security(ntt): make Barrett reduction branchless, closing the constant-time gap#55
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…t-time gap Replaces priv_barrett's data-dependent canonicalization (if r<0 ... if r>=q ...) with a branchless version using only unsigned arithmetic and a well-defined C99 boolean comparison, contributed by u/robchroma on r/C_Programming in response to the constant-time caveat this project had documented. Exhaustively verified against a % q for all 22,164,483 valid inputs in [0, 2*q^2] before adoption, not spot-checked. Rebuilt and re-ran the full test suite (335 tests) plus AddressSanitizer/UndefinedBehaviorSanitizer, clean under both, and recompiled under -std=c99 -pedantic-errors -Wall -Wextra -Werror on gcc and clang with zero warnings. Updates docs/algorithms/ntt.md and README.md to reflect that no known data-dependent branches remain in the transform, while keeping the honest caveat that this is not a formally audited cryptographic primitive. Credits robchroma in CONTRIBUTORS.md and adds a CHANGELOG entry.
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Summary
Direct response to feedback on r/C_Programming (Bman1296, robchroma, AttractiveDaddy) on the constant-time caveat documented in
docs/algorithms/ntt.md. u/robchroma proposed a branchless canonicalization forpriv_barrettthat avoids the implementation-defined-behavior problem with the reference Kyber technique.Verification
a % qfor all 22,164,483 valid inputs in[0, 2*q^2], zero mismatches (not spot-checked)-std=c99 -pedantic-errors -Wall -Wextra -Werroron both gcc and clang: zero warningsDocs
Updates the constant-time warning in
docs/algorithms/ntt.mdandREADME.mdto reflect the fix, while keeping the honest caveat that this isn't a formally audited cryptographic primitive. Credits robchroma inCONTRIBUTORS.md, adds a### Securityentry toCHANGELOG.md.Not done here (separate, larger decision)
This closes the specific constant-time gap that was documented and publicly flagged. It does not constitute a security audit of the NTT module as a whole.