fix(fiat-shamir): rejection-sample in sample_in_range to remove truncation bias#257
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lau90eth wants to merge 1 commit into
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fix(fiat-shamir): rejection-sample in sample_in_range to remove truncation bias#257lau90eth wants to merge 1 commit into
lau90eth wants to merge 1 commit into
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Description
sample_in_range()truncated field elements via bitmask, which biases the output whenever the field modulus is not an exact multiple of 2^bits (as already noted in the function's own doc comment).This replaces truncation with rejection sampling: field elements landing in the partial overflow block above the largest multiple of 2^bits are dropped and resampled, giving an exactly uniform distribution over [0, 2^bits) regardless of the modulus.
Related Issues
Closes #184
Testing
Added a statistical regression test (
sample_in_range_is_not_biased_toward_low_residues) verifying the low bits of accepted samples are close to uniform across 200k samples.