mantra-kmp/docs/shared-key-ceremony.md

# The shared key ceremony

A group creates a `t`-of-`n` FROST key by running ChillDKG over its NIP-17 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 group is the participant set, the member who opens the ceremony coordinates
it, and every protocol message travels as a gift-wrapped rumor on the same NIP-17
pipeline chat messages already use — so there is no second transport to operate.

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.

> **`pendingApproval` must mirror the gates in `advance` exactly.** 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 `#admins` room — a Marmot group whose id
is derived from the shared key. See [shared-key-derivation.md](./shared-key-derivation.md)
for how, and [marmot-membership.md](./marmot-membership.md) for how members are
added to it.

## Known gaps

- No ceremony has been run on a physical device.
- A member invited to the `#admins` 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 + 4` per ceremony — 19 lines for five
  members. Compact, but they dominate a transcript while a ceremony runs.