p521: fix ReduceNonZero<U576>
#1345
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daxpedda
changed the title
p521: use reduction in ReduceNonZero<U576>p521: fix ReduceNonZero<U576>
It's the mathematical equivalent of UB |
tarcieri
approved these changes
Aug 3, 2025
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I just realized from #1341 that P-521
ReduceNonZero<U576>wasn't implemented correctly.Because
U576can fit way more then2 * ORDER, doinginput - ORDER_MINUS_ONEdoesn't actually bring it below the modulus. We then useScalar::from_uint_unchecked(), which does require input below the modulus.I couldn't trigger a bug in practice however, not sure exactly how
fiat_p521_scalar_non_montgomery_domain_field_element()fails when its requirements aren't fulfilled.