The second half of a protocol Android did not have. With X3DH it can now agree
a secret, and with this it can hold a conversation.
Two ratchets: the symmetric one advances per message so a compromised key
reveals nothing earlier, the DH one advances on each direction change so a
conversation recovers from compromise rather than staying broken.
The tests run conversations rather than checking steps, because that is what
catches the bugs this code actually has. X3DH is a pure function — it agrees or
it does not. A ratchet can agree for six messages and diverge on the seventh
because one side counted a skipped message differently, and the symptom is only
that something will not decrypt. So: out-of-order delivery, a message lost
forever, a gap ACROSS a direction change (which is the whole reason "pn" is on
the wire), and four DH turns in sequence.
Three things that would each produce plausible-but-wrong bytes, so each is
tested rather than reasoned about:
- the sending chain advances BEFORE the key is used, so a caller that retries
cannot encrypt twice under one key — two GCM encryptions under one key and
nonce leak the XOR of both plaintexts
- the header's canonical JSON is hand-built, key order "dh, pn, n", no spaces,
because it goes into the AAD and three implementations must reproduce it
byte-for-byte
- the derived nonce is stripped from the wire rather than sent, so a receiver
cannot be handed a nonce that disagrees with the one its key implies
MAX_SKIP caps derivation at 1000: a header claiming a huge counter costs the
sender nothing and the receiver everything.
13 tests. Still missing before an app: a session layer, a wire client, and the
app itself.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
The Android engine had primitives and no protocol: no handshake, no ratchet, no
session. It could verify a safety number and could not start a conversation.
X3DH is the first half of fixing that.
The four agreements are mixed in the order the spec fixes, and DH4 is omitted
rather than zero-filled when the one-time prekey pool is drained. Both are
choices that produce 32 plausible bytes when wrong — the failure lands at the
far end as a message that will not decrypt, pointing nowhere near here, so the
tests check that the two sides AGREE rather than that either looks right.
Keys are JCA objects rather than raw bytes throughout, because Conscrypt will
not build an XDH key from raw bytes and wants a DER spec. Passing raw bytes
would work on this JVM and throw on the first tap on a phone, which this engine
has already been bitten by once.
Also here: AccountKdf, so Android can derive an account key and an address.
Checked against the vectors pinned from the enclave client — including the
non-ASCII and empty-passphrase cases, where a CharArray API and a UTF-8 wire
format quietly disagree.
⚠️ Still missing before an Android app can send a message: the Double Ratchet,
a session layer, a wire client, and the app itself.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
A relay's `user_id` is a row number it assigned itself, and the account
service's pseudonym is HMAC'd per product precisely so two services cannot
correlate their users. Neither survives a trip to a second relay, so nothing
addressed by either can cross a relay boundary — which is the first thing
federation needs, and the reason a deleted relay currently takes its users'
names with it.
So identity becomes a key the client derives and holds: Ed25519, stretched from
the passphrase under `flamenet/account/identity/v1`, alongside the per-product
keys that already work this way. The public half, Crockford base32, is the
address. Derived rather than generated, so the same passphrase reproduces the
same fnid on a device that never held it — re-register the same key at a new
relay and contacts who pinned it are still talking to the same person.
Relay: `users.account_key` / `users.fnid` (unique in the database, not just in
the handler), `POST /e2e/account/identity` to claim by proving possession, and
`GET /e2e/account/identity/:handle` to resolve. The claim signature binds the
audience, the local user id and the key itself; dropping any one of the three
lets a claim be replayed at another relay, lifted onto another account here, or
transplanted onto another key. Verification uses the very key being registered,
so the relay confirms possession and never acquires an opinion about whose key
it is. Rotation is refused rather than silently overwritten — a new address
strands every contact that pinned the old one, and that needs a migration
record they can follow.
Log leaves go v1 -> v2, binding a subject instead of a row number: `fnid:<b32>`
where an account has enrolled, `local:<audience>/<user_id>` where it has not.
The fallback is qualified rather than bare because a device can be registered
before its key is claimed, and the alternatives were worse in both directions —
skipping those devices leaves a hole in the one record that must not have
holes, and logging them unqualified produces leaves that mean different things
depending on origin with nothing in the leaf to say so.
This is not a server-side change. All three engines hash the leaf and two of
them verify it against the relay, and a published vector pins the exact bytes,
so the format moves in one commit or not at all. Existing v1 leaves keep their
hashes and are backfilled to a self-describing `legacy-v1/` subject: the log is
append-only, and a format change is not a licence to rewrite history. The
version prefix sits inside the hashed bytes so a v1 leaf recomputed under v2
rules fails rather than verifying against a different subject.
Green: engine conformance 80, relay §8 suite 235 (10 new), JS<->Swift interop
74, Kotlin published vectors.
Two things found on the way, neither addressed here:
- iOS never calls `enforceKeyLog`. It and `requireKeyLog` are both declared in
E2EInbox and nothing reads either, so §19 is dead code on that client while
the JS one wires it. The subject is threaded through ready for a caller.
- `engine/test/interop/build.sh` prefers ~/Code/flamenetmessenger over the
checkout it is run from, so in a worktree it silently builds the wrong
sources and reports a *skip* rather than a failure. Set FNMSG_APP_DIR.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Raw 32-byte keys on the wire, X.509 SubjectPublicKeyInfo in the JCA -- so
every crossing needs the 12-byte DER prefix added or stripped. Written out
as a constant rather than parsed: a DER parser here would be more code than
the thing it parses, and one more place to get a length wrong.
Refuses an all-zero shared secret. X25519 produces all zeros for a
small-order peer key, and a peer that sends one forces both sides to a
secret it already knows. RFC 7748 says implementations MAY check; when the
peer's key arrives over a relay nobody trusts, MAY is not the right reading.
All four small-order points from §6.1 are tested.
⚠️ XDH is API 33+ on Android. Older devices throw NoSuchAlgorithmException
at the first call, and the app must either bundle a provider or refuse to
run -- a decision nobody has made, so it is documented rather than papered
over.
⚠️ These run on the JVM against SunEC. Android routes the same calls through
Conscrypt. Passing here says the encoding is right and nothing about a
device.
Kotlin engine: 27 tests -> 33.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Checked against RFC 5869 and NIST CAVP rather than against our JavaScript
engine. An implementation validated only against our other implementation
inherits our misunderstandings, and finding where the first two quietly
agreed on something the spec does not say is the entire reason for writing
a third.
Three things pinned that a naive version gets wrong:
- An empty HKDF salt means HashLen zeros, not "no HMAC key". SecretKeySpec
throws on a zero-LENGTH key, so the naive version crashes on exactly the
case the app uses -- §12 derives its per-context secrets with an empty
salt.
- 255 blocks is a hard ceiling. The counter is one byte; a naive loop wraps
at 256 and silently returns key material it already returned.
- The AEAD box is nonce || ciphertext || tag. CryptoKit's `combined` emits
that layout and the JS engine assembles it by hand to match, so a box
built any other way decrypts to garbage on the other engines rather than
failing loudly.
HKDF is hand-written on Mac rather than taken from a provider: Android's
providers disagree about whether HKDF exists at all.
Kotlin engine: 9 tests -> 27.
⚠️ Still JVM-only. These prove nothing about Android, where crypto goes
through Conscrypt rather than the JVM's JCA, and nothing here has ever been
compiled against the Android SDK.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Android does not exist, and the first thing it needs is not a screen. It
needs the protocol implemented a third time — which is only worth
anything if the third implementation is checked against the spec and the
published vectors rather than against a reading of our own Swift.
So `android/engine` is a plain Kotlin/JVM library with no Android in it:
no SDK, no emulator, no device, and therefore gateable in CI on Linux
like everything else. It reads
engine/test/vectors/protocol-v1.json from its one location rather than
carrying a copy, because a vector transcribed into the test that checks
it is a vector that agrees with itself.
Covered so far: safety numbers (§7) and key transparency verification
(§19) — RFC 6962 leaf and node hashing, the unbalanced split, and
bottom-up audit paths. Nine tests, all green against the same file the
JavaScript and Swift engines answer to.
The §19 tests are the ones worth having. A top-down reading of a
bottom-up audit path yields the correct root for every left-spine index,
so it passes any suite that only checks index 0 — the vectors pin a
right-descending index, and the tests assert both that the published
path verifies and that the same path reversed does not.
android/README.md lists what is missing — X25519, X3DH, the ratchet,
sealed sender, both post-quantum halves, the relay client, the app — as
a table rather than as an omission. Android stays honestly marked "not
started" on the site until that list is shorter.
Bytecode targets 17 without demanding a JDK 17 be installed: a
jvmToolchain(17) fails on any newer JDK, which is a poor trade for a
library with no JDK 17-specific behaviour.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
