already received, so they cost no traffic and cannot disagree with the ritual they
describe. They render as system lines rather than bubbles — attributing "a shared
key ceremony started" to the coordinator would read as something they said.
Wording describes what a step accomplishes, not what it is called. "sent their contribution to the key" is useful in a group chat; "sent pmsg1" is not, and the protocol names are on the shared-key screen for anyone who wants them.
Idempotency is by construction, not de-duplication. ChatMessage has no key
to make a second insert a no-op — its id is autogenerated — while ritual messages
arrive repeatedly: relays redeliver, and replayStoredMessages feeds the whole
backlog through record() again on every resume. So every announce is guarded by
reading the row it is about to write over. DkgParticipantMessage absorbs the
redelivery itself, being keyed on (sessionId, participantPublicKey, kind); a chat
row cannot.
Request lines carry their step in the message type — one type per step, not one type for all three. The type is the only thing a transcript keeps: a line drawn days later has no session to ask what was being requested. Sharing one type left every request wearing the same icon.
Whether a request was answered is read from the transcript rather than the
session: approving is the only thing that causes the step to be published, and
publishing writes an authored line. That keeps a room that has run more than one
ceremony correct, since ChatMessage has no session id to disambiguate with.
Ordering
ChillDKG has no session-params object to agree on out of band. Every step takes the host public keys and the threshold and hashes them into the session identity, so a group that orders its participants differently on different devices does not get a weaker key — it gets no key.
Each device derives that order independently by sorting the host keys bytewise,
and orders each round's messages by their sender's host key to match. Sorting is
the only ordering every device can arrive at without being told. Nothing in the
protocol checks this, so ChillDkgRitualOrderingTest does.
After it completes
The group has a threshold public key; each device keeps its own share, restorable from that member's wallet backup and nobody else's.
From there the coordinator can create the group's admin room — a Marmot group whose id is derived from the shared key. See shared-key-derivation.md for how, and marmot-membership.md for how members are added to it.
It is named # and then the group's name — #Ekklesia for a group called
Ekklesia. Every Marmot room on a device carries that mark and no NIP-17 room does,
which is the only thing on a room list that says which of the two a row is. The
rule is one function, MarmotGroupName.of, applied where a room is minted rather
than where it is drawn: the name is baked into the epoch-0 MarmotGroupData every
member is welcomed with, so a name added at display time would be a name this
device alone could see. It replaces Ekklesia (#admins), which marked only the
admin room, left every subgroup unmarked, and sorted nowhere near the group it
belonged to.
Known gaps
- No ceremony has been run on a physical device.
- A member invited to the admin room whose Welcome never goes out is indistinguishable, from the coordinator's side, from one who joined.
- The ritual's request chat lines are
3n + 4per ceremony — 19 lines for five members. Compact, but they dominate a transcript while a ceremony runs.