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Constant-time field inversion

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WARNING: THIS IS NOT ACTUALLY CONSTANT TIME YET!

The jubjub and bls12_381 crates will replace our constant-time usages,
but we NEED to fix ff_derive because other users will expect it to
implement the Field trait correctly.
This commit is contained in:
Jack Grigg
2019-05-14 14:18:37 +01:00
parent e85a9f309f
commit 40749da9a7
25 changed files with 243 additions and 221 deletions
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13
zcash_primitives/src/jubjub/montgomery.rs
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@@ -139,11 +139,11 @@ impl<E: JubjubEngine, Subgroup> Point<E, Subgroup> {
{
let mut tmp = E::Fr::one();
tmp.sub_assign(&y);
u.mul_assign(&tmp.inverse().unwrap())
u.mul_assign(&tmp.invert().unwrap())
}
let mut v = u;
v.mul_assign(&x.inverse().unwrap());
v.mul_assign(&x.invert().unwrap());
// Scale it into the correct curve constants
v.mul_assign(params.scale());
@@ -226,7 +226,8 @@ impl<E: JubjubEngine, Subgroup> Point<E, Subgroup> {
}
{
let tmp = self.y.double();
delta.mul_assign(&tmp.inverse().expect("y is nonzero so this must be nonzero"));
// y is nonzero so this must be nonzero
delta.mul_assign(&tmp.invert().unwrap());
}
let mut x3 = delta.square();
@@ -272,10 +273,8 @@ impl<E: JubjubEngine, Subgroup> Point<E, Subgroup> {
{
let mut tmp = other.x;
tmp.sub_assign(&self.x);
delta.mul_assign(
&tmp.inverse()
.expect("self.x != other.x, so this must be nonzero"),
);
// self.x != other.x, so this must be nonzero
delta.mul_assign(&tmp.invert().unwrap());
}
let mut x3 = delta.square();