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test(marmot): MDK-authored Welcome interop vector + decryptor

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Add a Rust generator under quartz/tools/mdk-vector-gen that emits a
Welcome, joiner KeyPackage, committer signer_pub, joiner's three
private keys, and a post-join MLS-Exporter KAT, using the same
openmls 0.8 backend MDK / whitenoise use. Commit its output at
quartz/src/commonTest/resources/mls/mdk-welcome.json.

MdkWelcomeInteropTest consumes the vector and proves Amethyst can:
  - parse a KeyPackage OpenMLS authored and verify its self-signature;
  - run the joiner's processWelcome end-to-end (HPKE unwrap of group
    secrets, welcome-key derivation, AEAD GroupInfo decrypt, ratchet
    tree decode, signer leaf lookup, GroupInfoTBS Ed25519 verify);
  - derive MLS-Exporter("marmot", "group-event", 32) byte-for-byte
    identically to openmls — the exact exporter Marmot uses for the
    outer ChaCha20 wrap on kind:445 events.

testBobDecryptsMdkApplicationMessagesFromAlice is @Ignored with a
detailed TODO because Amethyst's MlsGroup.encrypt/decrypt skip the
RFC 9420 §6.3.1 PrivateMessageContent framing (application_data<V> +
FramedContentAuthData + padding). Amethyst ↔ Amethyst works (both
sides omit it) but every cross-implementation PrivateMessage fails.
Tracked as a follow-up.

https://claude.ai/code/session_01HfHdd5S5rvxUW2ihEpLGJr
This commit is contained in:
Claude
2026-04-21 05:04:41 +00:00
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# mdk-vector-gen
Rust helper that emits MLS interop test vectors from the same openmls 0.8
backend MDK/whitenoise uses. Produces `quartz/src/commonTest/resources/mls/mdk-welcome.json`,
which [`MdkWelcomeInteropTest`] consumes to prove Amethyst can parse and
decrypt a Welcome + application messages authored by the Rust side.
## What the vector contains
- `joiner.init_priv` / `encryption_priv` / `signature_priv` / `signature_pub` —
all the private key material the joiner needs to drive
`MlsGroup.processWelcome`.
- `joiner.key_package` (MlsMessage-wrapped) and `key_package_raw`.
- `welcome` (MlsMessage-wrapped) — Alice's Welcome for Bob.
- `committer.signer_pub` — Alice's Ed25519 signature public key.
- `exporter.{label,context,length,secret}` — `MLS-Exporter("marmot",
"group-event", 32)` derived from Bob's post-join state. This is the
exporter Marmot uses to derive the outer ChaCha20 key for kind:445
events, so a match here means Amethyst's whole post-join key schedule
agrees with openmls byte-for-byte.
- `app_messages_alice_to_bob[]` — Alice-sent PrivateMessage bytes plus
the expected plaintext. (Amethyst decrypt currently skips
`PrivateMessageContent` framing; the matching test is `@Ignore`d
until that is fixed.)
## Regenerating
```
cd quartz/tools/mdk-vector-gen
cargo run --release > ../../src/commonTest/resources/mls/mdk-welcome.json
```
The generator uses fresh randomness each run, so the committed vector
changes on regeneration — that is fine because the Kotlin test only
asserts round-trip correctness against whatever is in the JSON. Commit
the regenerated file if you change the generator.