mantra-kmp/docs/frost-batch-signing.md

# Batch signing with FROST

How to have a group sign several events in one ceremony instead of one at a
time, phased, with the cryptographic constraint that shapes every phase stated
first.

The headline: **there is no such thing as one FROST signature over many
messages, and no way to reuse a nonce across them.** What can be batched is the
ceremony — the rounds, the group events, and the approval a human is asked for.
A batch of `k` events is `k` independent FROST instances run in lockstep,
sharing one signer set, one transcript and one prompt. The saving is transport
and UX, not crypto, and that is the saving worth having: today
[FrostSigningManager](../composeApp/src/commonMain/kotlin/press/mantra/compose/managers/FrostSigningManager.kt)
spends five MLS group events and one human decision per event signed.

## The constraint

A Schnorr partial signature is `s = k + e·x`, with `e = H(R‖P‖m)`. Two different
messages under the same nonce `R` give two equations in one unknown and the
secret share falls out. That is not a subtlety to be careful around; it is the
one way a t-of-n key is lost, and it is already what the two write-once rules on
[FrostSigningSession](../composeApp/src/commonMain/kotlin/press/mantra/compose/database/model/FrostSigningSession.kt)
exist to prevent.

So the rule every phase below is built to keep:

> **Every item in a batch has its own independent nonce, from every signer, and
> that nonce signs exactly one message for the life of the session.**

`fr.acinq.bitcoin.crypto.frost.Session.create` takes the message and the
aggregated nonce, so a batch is `k` `Session` objects sharing a signer set and a
`TweakCache`. There is no batch primitive in the library and none is needed.

What *is* shared across items, safely, and what is not:

| shared across the batch | per item |
|---|---|
| signer set (`signerIds`) | nonce seed (`nonceRandom`) |
| ceremony, threshold, participant count | secret + public nonce |
| derivation path and `TweakCache` | aggregated nonce |
| the human approval | `Session`, partial signature, signature |
| the chat transcript | the unsigned event and its id |

---

## Phase 1 — schema

**Half a day. No wire change, no behaviour change.**

Five columns move off `FrostSigningSession` onto a new child table: they are the
per-item ones in the table above.

```kotlin
@Entity(
    primaryKeys = ["sessionId", "itemIndex"],
    foreignKeys = [ForeignKey(
        entity = FrostSigningSession::class,
        parentColumns = ["id"],
        childColumns = ["sessionId"],
        onDelete = ForeignKey.CASCADE,
    )],
    indices = [Index("sessionId")],
)
data class FrostSigningItem(
    val sessionId: String,
    /** Position in the batch. Fixed at proposal; it is the wire ordering. */
    val itemIndex: Int,
    val unsignedEventJson: String,
    val eventId: HexKey,
    val nonceRandom: HexKey,
    val aggregatedNonce: HexKey? = null,
    val signature: HexKey? = null,
)
```

`itemIndex` is load-bearing rather than cosmetic: it is the order every device
joins nonces and partial signatures in, so two devices that disagree about it
produce aggregates nobody can verify. It is fixed by the proposal and never
re-sorted. Spelled `itemIndex` rather than `index` because `index` needs quoting
in every hand-written query it appears in, and one missing backtick is a compile
error at best.

**No `itemCount` column.** The count is `SELECT COUNT(*) FROM FrostSigningItem
WHERE sessionId = :id`, for the same reason `signerIds` is derived from the
ceremony's participant order rather than stored: a denormalised count is one
more thing that can disagree with the session it describes.

### Migration 9 → 10

Not an auto-migration. Room can add a table but cannot backfill one, and this
migration has to move data before it drops the columns it came from. Manual, in
the shape of `MIGRATION_3_4` — the existing precedent for a migration that is
about data rather than shape.

```sql
CREATE TABLE FrostSigningItem (...);
INSERT INTO FrostSigningItem (sessionId, itemIndex, unsignedEventJson, eventId,
                              nonceRandom, aggregatedNonce, signature)
     SELECT id, 0, unsignedEventJson, eventId, nonceRandom, aggregatedNonce, signature
       FROM FrostSigningSession;
ALTER TABLE FrostSigningSession DROP COLUMN unsignedEventJson;  -- and the other four
```

**`DROP COLUMN`, not a table rebuild.** The usual SQLite way to remove columns —
create a new table, copy, drop the old one, rename — is unsafe here and quietly
so. `FrostSignerMessage` and the new `FrostSigningItem` both reference
`FrostSigningSession(id)` `ON DELETE CASCADE`, and `DROP TABLE` fires cascades:
with foreign keys enforced it would delete every signer message and every item
the migration had just written. Whether it does depends on Room having turned
foreign keys off around the migration, which is not worth depending on when
`ALTER TABLE ... DROP COLUMN` cannot go wrong. It needs SQLite 3.35 and columns
free of indices and constraints; these five qualify, and `getRoomDatabase` pins
`BundledSQLiteDriver` on every platform, so the SQLite version is ours rather
than the host's.

**Why the backfill has to be exact.** A session in flight at upgrade time holds
its nonce seed and, possibly, its aggregated nonce in those columns. Losing
either means regenerating a different nonce on the next pass — publishing a
second partial signature over the same message against a different aggregate,
which is the extraction case. Copying them verbatim into item 0 means an
in-flight session resumes as though nothing happened. A migration that dropped
them and let sessions restart would be the one dangerous way to write this.

Update the entity list and `version = 10` in
[MantraDatabase.kt:175](../composeApp/src/commonMain/kotlin/press/mantra/compose/database/MantraDatabase.kt),
regenerate `composeApp/schemas/10.json`, and add the DAO methods:
`getItems(sessionId)`, `observeItems(sessionId)`, `upsertItems(List<FrostSigningItem>)`,
`countItems(sessionId)`, `countSignedItems(sessionId)`.

**Test:** extend
[FrostSigningSessionDaoJvmTest](../composeApp/src/jvmTest/kotlin/press/mantra/compose/database/dao/FrostSigningSessionDaoJvmTest.kt)
with item CRUD, the `(sessionId, itemIndex)` dedupe, index ordering and the
cascade delete. Add a migration test that runs the migration over a v9 database
holding a half-finished session and asserts the seed and aggregate survive at
index 0 — the values, not merely that a row appeared.

---

## Phase 2 — vectorise the manager at k = 1

**Two days. Still no wire change, still no API change.**

The whole point of this phase is that **every existing test passes unchanged**.
`proposeSigning` still takes one event, still writes one item, and the messages
on the wire are byte-identical to today's. What changes is that `advance()`
loops.

In [FrostSigningManager.advance](../composeApp/src/commonMain/kotlin/press/mantra/compose/managers/FrostSigningManager.kt):

1. Load `items` once, ordered by `index`.
2. Nonce generation becomes a `map` over items, each with its own
   `session.nonceRandom` → `item.nonceRandom` and its own `message`.
3. `Session.create` becomes one per item; the `signerIds`, `publicShares`,
   `nParticipants`, `threshold` and `tweakCache` arguments are the same for all
   of them.
4. `sign` and `aggregateSigs` become per item.
5. `complete()` verifies each item's signature against `item.eventId` and calls
   `applySignedEvent` for each.

Three invariants to establish here, because Phase 3 depends on all three:

- **The signer set and every aggregated nonce are one write-once unit.** The
  coordinator writes `session.signerIds` and all `k` `item.aggregatedNonce`
  values in a single `@Transaction`, so "some items aggregated" is unreachable.
  `signerIds != null` stays the gate the rest of `advance()` reads, exactly as
  today.
- **Signatures likewise.** All `k` land in one transaction; `countSignedItems ==
  countItems` is the settled test, replacing today's `session.signature != null`
  in both `advance()` and `isAwaitingApproval`.
- **Item order is the wire order.** Every join and split goes through one pair
  of helpers, never an ad-hoc `map` at a call site.

### The cost that appears here

`advance()` regenerates nonces and creates FROST sessions **unconditionally on
every inbound message**, before checking whether this device has already
published. At k=1 that is one native call per message and nobody notices. At
k=64 it is 64 nonce generations, 64 `Session.create`s and 64 signs on every
message the group sends — several hundred native calls to discover there is
nothing to do.

Fix it in this phase, while it is still cheap to verify: generate nonces on
demand, and leave before building any `Session` when this device is neither
signing nor aggregating. Both are pure optimisations at k=1, which is the point
of doing them before k>1 exists.

**Generate them lazily, not behind a guard.** The obvious version — compute the
nonces only when `ownNonce == null || (isSigner() && ownPartial == null)` — is
wrong, and wrong in a way that passes a reading and fails every test. The
coordinator settles the signer set *further down the same pass*, so `isSigner()`
read at the top of `advance()` is false on the pass where the coordinator is
about to become a signer, and the nonces it then needs were never generated. A
`by lazy` has no such prediction to make: it generates at first use, at most
once per pass, and never on a pass with nothing to publish.

### The payload codec, early

`joinPayload`/`splitPayload` land here rather than in Phase 3, because at k=1
they are the identity — a one-element comma join is the bare value — so they
change no byte on the wire and leave Phase 3 to the proposal encoding alone.

**Test:** existing tests are the test. If
[FrostSigningRoundTest](../composeApp/src/commonTest/kotlin/press/mantra/compose/managers/FrostSigningRoundTest.kt)
and
[SignedGroupKeyStateTest](../composeApp/src/jvmTest/kotlin/press/mantra/compose/managers/SignedGroupKeyStateTest.kt)
pass without edits beyond the schema move, the vectorisation is faithful.

---

## Phase 3 — k > 1 on the wire

**Two days. This is the phase with the compatibility trap in it.**

### Payload encoding

*(Landed in Phase 2 — see above. Restated here because the rest of this phase
depends on it.)*

`FrostSignerMessage` keeps its `(sessionId, signerPublicKey, kind)` primary key
and **one row carries all `k` values**, comma-joined — the same encoding
`FrostSigningSession.signerIds` already uses.

Per-item message rows were the obvious alternative and are worse. The current
key is what makes a redelivered message overwrite rather than accumulate, which
is what keeps the coordinator's signer set the right length; splitting by item
multiplies the ways a partial delivery can look like a complete one. One group
event carrying a signer's whole contribution also matches the transport: a
signer publishes all `k` nonces or none.

The parse is strict. A payload whose element count is not the session's item
count is dropped, not truncated and not padded:

```kotlin
private fun splitPayload(payload: String, expected: Int): List<String>? =
    payload.split(",").takeIf { it.size == expected }
```

Note *where* this runs. `record()` stores payloads without parsing them, which
is what lets a nonce arrive before its proposal; the count check therefore
belongs in `orderedNonces` and `orderedPartialSignatures`, where the session —
and so the count — is known. Do not move it earlier to "fail faster"; that
breaks the out-of-order replay that `replayStoredMessages` exists for.

### Proposal encoding, and the compatibility trap

The proposal's content becomes a JSON **array** of unsigned events — but only
when `k > 1`.

A single-event session must keep serialising as a bare JSON object, byte for
byte as today. The reason is what an old build does with each form:

| old build receives | outcome |
|---|---|
| bare object (k=1) | signs it, exactly as now |
| JSON array (k>1) | `Event.fromJsonOrNull` returns null → "does not carry an event; dropping" |

That is the correct failure. An old device refuses a batch rather than
mis-signing part of one, and a group with mixed versions keeps single signing
working throughout the rollout. Emitting an array unconditionally would break
every k=1 session for old devices and buy nothing.

`acceptProposal` accepts both: array if the content starts with `[`, otherwise a
one-element list. New devices understand old proposals forever; old devices
understand new single proposals forever.

### `acceptProposal` over a list

Each element is rebuilt from its own fields under the room's path and checked
against the id it claims — the existing check, per item, and the whole proposal
is dropped if any one fails. The write-once rule extends from "the event this
session signs" to "the ordered list of events this session signs": a second
proposal under the same id whose list differs anywhere is logged and ignored,
never applied.

### A cap on `k`

Enforce a maximum batch size — 64 is a sane starting number — in
`proposeSigning` **and independently in `acceptProposal`**.

The second check is the one that matters. Without it a proposer can hand every
member a batch of arbitrary size and have them do unbounded native work and
publish an unbounded group event, from a single message. The proposal is the one
place in this protocol where a remote party decides how much work everyone else
does, and it is currently bounded only by never having more than one item.

Size it against the MLS group event limit rather than picking a round number:
each signer's nonce message is `k × 133` bytes of hex-and-commas, the partial
message `k × 65`, and the proposal itself carries `k` whole events, which is the
term that actually binds. 64 to start.

### `proposeSigningBatch` on the manager, here

The public API on the manager lands in this phase rather than the next one, for
a plain reason: there is no other way to produce a `k > 1` session, so without it
everything above ships untested. `proposeSigning` becomes its one-event form and
keeps its signature, so no caller moves. Phase 4 is then the repository, the call
sites and the failure policy.

That also makes this the phase where the batch is proved end to end — a `k = 3`
session between two devices over two databases, plus the negative test that no
two items share a nonce. Both are described under Phase 6 and are worth reading
there; they simply run here, because this is where the thing they test exists.

---

## Phase 4 — the batch API and its call sites

**A day.**

*(`FrostSigningManager.proposeSigningBatch` itself landed in Phase 3 — see
above. This phase is what surrounds it.)*

```kotlin
suspend fun proposeSigningBatch(
    database: MantraDatabase,
    localChatRoom: LocalChatRoom,
    userPublicKey: HexKey,
    events: List<EventTemplate<*>>,   // each carries its own createdAt
    key: DkgSession? = null,
): FrostSigningSession
```

`proposeSigning` stays, as the one-element call into it, so
[AddDialectViewModel](../composeApp/src/commonMain/kotlin/press/mantra/compose/ui/view/model/AddDialectViewModel.kt),
[AddArtifactViewModel](../composeApp/src/commonMain/kotlin/press/mantra/compose/ui/view/model/AddArtifactViewModel.kt)
and
[GroupKeyStateManager.propose](../composeApp/src/commonMain/kotlin/press/mantra/compose/managers/GroupKeyStateManager.kt)
need no edit at all. `GroupKeyStateManager` in particular should never batch: a
room's key state is the statement every other session is opened against, and
bundling it with anything else would make it as available as its worst
co-passenger.

Mirror both on
[FrostSigningRepository](../composeApp/src/commonMain/kotlin/press/mantra/compose/repository/FrostSigningRepository.kt),
including the `NO_OP` implementation. `signedEvent(session): Event?` becomes
`signedEvents(session): List<Event>`.

### Nonce seeds

`k` independent 32-byte seeds, generated at proposal, one per item. Deriving
them from a single session seed by index would work and save nothing worth
having: independent seeds mean an off-by-one in index handling produces a
session that fails to aggregate, rather than one that signs two messages under
one nonce.

### Failure policy: all or nothing

A batch fails whole. If any item's aggregation fails, `fail()` runs as it does
today, nothing is applied, and the group is told once.

This is a real cost and should be stated where callers can see it: **a batch is
only as available as its worst item**, so bundling unrelated events makes both
less likely to get signed. Batch things that belong together.

The retry rule is the one that must not be got wrong, and deserves a comment on
`proposeSigningBatch` itself: **a retry is a new session id with new seeds.**
Never re-propose a failed batch under its own id, and never reuse an item's
`nonceRandom`. This is true of single sessions today; batches make it more
tempting to get wrong, because a batch that failed on item 5 looks like it has
four perfectly good nonces going spare. It does not — those four have already
been published against an aggregate.

Per-item partial success is deliberately out of scope. It would need mixed-state
UI, a transcript that can say "3 of 5", and a `complete()` that applies a subset,
which is a lot of surface for an outcome that indicates a bug or a dishonest
coordinator rather than a normal ending.

---

## Phase 5 — UI

**Two days, most of it in the screen.**

- `FrostSigningUIState.Loaded` gains `items: List<FrostSigningItem>`.
- [FrostSigningViewModel](../composeApp/src/commonMain/kotlin/press/mantra/compose/ui/view/model/FrostSigningViewModel.kt)
  combines `observeItems` into its existing `combine`; nothing else changes,
  since it already re-renders on every session write.
- [FrostSigningScreen](../composeApp/src/commonMain/kotlin/press/mantra/compose/ui/composable/FrostSigningScreen.kt)
  renders a list where it renders one event today.

**The approval gate must show every event, not a count.** The argument in
`FrostSigningManager`'s own header — one approval rather than three, because the
event is fixed before the member is asked — only holds if the member can see
what they are approving. "Sign 12 events?" behind a chevron is a worse prompt
than the twelve prompts it replaces. A member who cannot scroll the whole list
should not be able to approve it.

`FrostSigningRoute` is unchanged: it already carries a session id, and a batch is
one session.

### Transcript

No new `ChatMessage` types. The existing `TYPE_FROST_*` constants,
`FROST_REQUEST_FULFILMENTS`, `FROST_SETTLEMENTS` and `FROST_TYPES` in
[ChatMessage.kt:220](../composeApp/src/commonMain/kotlin/press/mantra/compose/database/model/ChatMessage.kt)
all work as they stand — one line per step, per member, whatever `k` is. Only
the wording in `announceStep` and `announceStarted` gains a count: "signed their
part of 12 events". This is worth checking rather than assuming, because a new
type would need adding to five sets and would silently render as a chat bubble
if missed from `FROST_TYPES`.

---

## Phase 6 — the test that actually proves it

**A day, and do not skip it.**

*(The first two ran in Phase 3, where the batch first existed. Kept here because
this is the section anybody adding to these tests will read.)*

Extend
[SignedGroupKeyStateTest](../composeApp/src/jvmTest/kotlin/press/mantra/compose/managers/SignedGroupKeyStateTest.kt),
which already runs a real signing session between two devices over two
databases, with a `k = 3` batch: both devices reach `COMPLETE`, all three
signatures verify against their own event ids under the room's key, all three
events are applied locally on both, and the whole thing costs five messages from
the coordinator and two from the other signer rather than one round per event.

**The negative one that matters**: assert the three aggregated nonces are
pairwise distinct, and that the three seeds are. Every positive test above still
passes if two items share a nonce — the signatures verify perfectly well; what
sharing costs is the secret share, to anyone who sees both partial signatures.
It is the cheapest possible guard against the one mistake in this document that
loses the key, and it catches an index bug nothing else here would.

Then, still to write:

- a wrong-length payload is dropped rather than truncated;
- a second proposal under the same session id with a changed item is ignored;
- a `k = 3` batch in
  [FrostSigningRoundTest](../composeApp/src/commonTest/kotlin/press/mantra/compose/managers/FrostSigningRoundTest.kt),
  against real FROST and no database, for the same reason that file exists at
  all: the library calls are checked without a database in the way.

---

## Phase 7 — rollout

**No code.**

Nothing here needs a feature flag. `k = 1` is the entire behaviour of the app
after Phase 4, byte-identical on the wire to the app before Phase 1, and no
caller batches anything until one is written to. Phases 1 and 2 are shippable on
their own and worth shipping on their own — they are a schema move and a
refactor, and landing them apart from the wire change means a bisect over a
signing bug lands on one or the other rather than on all of it.

Before the first caller batches, confirm the group is on a build that
understands array proposals. There is no negotiation for this and adding one is
not worth it; the failure mode is a batch that never reaches threshold and is
abandoned, which is visible in the transcript and costs nothing but a retry.

---

## Appendix — what was considered and rejected

**One signature over `k` messages.** Does not exist for Schnorr. Aggregate
signature schemes that do this (BLS) are a different curve and a different
verification story, and nostr verifies BIP-340.

**One nonce, `k` messages.** This is the extraction attack, described at the top.

**Nonce pre-processing** — the FROST paper's own batching, where signers
pre-publish a list of π nonce commitments before any message is known, and each
later signature consumes one commitment per signer in a single online round. The
library supports it: `SecretNonce.generate` takes `message` as nullable.

Rejected for now, and it is worth writing down why, because it is the thing
somebody will suggest next. It cuts *latency* rather than message count, which
is not the complaint. And it costs the property this whole design rests on:
today a nonce seed is safe to store and regenerate from **because** a session
signs one fixed message and cannot be made to sign another. Banked nonces have
no message to be bound to at generation time, so their safety moves from a
structural argument to a used/unused ledger that must be right across crashes,
redeliveries and two devices. A bug there leaks a share. Revisit only if
round-trip latency becomes the actual problem.

**`Frost.deterministicSign` (BIP-445)**, where a signer going last derives its
nonce from the other signers' aggregate and persists nothing, is orthogonal to
batching but relevant to the same file. It would let one signer per session hold
no nonce state at all. Not part of this work.

**Signing one manifest event that commits to `k` items** — a list of ids, or a
Merkle root. One signature, no protocol change, and by far the cheapest thing on
this page. Rejected as the general answer because each item stops carrying its
own verifiable signature, and the whole point of
[shared-key-derivation.md](./shared-key-derivation.md) is that a reader holding
one signed dialect can check it against the room it was found in without a
lookup. Still the right answer for any case where the items are only ever read
together.