fix: branchless modular arithmetic to close NTT timing side-channel#1210
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fix: branchless modular arithmetic to close NTT timing side-channel#1210BAder82t wants to merge 1 commit into
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Closes openfheorg#1198. MultipartyDecryptFusion calls INTT on secret-correlated polynomial b = cv[0] + s*cv[1] + noise. Secret-dependent conditional branches in the butterfly (e.g. if (loVal < omegaFactor) loVal += mod) are timing oracles attackable via adaptive-chosen-ciphertext. New utils/constanttime.h adds SubIfGE, ModSubFast, AddIfNeg using the branchless mask idiom (no asm barriers, vectorisation preserved). Ten functions in ubintnat.h patched. All nine butterfly sites in transformnat-impl.h made branchless; #ifdef __GNUC__ compiler-dispatch splits and if (omegaFactor != zero) shortcut removed. NTT test added for zero-coefficient input path. Benchmarks vs original (clang -O3, Apple M-series): ModAddFast scalar: -3% | NTT n=4096: +4% | NTT n=16384: +5% (PR openfheorg#1177 asm-barrier approach regressed NTT by 49-52%) Known pre-existing limitations documented in constanttime.h: WASM: MultD carry-detect branches remain data-dependent. Non-power-of-2 rings: Bluestein INTT path not covered.
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Summary
Closes #1198.
MultipartyDecryptFusioncalls INTT on secret-correlated polynomialb = cv[0] + s·cv[1] + noiseif (loVal < omegaFactor) loVal += modulus) leak key bits via timing__asm__ volatilebarriers but caused 1.5–1.9× NTT regressionThis PR uses the branchless mask idiom instead — no barriers, vectorisation preserved.
Changes
utils/constanttime.h—SubIfGE,ModSubFast,AddIfNegCT primitivesubintnat.h— 10 functions patched:ModAddFast/Eq,ModSubFast/Eq,ModMulFastConst/Eq,ModMulFast/Eqtransformnat-impl.h— all 9 butterfly sites made branchless;#ifdef __GNUC__splits removedUnitTestNTT.cpp— test added for zero-coefficient input pathBenchmarks (Apple M-series, clang -O3)
2051/2051 tests pass.
Known limitations (pre-existing)
MultDcarry-detect branches remain data-dependent; documented inconstanttime.h