{"id":"RUSTSEC-2026-0385","summary":"Non-constant-time mask generation and `u128` comparisons","details":"## Summary\n\nThe `ctutils::Choice` type has mask generation functions for each unsigned\ninteger size (`u8`, `u16`, `u32`, `u64`, `u128`). These are `const fn` and\ntherefore can't reuse the `asm!`-backed mask generation in the `cmov` crate\nthat's otherwise used to ensure constant-time.\n\nIn releases prior to v0.4.4, the generated code from these functions could\ninclude a branch on commonly used platforms. This has been corrected in this\nrelease and now verified on several platforms using BINSEC.\n\nAdditionally, `u128` comparison operations provided via `ctutils::Choice` and\nintended to work in constant-time were generating branches on 32-bit targets.\nThis has been resolved via a special case implementation of these functions\nfor such targets.\n\n## Impact\n\nBranch instructions on secret values can potentially leak such values via\ncovert channels. This can enable attackers to potentially recover such secrets.\n\n## Resolution\n\nThis has been resolved by calling `Choice::to_u8()` when generating the mask,\nwhich in turn uses `black_box` as an optimization barrier. Verification that\nthis resulted in the desired change in the generated assembly across a variety\nof target CPU architectures was performed by BINSEC, which now runs\nautomatically in CI using `cargo checkct`.","modified":"2026-10-23T08:30:02.389575143Z","published":"2026-10-23T12:00:00Z","related":["RUSTSEC-2026-0003"],"database_specific":{"license":"CC-BY-4.0"},"references":[{"type":"PACKAGE","url":"https://crates.io/crates/ctutils"},{"type":"ADVISORY","url":"https://rustsec.org/advisories/RUSTSEC-2026-0385.html"},{"type":"WEB","url":"https://github.com/RustCrypto/utils/pull/1569"}],"affected":[{"package":{"name":"ctutils","ecosystem":"crates.io","purl":"pkg:cargo/ctutils"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.0.0-0"},{"fixed":"0.4.4"}]}],"ecosystem_specific":{"affected_functions":null,"affects":{"functions":[],"arch":[],"os":[]}},"database_specific":{"informational":null,"source":"https://github.com/rustsec/advisory-db/blob/osv/crates/RUSTSEC-2026-0385.json","categories":["crypto-failure"],"cvss":null}}],"schema_version":"1.9.0"}