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Add p constant for subtraction

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Henry de Valence 2017-03-12 23:18:31 -07:00
parent 08ef9e5d1e
commit 3eab4acbd8
2 changed files with 17 additions and 5 deletions
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3
src/constants.rs
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@ -26,6 +26,9 @@ use curve::CompressedEdwardsY;
use curve::EdwardsBasepointTable;
use scalar::Scalar;
#[cfg(feature="radix_51")]
pub const p: FieldElement = FieldElement([2251799813685229, 2251799813685247, 2251799813685247, 2251799813685247, 2251799813685247]);
#[cfg(feature="radix_25_5")]
pub const d: FieldElement = FieldElement([
-10913610, 13857413, -15372611, 6949391, 114729,

19
src/field.rs
View file

@ -244,11 +244,11 @@ impl<'b> SubAssign<&'b FieldElement> for FieldElement {
fn sub_assign(&mut self, _rhs: &'b FieldElement) {
// To avoid underflow, first add p
// XXX how many copies should we add to preserve headroom?
self.0[0] += 2251799813685229;
self.0[1] += 2251799813685247;
self.0[2] += 2251799813685247;
self.0[3] += 2251799813685247;
self.0[4] += 2251799813685247;
self.0[0] += constants::p.0[0];
self.0[1] += constants::p.0[1];
self.0[2] += constants::p.0[2];
self.0[3] += constants::p.0[3];
self.0[4] += constants::p.0[4];
// then subtract _rhs
self.0[0] -= _rhs.0[0];
self.0[1] -= _rhs.0[1];
@ -338,6 +338,15 @@ impl FieldElement {
self[i] = -self[i];
}
}
#[cfg(feature="radix_51")]
pub fn negate(&mut self) {
// XXX how many copies of p
self.0[0] = constants::p.0[0] - self.0[0];
self.0[1] = constants::p.0[1] - self.0[1];
self.0[2] = constants::p.0[2] - self.0[2];
self.0[3] = constants::p.0[3] - self.0[3];
self.0[4] = constants::p.0[4] - self.0[4];
}
/// Construct the additive identity
#[cfg(feature="radix_25_5")]