---
id: RUSTSEC-2026-0385
title: Non-constant-time mask generation and `u128` comparisons
summary: Non-constant-time mask generation and `u128` comparisons
severity: none
vendor: ctutils
product: ctutils
ecosystem: rust
affected:
  - 'ctutils >= 0.0.0-0, < 0.4.4'
patched:
  - ctutils 0.4.4
published: '2026-10-23'
updated: '2026-10-23'
sourceUpdated: '2026-10-23T08:30:02.389575143Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/RUSTSEC-2026-0385'
references:
  - url: 'https://crates.io/crates/ctutils'
  - url: 'https://rustsec.org/advisories/RUSTSEC-2026-0385.html'
  - url: 'https://github.com/RustCrypto/utils/pull/1569'
tags:
  - osv
  - rust
ingestedAt: '2026-10-23T11:58:17.249Z'
---

## Overview

## Summary

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`.

## Affected packages

- `ctutils >= 0.0.0-0, < 0.4.4`

## Remediation

Upgrade to a patched release:

- `ctutils 0.4.4`