A new `:obfs4` module, a plain JVM library like `:engine` so its tests run
without an emulator -- which matters here because the only tests that mean
anything are vector comparisons against lyrebird.
Far smaller than the Swift port, for a reason worth recording: the Swift
implementation hand-writes SipHash-2-4, XSalsa20, Poly1305 and its own
field arithmetic because CryptoKit exposes none of them. The JVM does not
have that hole. BouncyCastle -- already a dependency of :engine, for the
same class of reason -- has X25519Field, XSalsa20Engine, Poly1305 and
SipHash, vetted and constant-time where it matters.
So the two clients do NOT share an implementation. They share the vectors
and the live interop harness, which is what actually keeps them honest:
two independent implementations agreeing with the same reference is a
stronger statement than one agreeing with itself twice.
Two things BouncyCastle would not do, both found by vectors rather than
by reading.
X25519Field.sqrtRatioVar is not a drop-in. Elligator needs the root even
when the ratio is NOT square -- it is sqrt(i*u/v) then, and the algorithm
uses it to pick a branch rather than to detect an error. BC only promises
a usable root in the square case, and handing its output to the map
produced zeros for 120 of 256 vectors. The exponent chain and branch
selection are written out here; the field arithmetic underneath is still
BouncyCastle's.
X25519Field.cmov did nothing at all with a condition of 1. Both tweak
parities produced the SAME representative -- a map that still
round-trips, still looks uniform, and only ever emits one of the two
valid answers. The failing assertion said "wrong bytes"; it took a
diagnostic printing both candidates to see they were identical. Replaced
with an explicit mask so the convention is stated rather than assumed.
Behind no setting yet because nothing calls it: this is the crypto
foundation only. The transport, the OkHttp socket factory and the
user-facing switch come next, and the switch defaults to off.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
