Comparison against BoringSSL-generated test vectors showed mismatches
with the pure Go implementation of curve25519. The problem was narrowed
down to a missing mask in feFromBytes(). This diff adds the mask,
bringing this back in line with the reference implementation and
RFC 7748:
When receiving such an array, implementations of X25519 (but not
X448) MUST mask the most significant bit in the final byte. This is
done to preserve compatibility with point formats that reserve the
sign bit for use in other protocols and to increase resistance to
implementation fingerprinting.
Fixesgolang/go#30095
Change-Id: If7efc0e2acd6efb761d6e3cb89cec359d7d81cb1
Reviewed-on: https://go-review.googlesource.com/c/161257
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Michael McLoughlin
193df9c0f0