curve: use subtle::Choice for constant-time fixes
#665
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Alternative to #659/#661 and #662 which leverages `subtle::Choice` and `subtle::ConditionallySelectable` as the optimization barriers. Really the previous masking was there to conditionally add the scalar field modulus on underflow, so instead of that, we can conditionally select zero or the modulus using a `Choice` constructed from the underflow bit.
tarcieri
commented
Jun 25, 2024
isislovecruft
approved these changes
Jun 25, 2024
rozbb
approved these changes
Jun 25, 2024
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Alternative to #659/#661 and #662 which leverages
subtle::Choiceandsubtle::ConditionallySelectableas the optimization barriers.Really the previous masking was there to conditionally add the scalar field modulus on underflow, so instead of that, we can conditionally select zero or the modulus using a
Choiceconstructed from the underflow bit.