short-weierstrass 0.1.0

A library for working with curves in a short Weierstrass form
Documentation 
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use core::ops::*;

use subtle::{Choice, CtOption, ConstantTimeEq, ConditionallySelectable, ConditionallyNegatable};
use zeroize::DefaultIsZeroes;

use rand_core::RngCore;

use group::ff::Field;

use crate::{ShortWeierstrass, Projective};

/// A point represented with affine coordinates.
#[derive(Debug)]
pub struct Affine<C: ShortWeierstrass> {
  pub(crate) x: C::FieldElement,
  pub(crate) y: C::FieldElement,
}
impl<C: ShortWeierstrass> Clone for Affine<C> {
  fn clone(&self) -> Self {
    *self
  }
}
impl<C: ShortWeierstrass> Copy for Affine<C> {}

impl<C: ShortWeierstrass> Affine<C> {
  pub fn try_from(projective: Projective<C>) -> CtOption<Self> {
    projective.z.invert().map(|z_inv| Self { x: projective.x * z_inv, y: projective.y * z_inv })
  }
}

impl<C: ShortWeierstrass> Default for Affine<C> {
  fn default() -> Self {
    C::GENERATOR
  }
}
impl<C: ShortWeierstrass> DefaultIsZeroes for Affine<C> {}

impl<C: ShortWeierstrass> ConstantTimeEq for Affine<C> {
  fn ct_eq(&self, other: &Self) -> Choice {
    self.x.ct_eq(&other.x) & self.y.ct_eq(&other.y)
  }
}
impl<C: ShortWeierstrass> PartialEq for Affine<C> {
  fn eq(&self, other: &Self) -> bool {
    self.ct_eq(other).into()
  }
}
impl<C: ShortWeierstrass> Eq for Affine<C> {}

impl<C: ShortWeierstrass> ConditionallySelectable for Affine<C> {
  fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
    let x = C::FieldElement::conditional_select(&a.x, &b.x, choice);
    let y = C::FieldElement::conditional_select(&a.y, &b.y, choice);
    Affine { x, y }
  }
}

impl<C: ShortWeierstrass> Neg for Affine<C> {
  type Output = Self;
  fn neg(mut self) -> Self::Output {
    self.y = -self.y;
    self
  }
}

impl<C: ShortWeierstrass> ConditionallyNegatable for Affine<C> {
  fn conditional_negate(&mut self, negate: Choice) {
    self.y = <_>::conditional_select(&self.y, &-self.y, negate);
  }
}

impl<C: ShortWeierstrass> Affine<C> {
  /// Create an affine point from `x, y` coordinates, without performing any checks.
  ///
  /// This should NOT be used. It is solely intended for trusted data at compile-time. It MUST NOT
  /// be used with any untrusted/unvalidated data.
  pub const fn from_xy_unchecked(x: C::FieldElement, y: C::FieldElement) -> Self { Self { x, y } }

  /// The `x, y` coordinates of this point.
  pub fn coordinates(self) -> (C::FieldElement, C::FieldElement) {
    (self.x, self.y)
  }

/// Sample a random on-curve point with an unknown discrete logarithm w.r.t. any other points.
pub fn random(mut rng: impl RngCore) -> Self {
  loop {
    let x = C::FieldElement::random(&mut rng);
    let y_square = ((x.square() + C::A) * x) + C::B;
    let Some(mut y) = Option::<C::FieldElement>::from(y_square.sqrt()) else { continue };
    if (rng.next_u64() % 2) == 1 {
      y = -y;
    }
    return Self { x, y };
  }
    }
}