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ctutils: Thumb-1 constant-time hardening - #1580
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Extends `cargo checkct` coverage to include comparisons and conditional negation, and corrects constant-time issues impacting Thumb-1 targets (e.g. Cortex-M0).
This was referenced Oct 28, 2026
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## Security - Thumb-1 (Cortex-M0) and RISCV32 constant-time hardening (#1580)
tarcieri
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Oct 29, 2026
Uses newly hardened `ctutils::{CtLt, CtSelect}` (RustCrypto/utils#1580)
for conditional subtraction in `small_reduce`, preventing
secret-dependent branches on Thumb-1 as well as avoiding the `psubusw`
instruction on `x86_64` which was blocking BINSEC analysis.
Making this fix pass `checkct` requires a bump to `ctutils` v0.4.7. It
also makes `ctutils` a hard dependency of `module-lattice`, though it's
currently a conditional feature and needs to remain so for backwards
compatibility. A TODO notes the next breaking release should remove the
legacy feature.
The issue as well as the fix were confirmed using `cargo checkct`
analysis of the `ml-kem` crate, including some newly expanded coverage
included in this PR.
tarcieri
added a commit
to RustCrypto/KEMs
that referenced
this pull request
Oct 29, 2026
Uses newly hardened `ctutils::{CtLt, CtSelect}` (RustCrypto/utils#1580)
for conditional subtraction in `small_reduce`, preventing
secret-dependent branches on Thumb-1 as well as avoiding the `psubusw`
instruction on `x86_64` which was blocking BINSEC analysis.
Making this fix pass `checkct` requires a bump to `ctutils` v0.4.7. It
also makes `ctutils` a hard dependency of `module-lattice`, though it's
currently a conditional feature and needs to remain so for backwards
compatibility. A TODO notes the next breaking release should remove the
legacy feature.
The issue as well as the fix were confirmed using `cargo checkct`
analysis of the `ml-kem` crate, including some newly expanded coverage
included in this PR.
tarcieri
added a commit
to RustCrypto/KEMs
that referenced
this pull request
Oct 29, 2026
Uses newly hardened `ctutils::{CtLt, CtSelect}` (RustCrypto/utils#1580)
for conditional subtraction in `small_reduce`, preventing
secret-dependent branches on Thumb-1 as well as avoiding the `psubusw`
instruction on `x86_64` which was blocking BINSEC analysis.
Making this fix pass `checkct` requires a bump to `ctutils` v0.4.7. It
also makes `ctutils` a hard dependency of `module-lattice`, though it's
currently a conditional feature and needs to remain so for backwards
compatibility. A TODO notes the next breaking release should remove the
legacy feature.
The issue as well as the fix were confirmed using `cargo checkct`
analysis of the `ml-kem` crate, including some newly expanded coverage
included in this PR.
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Extends
cargo checkctcoverage to include comparisons and conditional negation, and corrects constant-time issues impacting Thumb-1 targets (e.g. Cortex-M0).