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Check the equality of EdwardsPoints in the projective coordinates #226

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merged 1 commit into from May 21, 2019
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Check the equality of EdwardsPoints in the projective coordinates #226

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9 changes: 8 additions & 1 deletion src/edwards.rs
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Expand Up @@ -394,7 +394,14 @@ impl ConditionallySelectable for EdwardsPoint {

impl ConstantTimeEq for EdwardsPoint {
fn ct_eq(&self, other: &EdwardsPoint) -> Choice {
self.compress().ct_eq(&other.compress())
// We would like to check that the point (X/Z, Y/Z) is equal to
// the point (X'/Z', Y'/Z') without converting into affine
// coordinates (x, y) and (x', y'), which requires two inversions.
// We have that X = xZ and X' = x'Z'. Thus, x = x' is equivalent to
// (xZ)Z' = (x'Z')Z, and similarly for the y-coordinate.

(&self.X * &other.Z).ct_eq(&(&other.X * &self.Z))
& (&self.Y * &other.Z).ct_eq(&(&other.Y * &self.Z))
}
}

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