The seventeen Kotlin tests that landed with X3DH and the ratchet prove the
engine agrees with itself. That is not the property anyone needs. Two engines
written by the same person agree about the same misreadings of the spec, and
every one of those shows up months later as "the message will not decrypt" on
someone else's phone. So this wires the Kotlin engine up against the web
engine, which is normative, in both directions.
23 assertions, all passing: v1 X3DH both ways, identical associated data,
alternating ratchet turns past the handshake, out-of-order and permanently
lost messages, a drained one-time prekey pool, tamper rejection, safety
numbers, and the canonical transparency-log leaf.
Three things worth recording.
Ed25519 was missing, and the engine could derive an account address but never
sign a claim for one. The JCA offers no route from a private EdDSA key to its
public half, so the RFC 8032 keygen is done here -- and verified against the
RFC 8032 test vectors, not against a round-trip through our own code. A
round-trip proves only that the file agrees with itself, and every wrong clamp,
endianness and sign convention agrees with itself perfectly. The scalar
multiplication uses BigInteger and is therefore not constant-time; it runs once
at sign-in, in the user's own process, and signing deliberately stays with the
platform provider.
The harness reports a build failure as a failure, never a skip. The Swift
harness beside it once printed "skipped" -- which reads as green -- while
silently building a different checkout. Only a missing JDK skips here, and the
resolved path is printed either way.
Each section is wrapped so a divergence is a named failure rather than a crash.
That was not theoretical: breaking the Kotlin message-key derivation on purpose
crashed the process with a stack trace and skipped every later section. It now
reports four failing sections and finishes the run. The negative control is how
this was checked at all -- a suite that passes on its first run and has never
been seen to fail is not evidence of anything.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
