b99cb8fcd5
First docs in the repo -- README.md is still the stock KMP template. Three documents plus an index, covering the parts whose behaviour is not recoverable by reading the code: where the reasoning lives in a protocol, where a failure mode is silent, or where a decision looked arbitrary and was not. marmot-membership.md is the one that earns its place. Everything about adding a member compiles, the invite reports success, and a member simply never appears -- and the reason is never in the invite code. It records that inviteMemberToChatRoom hardcodes isOneMemberInitialGroupCreation = false and that ChatRepository does not expose it, so every group invite takes the deferred-welcome path including the first, when the group is still just its creator and the commit has no audience at all. Then why that is silent rather than noisy: MarmotInboundManager refuses future-epoch messages outright, on both wire formats, with no queue and no replay, so a commit arriving before its recipient's welcome is dropped and that member never advances. EPOCH_RETENTION_WINDOW retains past epochs and does nothing for messages from ahead. Three options are set out with the per-invite correctness table, including the honest limit that the recommended one narrows the race without closing it. shared-key-derivation.md argues why the paths are not BIP32 -- no chain code exists, hardened derivation is impossible rather than unimplemented, and a FROST tweak takes the scalar as input so the chain code leaves the problem entirely. It records the x-only serialisation trap avoided by choosing the scalar directly, and states the rule that must not be broken: never reconstruct a derived key in the clear, because k = k' - t hands over the group key rather than one derived key. shared-key-ceremony.md covers the seven kinds, the three approval gates and why the coordinator's aggregations are deliberately not among them, faults as values rather than exceptions, and the transcript's idempotency-by-construction. It also writes down the invariant that produces no error when broken: pendingApproval must mirror the gates in advance, or the screen offers an approval that does nothing -- or none while the ritual sits still. Every factual claim was checked against the source rather than recalled, which turned up one correction worth having: there are two future-epoch refusals, for PrivateMessage and for Commit, so the drop covers both wire formats and not just one. Each document leads with the failure mode rather than the architecture, on the grounds that a failure is what sends somebody to docs in the first place, and each lists its known gaps -- including that none of this has run on a physical device. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
153 lines
6.4 KiB
Markdown
153 lines
6.4 KiB
Markdown
# Deriving keys from a group's shared key
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`SharedKeyDerivation` turns a group's ChillDKG threshold key into further keys the
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group can sign with, at paths that look like BIP32 but deliberately are not.
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## What it produces
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```kotlin
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val derived = SharedKeyDerivation.derive(thresholdPublicKey) // default m/9420/0/0
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derived.publicKey // XonlyPublicKey — 32 bytes, the form nostr and Marmot use
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derived.cache // TweakCache — required to sign
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derived.hex // publicKey as hex
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```
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**The cache is not an optimisation.** A FROST signing session has to be created
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with a cache carrying the same tweaks, or the partial signatures aggregate to
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something that verifies against a different key. Code that takes only
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`publicKey` and later tries to sign will fail in a way that is tedious to diagnose
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from the outside, because the signature is valid — just not for the key you
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expected.
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Everything is a pure function of the threshold key and the path, so every member's
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device computes the same result with no agreement round and nothing to store.
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Rederive rather than persist.
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## Why not BIP32
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The paths read like BIP32 and are walked the same way, index by index. They are
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not BIP32, and the difference matters.
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**A BIP32 node is a key *and* a chain code. ChillDKG produces no chain code.**
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`ParticipantFinalizeResult` gives you `thresholdPublicKey`, `secretShare`,
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`publicShares` and `recovery` — no chain code, because ChillDKG is not a BIP32
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ceremony.
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**Hardened derivation is impossible here, not merely unimplemented.** It is:
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```
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I = HMAC-SHA512(c_par, 0x00 || ser256(k_par) || ser32(i))
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```
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which takes the parent *private* key. In a FROST group nobody holds that; it
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exists only as shares. No member, and no quorum of members short of reconstructing
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the secret, can perform it. So `m/44'/1237'/0'/0/0` — the NIP-06 nostr path — is
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not derivable from a threshold key by anyone.
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**Non-hardened derivation is available, as an additive tweak.**
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```
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t = HMAC-SHA512(c_par, serP(K_par) || ser32(i))[0:32]
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K' = K + t·G
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```
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which is exactly what `TweakCache.tweak` does. But note where the chain code
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appears: only in *computing* `t`. A FROST tweak takes `t` as an input, so
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**choosing the scalar directly removes the chain code from the problem entirely.**
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That is what this does:
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```
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t = SHA256("mantra/shared-key/tweak/v1" || parentXonlyKey || index-as-4-bytes)
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```
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Each scalar commits to the key being tweaked as well as the index, so steps cannot
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be reordered or replayed at a different depth to reach the same key.
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`listOf(0L)` and `listOf(0L, 0L, 0L)` do not collide — there is a test for it.
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### What avoiding BIP32 also avoids
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With x-only keys there is no single obvious `serP(K_par)`: BIP32 serialises
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compressed 33-byte keys, BIP340 uses 32-byte x-only, and the parity byte has to
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come from somewhere. Two devices picking different conventions would **silently
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derive different keys** rather than fail. Choosing the tweak input ourselves makes
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the domain separation explicit and removes that class of bug.
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Nothing is lost in exchange. No external tool can derive these children anyway —
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none of them has the chain code, and nostr has no way to publish one. An npub is
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bare bech32 over a 32-byte key with no chain code, depth or parent fingerprint;
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NIP-06 uses BIP32 internally but discards everything except the leaf public key.
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## The security property this inherits
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Additive tweaking is what non-hardened BIP32 does, and it carries the same
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weakness. Because `t` is publicly computable:
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```
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k' = k + t ⟹ k = k' − t
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```
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**Anyone who learns one derived private key recovers the group's threshold key**
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and can sign as the group with no quorum at all — defeating the entire point of
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the ceremony. In ordinary BIP32 this is why BIP44 hardens the first three levels:
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a leaked leaf costs you one account, not the wallet. That defence is unavailable
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here.
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The mitigating factor is that a derived private key does not normally exist:
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reconstructing one needs `t` members to collude, at which point they already have
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the parent. So the rule is narrow and absolute:
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> **Never reconstruct a derived key in the clear.** Any code path that could — an
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> export, a "reveal private key" screen, a test helper, a debugging convenience —
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> leaks the group key, not just the key it appears to expose.
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If you need many keys that cannot be linked back to one another, derivation is the
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wrong tool: run a ceremony per key. Each output is then independent and no single
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leak reaches the others.
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## Paths
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`derive` and `marmotGroupId` both take `path: List<Long>`, defaulting to
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`MARMOT_ADMIN_GROUP_PATH` (`m/9420/0/0`). Any depth works.
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`9420` is arbitrary and has to stay put: the derived key *is* the `#admins` room's
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id, so changing the path orphans every room already created — members would derive
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a different id and stop finding the room at all.
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There is no string-path parser for input. Paths are written as lists at the call
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site. If one is added it must reject `'` outright rather than accepting a hardened
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path it cannot honour.
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## Recording the path
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MIP-01's group data is a fixed TLS schema — version, `nostrGroupId`, name,
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description, `adminPubkeys`, relays, four image fields, `disappearingMessageSecs`.
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There is no extension map, and inventing a field would emit bytes other Marmot
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clients cannot decode.
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So the path rides in the description, which is the only free text MIP-01 offers:
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```
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Admins of Ubuntu Collective.
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Shared key path: m/9420/0/0
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```
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`formatPath`, `parsePath` and `describe` round-trip this. The marker sits on its
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own line and `parsePath` scans lines for it, so somebody rewriting the rest of the
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description does not cost the group the record of how its key was derived.
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Worth storing even though the path is currently a constant: it is what rebuilds
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the `TweakCache` a signing session needs, and recomputing from the constant only
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holds while the constant never changes. A room that records the path it was made
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under lets a later scheme coexist with rooms already created.
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`parsePath` refuses hardened indices — `m/9420'/0/0` returns null. A hardened path
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cannot have been walked here, so acting on one would derive something other than
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what the room claims.
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Consequence worth knowing: the path is visible to anyone in the group, in any
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Marmot client, since description is user-facing text. The path is not a secret and
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the key it derives from is not published, but the room does announce how it was
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made.
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