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Deferred Montgomery reductions #20

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Deferred Montgomery reductions #20

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fea0013
Introduce FpWide type to handle unreduced products.
ebfull Nov 22, 2019
cff96bf
Defer Montgomery reductions during Fp2 multiplication.
ebfull Nov 22, 2019
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Defer Montgomery reductions during Fp2 multiplication.
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@ebfull
ebfull committed Nov 22, 2019
commit cff96bf5ce07b89100f0ca4fb3bff33379df2f31
20 changes: 12 additions & 8 deletions src/fp2.rs
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Original file line number Diff line number Diff line change
@@ -209,14 +209,18 @@ impl Fp2 {
// c0 = v0 + v1
// c1 = (a0 + a1) * (b0 + b1) - v0 + v1

let v0 = (&self.c0).mul(&rhs.c0);
let v1 = (&(&self.c1).neg()).mul(&rhs.c1);
let c0 = (&v0).add(&v1);
let c1 = (&(&self.c0).add(&self.c1)).mul(&(&rhs.c0).add(&rhs.c1));
let c1 = (&c1).sub(&v0);
let c1 = (&c1).add(&v1);

Fp2 { c0, c1 }
let v0 = (&self.c0).mul_unreduced(&rhs.c0); // v0 has magnitude 1
let v1 = (&(&self.c1).neg()).mul_unreduced(&rhs.c1); // v1 has magnitude 1
let c0 = v0.add(&v1); // c0 has magnitude 2
let c1 = (&(&self.c0).add(&self.c1)).mul_unreduced(&(&rhs.c0).add(&rhs.c1)); // c1 has magnitude 1
let c1 = c1.add(&v1); // c1 has magnitude 2
let v0 = v0.negate(); // v0 has magnitude 2 (it could have been zero)
let c1 = c1.add(&v0); // c1 has magnitude 4

Fp2 {
c0: Fp::montgomery_reduce(c0),
c1: Fp::montgomery_reduce(c1),
}
}

pub const fn add(&self, rhs: &Fp2) -> Fp2 {