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Refactor code for finding affine points from x-coordinates.

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This commit is contained in:
Sean Bowe
2017-10-04 11:43:42 -06:00
parent f9d4c8b6aa
commit 931257599d
1 changed files with 39 additions and 38 deletions
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77
src/bls12_381/ec.rs
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@@ -85,6 +85,33 @@ macro_rules! curve_impl {
} }
impl $affine { impl $affine {
/// Constructs an affine point with the lexicographically smallest
/// y-coordinate, given an x-coordinate, so long as the x-coordinate
/// exists on the curve. The point is not guaranteed to be in the
/// prime order subgroup.
fn get_point_from_x(x: $basefield) -> Option<$affine> {
// Compute x^3 + b
let mut x3b = x;
x3b.square();
x3b.mul_assign(&x);
x3b.add_assign(&$affine::get_coeff_b());
x3b.sqrt().map(|y| {
let mut negy = y;
negy.negate();
$affine {
x: x,
y: if y < negy {
y
} else {
negy
},
infinity: false
}
})
}
fn is_on_curve(&self) -> bool { fn is_on_curve(&self) -> bool {
if self.is_zero() { if self.is_zero() {
true true
@@ -781,26 +808,13 @@ pub mod g1 {
// Interpret as Fq element. // Interpret as Fq element.
let x = Fq::from_repr(x).map_err(|e| GroupDecodingError::CoordinateDecodingError("x coordinate", e))?; let x = Fq::from_repr(x).map_err(|e| GroupDecodingError::CoordinateDecodingError("x coordinate", e))?;
// Compute x^3 + b match G1Affine::get_point_from_x(x) {
let mut x3b = x; Some(mut p) => {
x3b.square(); if greatest {
x3b.mul_assign(&x); p.negate();
x3b.add_assign(&G1Affine::get_coeff_b()); }
// Attempt to compute y Ok(p)
match x3b.sqrt() {
Some(y) => {
let mut negy = y;
negy.negate();
// Get the parity of the sqrt we found.
let parity = y > negy;
Ok(G1Affine {
x: x,
y: if parity == greatest { y } else { negy },
infinity: false
})
}, },
None => { None => {
// Point must not be on the curve. // Point must not be on the curve.
@@ -1307,26 +1321,13 @@ pub mod g2 {
c1: Fq::from_repr(x_c1).map_err(|e| GroupDecodingError::CoordinateDecodingError("x coordinate (c1)", e))? c1: Fq::from_repr(x_c1).map_err(|e| GroupDecodingError::CoordinateDecodingError("x coordinate (c1)", e))?
}; };
// Compute x^3 + b match G2Affine::get_point_from_x(x) {
let mut x3b = x; Some(mut p) => {
x3b.square(); if greatest {
x3b.mul_assign(&x); p.negate();
x3b.add_assign(&G2Affine::get_coeff_b()); }
// Attempt to compute y Ok(p)
match x3b.sqrt() {
Some(y) => {
let mut negy = y;
negy.negate();
// Get the parity of the sqrt we found.
let parity = y > negy;
Ok(G2Affine {
x: x,
y: if parity == greatest { y } else { negy },
infinity: false
})
}, },
None => { None => {
// Point must not be on the curve. // Point must not be on the curve.