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sections that argued the old collision -- "The collision this buys" and "Why a subgroup cannot be the whole group was withdrawn" -- keep their reasoning and gain the end of it: `(room, parent)` stopped telling two subgroups of one parent apart, so the lookup moved to `(room, parent,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