tree ed9a864a5c3c9c9bf39fa2caa10cb4cb28eea8cc
parent 6cf98208371e5c2c8b9d34ce3b8c452ea90e2963
author David Benjamin <davidben@google.com> 1685127987 -0400
committer Boringssl LUCI CQ <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com> 1685568779 +0000

Add a value barrier when checking for point doubling.

Many of our point addition functions internally check for the doubling
case and branch because the addition formulas are incomplete. This
branch is fine because the multiplication formulas are arranged to not
hit this case. However, we don't want to leak the couple of intermedate
values that determine whether to branch. Previously, we ran into this
with https://boringssl-review.googlesource.com/c/boringssl/+/36465.

This wasn't sufficient. The compiler understands if (a & b) enough to
compile into two branches. Thanks to Moritz Schneider, Nicolas Dutly,
Daniele Lain, Ivan Puddu, and Srdjan Capkun for reporting this!

Fix the leak by adding a value barrier on the final value. As we're also
intentionally leaking the result of otherwise secret data flow, I've
used the constant_time_declassify functions, which feed into our
valgrind-based constant-time validation and double as barriers.

Accordingly, I've also added some CONSTTIME_SECRET markers around the
ECDSA nonce value, so we can check with valgrind the fix worked. The
marker really should be at a lower level, at ec_random_nonzero_scalar or
further (maybe RAND_bytes?), but for now I've just marked the nonce.
To then clear valgrind, add constant_time_declassify in a few other
places, like trying to convert infinity to affine coordinates. (ECDH
deals with secret points, but it is public that the point isn't
infinity.)

Valgrind now says this code is constant-time, at least up to compilation
differences introduced by the annotations. I've also inspected the
compiler output. This seems to be fine, though neither test is quite
satisfying. Ideally we could add annotations in ways that don't
influence compiler output.

Change-Id: Idfc413a75d92514717520404a0f5424903cb4453
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/60225
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>