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Fix comment on Montgomery conversion. #231

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Feb 13, 2019
Merged

Fix comment on Montgomery conversion. #231

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6 changes: 5 additions & 1 deletion src/edwards.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -450,14 +450,18 @@ impl EdwardsPoint {
/// Convert this `EdwardsPoint` on the Edwards model to the
/// corresponding `MontgomeryPoint` on the Montgomery model.
///
/// This function has one exceptional case; the identity point of
/// the Edwards curve is sent to the 2-torsion point \\((0,0)\\)
/// on the Montgomery curve.
///
/// Note that this is a one-way conversion, since the Montgomery
/// model does not retain sign information.
pub fn to_montgomery(&self) -> MontgomeryPoint {
// We have u = (1+y)/(1-y) = (Z+Y)/(Z-Y).
//
// The denominator is zero only when y=1, the identity point of
// the Edwards curve. Since 0.invert() = 0, in this case we
// compute u = 0, the identity point of the Montgomery line.
// compute the 2-torsion point (0,0).
let U = &self.Z + &self.Y;
let W = &self.Z - &self.Y;
let u = &U * &W.invert();
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