RUSTSEC-2026-0385: Non-constant-time mask generation and `u128` comparisons
10 affectedSummary
The ctutils::Choice type has mask generation functions for each unsigned
integer size (u8, u16, u32, u64, u128). These are const fn and
therefore can't reuse the asm!-backed mask generation in the cmov crate
that's otherwise used to ensure constant-time.
In releases prior to v0.4.4, the generated code from these functions could include a branch on commonly used platforms. This has been corrected in this release and now verified on several platforms using BINSEC.
Additionally, u128 comparison operations provided via ctutils::Choice and
intended to work in constant-time were generating branches on 32-bit targets.
This has been resolved via a special case implementation of these functions
for such targets.
Impact
Branch instructions on secret values can potentially leak such values via covert channels. This can enable attackers to potentially recover such secrets.
Resolution
This has been resolved by calling Choice::to_u8() when generating the mask,
which in turn uses black_box as an optimization barrier. Verification that
this resulted in the desired change in the generated assembly across a variety
of target CPU architectures was performed by BINSEC, which now runs
automatically in CI using cargo checkct.