A session is post-quantum only when both ends are, so with every Android
device registering `proto: 1` the iPhone implementation protected
approximately no real conversation. The property was true of the code
and false of the product. This is the other half.
ML-KEM-768 over Bouncy Castle's low-level `pqc.crypto.mlkem`, the same
way Ed25519 already uses its `rfc8032` class: not the JCA provider,
which is awkward to register on Android and which R8 cannot shrink. The
platform offers nothing here — Conscrypt has no ML-KEM at any API level
— and hand-rolling a lattice KEM to avoid a dependency would be a far
worse trade than the one Ed25519 already made. No new dependency: the
jar was already in the build for Ed25519.
Keys are stored as the 64-byte FIPS 203 seed, never the expanded
decapsulation key — 64 bytes in the vault instead of 2400, and the same
representation iOS persists, which is what lets a history export cross
platforms. That is also the thing most likely to be silently wrong, so
it is checked against the published vector rather than against another
run of our own code: the seed in `pqx3dh-v2.json` derives the public key
the normative JavaScript engine derived, and the full responder side
reproduces the published session key and associated data byte for byte.
Everything else here would pass with an implementation that is
internally consistent and agrees with nobody.
Negotiation refuses in three places, and each is a substitution
somebody would otherwise get for free. A KEM prekey whose signature does
not verify is treated as ABSENT, so a relay handing out a key of its own
drops the session to v1 rather than to its key — the same substitution
`spk_sig` stops classically, and worth more here because it buys the
whole post-quantum half. A `proto` claim with no `caps_sig` reads as v1,
so a relay cannot advertise a v2 device as v1-only and strip the
post-quantum half off every new session. And the one-time id is signed
big-endian and inside the signature, so a renumbered key falls back to
the last-resort one instead of being spent.
Both pools are refilled together, and that is not tidiness. They drain
at the same rate — one key per new session — so a refill that topped up
only the classical pool would let the post-quantum one run dry and push
every later session onto the last-resort key, every one of them sharing
a single KEM key, with nothing anywhere reporting a downgrade.
Mixed fleets work in both directions and a spent prekey is not an error:
a duplicate delivery of a v2 prekey envelope names a one-time key this
device has already consumed, and throwing there would turn a
retransmission into a dead conversation.
Two test fixtures had to say what they are rather than answer by order.
Sending now asks for a transparency-log head on its own, so a queue
matched purely by position handed that request the device listing and
failed on a parse error nowhere near the cause.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
