Bootstrap: reject inputs with fewer limbs than their NoiseLevel needs instead of returning garbage - #37
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Bootstrap: reject inputs with fewer limbs than their NoiseLevel needs instead of returning garbage#37seyfal wants to merge 1 commit into
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… instead of returning garbage ModRaise checked `getLevel() - NoiseLevel + 1 >= 1` only with assert(), which is compiled out in Release builds. A ciphertext at level 1 with NoiseLevel 2, or at level 0 with NoiseLevel 1, then goes through the raise: the pending rescale (and the raise's own multScalar + rescale) run on a single-limb polynomial, the level drops to -1 and the bootstrap completes normally but returns noise (OpenFHE's Decrypt reports "approximation error is too high"). Throw std::invalid_argument naming the level, the NoiseLevel and the required minimum (level >= NoiseLevel; for prescaled inputs level == NoiseLevel - 1), and document the precondition on Bootstrap() in Bootstrap.cuh. Valid inputs are unchanged. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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Bootstrap()on a ciphertext at level 1 with NoiseLevel 2, or at level 0 with NoiseLevel 1, runs to completion without any error and returns a ciphertext at the normal post-bootstrap level, but the content is noise (OpenFHE'sDecryptreports "the approximation error is too high"). Level 2 / NoiseLevel 2 and level 1 / NoiseLevel 1 bootstrap correctly.Cause:
ModRaise(src/CKKS/Bootstrap.cu) checksgetLevel() - NoiseLevel + 1 >= 1only withassert(), which is compiled out in Release builds. The raise then consumes the pending rescale of a NoiseLevel-2 input and one more limb (multScalar+rescalebeforedropToLevel(0)), so an input with fewer limbs than that rescales a single-limb polynomial (LimbPartition::rescale's ownassert(limbsize > 1)is compiled out too) and the raise works on a level -1 ciphertext.This PR turns the check into a real one:
std::invalid_argumentnaming the level, the NoiseLevel and the required minimum (level >= NoiseLevel; forprescaled == true, level == NoiseLevel - 1), and documents the precondition onBootstrap()inBootstrap.cuh. Valid inputs are unchanged.Tested on v2.1.3 (786c760), CUDA 13.0, H200 with N=2^16 / depth 25 / scaleModSize 52 / firstModSize 56 / 3 digits / FLEXIBLEAUTO / UNIFORM_TERNARY / 16384 slots, with the
examples/bootstrapparameters (N=2^12, full slots) and with the UNIFORM bootstrap configuration: the two bad cases (level 1 / NoiseLevel 2, level 0 / NoiseLevel 1) went from silent garbage to the exception; the three good cases are unchanged (max error 2.0e-3 to 2.2e-3 at our parameters, 3e-5 at the example parameters).The commit also applies cleanly on
OpenFHECompatTests.Fable 5.1 on behalf of Seyfal