Timing side-channel in ML-DSA decomposition

### Summary A timing side-channel was discovered in the Decompose algorithm which is used during ML-DSA signing to generate hints for the signature. ### Details The analysis was performed using a constant-time analyzer that examines compiled assembly code for instructions with data-dependent timing behavior. The analyzer flags: - **UDIV/SDIV instructions**: Hardware division instructions have early termination optimizations where execution time depends on operand values. The `decompose` function used a hardware division instruction to compute `r1.0 / TwoGamma2::U32`. This function is called during signing through `high_bits()` and `low_bits()`, which process values derived from secret key components: - `(&w - &cs2).low_bits()` where `cs2` is derived from secret key component `s2` - `Hint::new()` calls `high_bits()` on values derived from secret key component `t0` **Original Code**: ```rust fn decompose<TwoGamma2: Unsigned>(self) -> (Elem, Elem) { // ... let mut r1 = r_plus - r0; r1.0 /= TwoGamma2::U32; // Variable-time division on secret-derived data (r1, r0) } ``` ### Impact The dividend (`r1.0`) is derived from secret key material. An attacker with precise timing measurements could extract information about the signing key by observing timing variations in the division operation. ### Mitigation Integer division was replaced with a constant-time Barrett reduction.

Source published

12 Dec 2025 12:00:00 UTC

Source modified

12 Oct 2026 18:00:13 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

OSV · upstream source license applies

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CVSS_V3 · CVSS:3.1/AV:A/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N