Proving the v3 ratchet across engines turned up a bug in shipping it, and this
one would have broken every existing Android conversation.
`E2ESession.initiate` turned the v3 ratchet on whenever the peer offered a PQ
prekey. Every v2 device offers one — offering a prekey is a claim to §12, not to
§13. So a build with this change would attach `pqdh`/`pqct` to messages for
0.6, the released build, whose ratchet has no post-quantum anything.
That fails twice over, which is why it would have looked like a network
problem. Their root step derives without `ss` and under the `_v1` info string
while ours used `_v3`, so the keys diverge; and their canonical header omits the
two fields, so the AAD does not match either. The first message never opens and
never would.
Gated on a PROVEN claim now, the way the iOS engine already does it:
`chosen != null && bundle.proto >= 3`. `bundle.proto` on its own is the relay's
word for it, but `chosen` is non-null only when the capability signature over
that same proto verified (§12.7), so the pair is a claim the peer actually made.
The test asserts on the absence of a header field, which is the only way to
catch this — every message still decrypts in a self-test, because both sides of
one engine agree with each other about the mistake.
The interop driver now publishes a real v3 bundle: `pqspk` under
`FlamenetE2E_PQSPK_v2`, and `caps_sig` over `FlamenetE2E_Caps_v2 ‖ uint8(3)`.
Both were needed before the web engine would negotiate anything but v1 — which
is why the harness had been quietly proving v1 and nothing else. It can still
publish a v1 bundle on request, because the fleet still holds devices that speak
it and that coverage must not disappear the day v3 lands.
605 tests, 0 failures.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
