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Tweak how prime-field normalizes to the even square root

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Luke Parker
2025-09-04 20:47:59 -04:00
parent 5265cc69de
commit 2216ade8c4
1 changed files with 5 additions and 2 deletions
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7
crypto/prime-field/src/lib.rs
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@@ -419,7 +419,7 @@ macro_rules! odd_prime_field_with_specific_repr {
const ONE_MOD_EIGHT: bool = (MODULUS.as_words()[0] % 8) == 1;
const FIVE_MOD_EIGHT: bool = (MODULUS.as_words()[0] % 8) == 5;
let sqrt = if THREE_MOD_FOUR {
let mut sqrt = if THREE_MOD_FOUR {
const SQRT_EXP: UnderlyingUint =
MODULUS.shr_vartime(2).wrapping_add(&UnderlyingUint::ONE);
Self(self.0.pow(&SQRT_EXP))
@@ -449,7 +449,10 @@ macro_rules! odd_prime_field_with_specific_repr {
Self(upsilon * self.0 * (i - Self::ONE.0))
};
let sqrt = <_>::conditional_select(&sqrt, &-sqrt, sqrt.0.retrieve().is_odd());
// Normalize to the even choice of square root
// `let ()` is used to assert how `conditional_negate` operates in-place
let () = sqrt.conditional_negate(sqrt.is_odd());
CtOption::new(sqrt, sqrt.square().ct_eq(self))
}
fn sqrt_ratio(num: &Self, div: &Self) -> (Choice, Self) {