Add implementation of secq256k1

crypto/evrf/secq256k1/Cargo.toml

@@ -0,0 +1,39 @@
+[package]
+name = "secq256k1"
+version = "0.1.0"
+description = "An implementation of the curve secp256k1 cycles with"
+license = "MIT"
+repository = "https://github.com/serai-dex/serai/tree/develop/crypto/evrf/secq256k1"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+keywords = ["secp256k1", "secq256k1", "group"]
+edition = "2021"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
+[dependencies]
+rustversion = "1"
+hex-literal = { version = "0.4", default-features = false }
+
+rand_core = { version = "0.6", default-features = false, features = ["std"] }
+
+zeroize = { version = "^1.5", default-features = false, features = ["std", "zeroize_derive"] }
+subtle = { version = "^2.4", default-features = false, features = ["std"] }
+
+ff = { version = "0.13", default-features = false, features = ["std", "bits"] }
+group = { version = "0.13", default-features = false }
+
+generic-array = { version = "0.14", default-features = false }
+crypto-bigint = { version = "0.5", default-features = false, features = ["zeroize"] }
+
+k256 = { version = "0.13", default-features = false, features = ["arithmetic"] }
+
+ec-divisors = { path = "../divisors" }
+
+[dev-dependencies]
+hex = "0.4"
+
+rand_core = { version = "0.6", features = ["std"] }
+
+ff-group-tests = { path = "../../ff-group-tests" }

crypto/evrf/secq256k1/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2022-2024 Luke Parker
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

crypto/evrf/secq256k1/README.md

@@ -0,0 +1,3 @@
+# secq256k1
+
+An implementation of the curve secp256k1 cycles with.

crypto/evrf/secq256k1/src/backend.rs

@@ -0,0 +1,290 @@
+use zeroize::Zeroize;
+
+// Use black_box when possible
+#[rustversion::since(1.66)]
+use core::hint::black_box;
+#[rustversion::before(1.66)]
+fn black_box<T>(val: T) -> T {
+  val
+}
+
+pub(crate) fn u8_from_bool(bit_ref: &mut bool) -> u8 {
+  let bit_ref = black_box(bit_ref);
+
+  let mut bit = black_box(*bit_ref);
+  let res = black_box(bit as u8);
+  bit.zeroize();
+  debug_assert!((res | 1) == 1);
+
+  bit_ref.zeroize();
+  res
+}
+
+macro_rules! math_op {
+  (
+    $Value: ident,
+    $Other: ident,
+    $Op: ident,
+    $op_fn: ident,
+    $Assign: ident,
+    $assign_fn: ident,
+    $function: expr
+  ) => {
+    impl $Op<$Other> for $Value {
+      type Output = $Value;
+      fn $op_fn(self, other: $Other) -> Self::Output {
+        Self($function(self.0, other.0))
+      }
+    }
+    impl $Assign<$Other> for $Value {
+      fn $assign_fn(&mut self, other: $Other) {
+        self.0 = $function(self.0, other.0);
+      }
+    }
+    impl<'a> $Op<&'a $Other> for $Value {
+      type Output = $Value;
+      fn $op_fn(self, other: &'a $Other) -> Self::Output {
+        Self($function(self.0, other.0))
+      }
+    }
+    impl<'a> $Assign<&'a $Other> for $Value {
+      fn $assign_fn(&mut self, other: &'a $Other) {
+        self.0 = $function(self.0, other.0);
+      }
+    }
+  };
+}
+
+macro_rules! from_wrapper {
+  ($wrapper: ident, $inner: ident, $uint: ident) => {
+    impl From<$uint> for $wrapper {
+      fn from(a: $uint) -> $wrapper {
+        Self(Residue::new(&$inner::from(a)))
+      }
+    }
+  };
+}
+
+macro_rules! field {
+  (
+    $FieldName: ident,
+    $ResidueType: ident,
+
+    $MODULUS_STR: ident,
+    $MODULUS: ident,
+    $WIDE_MODULUS: ident,
+
+    $MULTIPLICATIVE_GENERATOR: literal,
+    $S: literal,
+    $ROOT_OF_UNITY: literal,
+    $DELTA: literal,
+  ) => {
+    use core::{
+      ops::{DerefMut, Add, AddAssign, Neg, Sub, SubAssign, Mul, MulAssign},
+      iter::{Sum, Product},
+    };
+
+    use subtle::{Choice, CtOption, ConstantTimeEq, ConstantTimeLess, ConditionallySelectable};
+    use rand_core::RngCore;
+
+    use crypto_bigint::{Integer, NonZero, Encoding, impl_modulus};
+
+    use ff::{Field, PrimeField, FieldBits, PrimeFieldBits, helpers::sqrt_ratio_generic};
+
+    use $crate::backend::u8_from_bool;
+
+    fn reduce(x: U512) -> U256 {
+      U256::from_le_slice(&x.rem(&NonZero::new($WIDE_MODULUS).unwrap()).to_le_bytes()[.. 32])
+    }
+
+    impl ConstantTimeEq for $FieldName {
+      fn ct_eq(&self, other: &Self) -> Choice {
+        self.0.ct_eq(&other.0)
+      }
+    }
+
+    impl ConditionallySelectable for $FieldName {
+      fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
+        $FieldName(Residue::conditional_select(&a.0, &b.0, choice))
+      }
+    }
+
+    math_op!($FieldName, $FieldName, Add, add, AddAssign, add_assign, |x: $ResidueType, y| x
+      .add(&y));
+    math_op!($FieldName, $FieldName, Sub, sub, SubAssign, sub_assign, |x: $ResidueType, y| x
+      .sub(&y));
+    math_op!($FieldName, $FieldName, Mul, mul, MulAssign, mul_assign, |x: $ResidueType, y| x
+      .mul(&y));
+
+    from_wrapper!($FieldName, U256, u8);
+    from_wrapper!($FieldName, U256, u16);
+    from_wrapper!($FieldName, U256, u32);
+    from_wrapper!($FieldName, U256, u64);
+    from_wrapper!($FieldName, U256, u128);
+
+    impl Neg for $FieldName {
+      type Output = $FieldName;
+      fn neg(self) -> $FieldName {
+        Self(self.0.neg())
+      }
+    }
+
+    impl<'a> Neg for &'a $FieldName {
+      type Output = $FieldName;
+      fn neg(self) -> Self::Output {
+        (*self).neg()
+      }
+    }
+
+    impl $FieldName {
+      /// Perform an exponentation.
+      pub fn pow(&self, other: $FieldName) -> $FieldName {
+        let mut table = [Self(Residue::ONE); 16];
+        table[1] = *self;
+        for i in 2 .. 16 {
+          table[i] = table[i - 1] * self;
+        }
+
+        let mut res = Self(Residue::ONE);
+        let mut bits = 0;
+        for (i, mut bit) in other.to_le_bits().iter_mut().rev().enumerate() {
+          bits <<= 1;
+          let mut bit = u8_from_bool(bit.deref_mut());
+          bits |= bit;
+          bit.zeroize();
+
+          if ((i + 1) % 4) == 0 {
+            if i != 3 {
+              for _ in 0 .. 4 {
+                res *= res;
+              }
+            }
+
+            let mut factor = table[0];
+            for (j, candidate) in table[1 ..].iter().enumerate() {
+              let j = j + 1;
+              factor = Self::conditional_select(&factor, &candidate, usize::from(bits).ct_eq(&j));
+            }
+            res *= factor;
+            bits = 0;
+          }
+        }
+        res
+      }
+    }
+
+    impl Field for $FieldName {
+      const ZERO: Self = Self(Residue::ZERO);
+      const ONE: Self = Self(Residue::ONE);
+
+      fn random(mut rng: impl RngCore) -> Self {
+        let mut bytes = [0; 64];
+        rng.fill_bytes(&mut bytes);
+        $FieldName(Residue::new(&reduce(U512::from_le_slice(bytes.as_ref()))))
+      }
+
+      fn square(&self) -> Self {
+        Self(self.0.square())
+      }
+      fn double(&self) -> Self {
+        *self + self
+      }
+
+      fn invert(&self) -> CtOption<Self> {
+        let res = self.0.invert();
+        CtOption::new(Self(res.0), res.1.into())
+      }
+
+      fn sqrt(&self) -> CtOption<Self> {
+        // (p + 1) // 4, as valid since p % 4 == 3
+        let mod_plus_one_div_four = $MODULUS.saturating_add(&U256::ONE).wrapping_div(&(4u8.into()));
+        let res = self.pow(Self($ResidueType::new_checked(&mod_plus_one_div_four).unwrap()));
+        CtOption::new(res, res.square().ct_eq(self))
+      }
+
+      fn sqrt_ratio(num: &Self, div: &Self) -> (Choice, Self) {
+        sqrt_ratio_generic(num, div)
+      }
+    }
+
+    impl PrimeField for $FieldName {
+      type Repr = [u8; 32];
+
+      const MODULUS: &'static str = $MODULUS_STR;
+
+      const NUM_BITS: u32 = 256;
+      const CAPACITY: u32 = 255;
+
+      const TWO_INV: Self = $FieldName($ResidueType::new(&U256::from_u8(2)).invert().0);
+
+      const MULTIPLICATIVE_GENERATOR: Self =
+        Self(Residue::new(&U256::from_u8($MULTIPLICATIVE_GENERATOR)));
+      const S: u32 = $S;
+
+      const ROOT_OF_UNITY: Self = $FieldName(Residue::new(&U256::from_be_hex($ROOT_OF_UNITY)));
+      const ROOT_OF_UNITY_INV: Self = Self(Self::ROOT_OF_UNITY.0.invert().0);
+
+      const DELTA: Self = $FieldName(Residue::new(&U256::from_be_hex($DELTA)));
+
+      fn from_repr(bytes: Self::Repr) -> CtOption<Self> {
+        let res = U256::from_le_slice(&bytes);
+        CtOption::new($FieldName(Residue::new(&res)), res.ct_lt(&$MODULUS))
+      }
+      fn to_repr(&self) -> Self::Repr {
+        let mut repr = [0; 32];
+        repr.copy_from_slice(&self.0.retrieve().to_le_bytes());
+        repr
+      }
+
+      fn is_odd(&self) -> Choice {
+        self.0.retrieve().is_odd()
+      }
+    }
+
+    impl PrimeFieldBits for $FieldName {
+      type ReprBits = [u8; 32];
+
+      fn to_le_bits(&self) -> FieldBits<Self::ReprBits> {
+        self.to_repr().into()
+      }
+
+      fn char_le_bits() -> FieldBits<Self::ReprBits> {
+        let mut repr = [0; 32];
+        repr.copy_from_slice(&MODULUS.to_le_bytes());
+        repr.into()
+      }
+    }
+
+    impl Sum<$FieldName> for $FieldName {
+      fn sum<I: Iterator<Item = $FieldName>>(iter: I) -> $FieldName {
+        let mut res = $FieldName::ZERO;
+        for item in iter {
+          res += item;
+        }
+        res
+      }
+    }
+
+    impl<'a> Sum<&'a $FieldName> for $FieldName {
+      fn sum<I: Iterator<Item = &'a $FieldName>>(iter: I) -> $FieldName {
+        iter.cloned().sum()
+      }
+    }
+
+    impl Product<$FieldName> for $FieldName {
+      fn product<I: Iterator<Item = $FieldName>>(iter: I) -> $FieldName {
+        let mut res = $FieldName::ONE;
+        for item in iter {
+          res *= item;
+        }
+        res
+      }
+    }
+
+    impl<'a> Product<&'a $FieldName> for $FieldName {
+      fn product<I: Iterator<Item = &'a $FieldName>>(iter: I) -> $FieldName {
+        iter.cloned().product()
+      }
+    }
+  };
+}

crypto/evrf/secq256k1/src/lib.rs

@@ -0,0 +1,13 @@
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![doc = include_str!("../README.md")]
+
+#[macro_use]
+mod backend;
+
+mod scalar;
+pub use scalar::Scalar;
+
+pub use k256::Scalar as FieldElement;
+
+mod point;
+pub use point::Point;

crypto/evrf/secq256k1/src/point.rs

@@ -0,0 +1,414 @@
+use core::{
+  ops::{DerefMut, Add, AddAssign, Neg, Sub, SubAssign, Mul, MulAssign},
+  iter::Sum,
+};
+
+use rand_core::RngCore;
+
+use zeroize::Zeroize;
+use subtle::{Choice, CtOption, ConstantTimeEq, ConditionallySelectable, ConditionallyNegatable};
+
+use generic_array::{typenum::U33, GenericArray};
+
+use group::{
+  ff::{Field, PrimeField, PrimeFieldBits},
+  Group, GroupEncoding,
+  prime::PrimeGroup,
+};
+
+use crate::{backend::u8_from_bool, Scalar, FieldElement};
+
+fn recover_y(x: FieldElement) -> CtOption<FieldElement> {
+  // x**3 + B since a = 0
+  ((x.square() * x) + FieldElement::from(7u64)).sqrt()
+}
+
+/// Point.
+#[derive(Clone, Copy, Debug, Zeroize)]
+#[repr(C)]
+pub struct Point {
+  x: FieldElement, // / Z
+  y: FieldElement, // / Z
+  z: FieldElement,
+}
+
+impl ConstantTimeEq for Point {
+  fn ct_eq(&self, other: &Self) -> Choice {
+    let x1 = self.x * other.z;
+    let x2 = other.x * self.z;
+
+    let y1 = self.y * other.z;
+    let y2 = other.y * self.z;
+
+    (self.x.is_zero() & other.x.is_zero()) | (x1.ct_eq(&x2) & y1.ct_eq(&y2))
+  }
+}
+
+impl PartialEq for Point {
+  fn eq(&self, other: &Point) -> bool {
+    self.ct_eq(other).into()
+  }
+}
+
+impl Eq for Point {}
+
+impl ConditionallySelectable for Point {
+  fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
+    Point {
+      x: FieldElement::conditional_select(&a.x, &b.x, choice),
+      y: FieldElement::conditional_select(&a.y, &b.y, choice),
+      z: FieldElement::conditional_select(&a.z, &b.z, choice),
+    }
+  }
+}
+
+impl Add for Point {
+  type Output = Point;
+  #[allow(non_snake_case)]
+  fn add(self, other: Self) -> Self {
+    // add-2015-rcb
+
+    let a = FieldElement::ZERO;
+    let B = FieldElement::from(7u64);
+    let b3 = B + B + B;
+
+    let X1 = self.x;
+    let Y1 = self.y;
+    let Z1 = self.z;
+    let X2 = other.x;
+    let Y2 = other.y;
+    let Z2 = other.z;
+
+    let t0 = X1 * X2;
+    let t1 = Y1 * Y2;
+    let t2 = Z1 * Z2;
+    let t3 = X1 + Y1;
+    let t4 = X2 + Y2;
+    let t3 = t3 * t4;
+    let t4 = t0 + t1;
+    let t3 = t3 - t4;
+    let t4 = X1 + Z1;
+    let t5 = X2 + Z2;
+    let t4 = t4 * t5;
+    let t5 = t0 + t2;
+    let t4 = t4 - t5;
+    let t5 = Y1 + Z1;
+    let X3 = Y2 + Z2;
+    let t5 = t5 * X3;
+    let X3 = t1 + t2;
+    let t5 = t5 - X3;
+    let Z3 = a * t4;
+    let X3 = b3 * t2;
+    let Z3 = X3 + Z3;
+    let X3 = t1 - Z3;
+    let Z3 = t1 + Z3;
+    let Y3 = X3 * Z3;
+    let t1 = t0 + t0;
+    let t1 = t1 + t0;
+    let t2 = a * t2;
+    let t4 = b3 * t4;
+    let t1 = t1 + t2;
+    let t2 = t0 - t2;
+    let t2 = a * t2;
+    let t4 = t4 + t2;
+    let t0 = t1 * t4;
+    let Y3 = Y3 + t0;
+    let t0 = t5 * t4;
+    let X3 = t3 * X3;
+    let X3 = X3 - t0;
+    let t0 = t3 * t1;
+    let Z3 = t5 * Z3;
+    let Z3 = Z3 + t0;
+    Point { x: X3, y: Y3, z: Z3 }
+  }
+}
+
+impl AddAssign for Point {
+  fn add_assign(&mut self, other: Point) {
+    *self = *self + other;
+  }
+}
+
+impl Add<&Point> for Point {
+  type Output = Point;
+  fn add(self, other: &Point) -> Point {
+    self + *other
+  }
+}
+
+impl AddAssign<&Point> for Point {
+  fn add_assign(&mut self, other: &Point) {
+    *self += *other;
+  }
+}
+
+impl Neg for Point {
+  type Output = Point;
+  fn neg(self) -> Self {
+    Point { x: self.x, y: -self.y, z: self.z }
+  }
+}
+
+impl Sub for Point {
+  type Output = Point;
+  #[allow(clippy::suspicious_arithmetic_impl)]
+  fn sub(self, other: Self) -> Self {
+    self + other.neg()
+  }
+}
+
+impl SubAssign for Point {
+  fn sub_assign(&mut self, other: Point) {
+    *self = *self - other;
+  }
+}
+
+impl Sub<&Point> for Point {
+  type Output = Point;
+  fn sub(self, other: &Point) -> Point {
+    self - *other
+  }
+}
+
+impl SubAssign<&Point> for Point {
+  fn sub_assign(&mut self, other: &Point) {
+    *self -= *other;
+  }
+}
+
+impl Group for Point {
+  type Scalar = Scalar;
+  fn random(mut rng: impl RngCore) -> Self {
+    loop {
+      let mut bytes = GenericArray::default();
+      rng.fill_bytes(bytes.as_mut());
+      let opt = Self::from_bytes(&bytes);
+      if opt.is_some().into() {
+        return opt.unwrap();
+      }
+    }
+  }
+  fn identity() -> Self {
+    Point { x: FieldElement::ZERO, y: FieldElement::ONE, z: FieldElement::ZERO }
+  }
+  fn generator() -> Self {
+    Point {
+      x: FieldElement::from_repr(
+        hex_literal::hex!("0000000000000000000000000000000000000000000000000000000000000001")
+          .into(),
+      )
+      .unwrap(),
+      y: FieldElement::from_repr(
+        hex_literal::hex!("0C7C97045A2074634909ABDF82C9BD0248916189041F2AF0C1B800D1FFC278C0")
+          .into(),
+      )
+      .unwrap(),
+      z: FieldElement::ONE,
+    }
+  }
+  fn is_identity(&self) -> Choice {
+    self.z.ct_eq(&FieldElement::ZERO)
+  }
+  #[allow(non_snake_case)]
+  fn double(&self) -> Self {
+    // dbl-2007-bl
+
+    let a = FieldElement::ZERO;
+
+    let X1 = self.x;
+    let Y1 = self.y;
+    let Z1 = self.z;
+
+    let XX = X1 * X1;
+    let ZZ = Z1 * Z1;
+    let w = (a * ZZ) + XX.double() + XX;
+    let s = (Y1 * Z1).double();
+    let ss = s * s;
+    let sss = s * ss;
+    let R = Y1 * s;
+    let RR = R * R;
+    let B = X1 + R;
+    let B = (B * B) - XX - RR;
+    let h = (w * w) -  B.double();
+    let X3 = h * s;
+    let Y3 = w * (B - h) - RR.double();
+    let Z3 = sss;
+
+    let res = Self { x: X3, y: Y3, z: Z3 };
+    // If self is identity, res will not be well-formed
+    // Accordingly, we return self if self was the identity
+    Self::conditional_select(&res, self, self.is_identity())
+  }
+}
+
+impl Sum<Point> for Point {
+  fn sum<I: Iterator<Item = Point>>(iter: I) -> Point {
+    let mut res = Self::identity();
+    for i in iter {
+      res += i;
+    }
+    res
+  }
+}
+
+impl<'a> Sum<&'a Point> for Point {
+  fn sum<I: Iterator<Item = &'a Point>>(iter: I) -> Point {
+    Point::sum(iter.cloned())
+  }
+}
+
+impl Mul<Scalar> for Point {
+  type Output = Point;
+  fn mul(self, mut other: Scalar) -> Point {
+    // Precompute the optimal amount that's a multiple of 2
+    let mut table = [Point::identity(); 16];
+    table[1] = self;
+    for i in 2 .. 16 {
+      table[i] = table[i - 1] + self;
+    }
+
+    let mut res = Self::identity();
+    let mut bits = 0;
+    for (i, mut bit) in other.to_le_bits().iter_mut().rev().enumerate() {
+      bits <<= 1;
+      let mut bit = u8_from_bool(bit.deref_mut());
+      bits |= bit;
+      bit.zeroize();
+
+      if ((i + 1) % 4) == 0 {
+        if i != 3 {
+          for _ in 0 .. 4 {
+            res = res.double();
+          }
+        }
+
+        let mut term = table[0];
+        for (j, candidate) in table[1 ..].iter().enumerate() {
+          let j = j + 1;
+          term = Self::conditional_select(&term, candidate, usize::from(bits).ct_eq(&j));
+        }
+        res += term;
+        bits = 0;
+      }
+    }
+    other.zeroize();
+    res
+  }
+}
+
+impl MulAssign<Scalar> for Point {
+  fn mul_assign(&mut self, other: Scalar) {
+    *self = *self * other;
+  }
+}
+
+impl Mul<&Scalar> for Point {
+  type Output = Point;
+  fn mul(self, other: &Scalar) -> Point {
+    self * *other
+  }
+}
+
+impl MulAssign<&Scalar> for Point {
+  fn mul_assign(&mut self, other: &Scalar) {
+    *self *= *other;
+  }
+}
+
+impl GroupEncoding for Point {
+  type Repr = GenericArray<u8, U33>;
+
+  fn from_bytes(bytes: &Self::Repr) -> CtOption<Self> {
+    // Extract and clear the sign bit
+    let sign = Choice::from(bytes[0] & 1);
+
+    // Parse x, recover y
+    FieldElement::from_repr(*GenericArray::from_slice(&bytes[1 ..])).and_then(|x| {
+      let is_identity = x.is_zero();
+
+      let y = recover_y(x).map(|mut y| {
+        y.conditional_negate(y.is_odd().ct_eq(&!sign));
+        y
+      });
+
+      // If this the identity, set y to 1
+      let y =
+        CtOption::conditional_select(&y, &CtOption::new(FieldElement::ONE, 1.into()), is_identity);
+      // Create the point if we have a y solution
+      let point = y.map(|y| Point { x, y, z: FieldElement::ONE });
+
+      let not_negative_zero = !(is_identity & sign);
+      // Only return the point if it isn't -0 and the sign byte wasn't malleated
+      CtOption::conditional_select(
+        &CtOption::new(Point::identity(), 0.into()),
+        &point,
+        not_negative_zero & ((bytes[0] & 1).ct_eq(&bytes[0])),
+      )
+    })
+  }
+
+  fn from_bytes_unchecked(bytes: &Self::Repr) -> CtOption<Self> {
+    Point::from_bytes(bytes)
+  }
+
+  fn to_bytes(&self) -> Self::Repr {
+    let Some(z) = Option::<FieldElement>::from(self.z.invert()) else {
+      return *GenericArray::from_slice(&[0; 33]);
+    };
+    let x = self.x * z;
+    let y = self.y * z;
+
+    let mut res = *GenericArray::from_slice(&[0; 33]);
+    res[1 ..].as_mut().copy_from_slice(&x.to_repr());
+
+    // The following conditional select normalizes the sign to 0 when x is 0
+    let y_sign = u8::conditional_select(&y.is_odd().unwrap_u8(), &0, x.ct_eq(&FieldElement::ZERO));
+    res[0] |= y_sign;
+    res
+  }
+}
+
+impl PrimeGroup for Point {}
+
+impl ec_divisors::DivisorCurve for Point {
+  type FieldElement = FieldElement;
+
+  fn a() -> Self::FieldElement {
+    FieldElement::from(0u64)
+  }
+  fn b() -> Self::FieldElement {
+    FieldElement::from(7u64)
+  }
+
+  fn to_xy(point: Self) -> Option<(Self::FieldElement, Self::FieldElement)> {
+    let z: Self::FieldElement = Option::from(point.z.invert())?;
+    Some((point.x * z, point.y * z))
+  }
+}
+
+#[test]
+fn test_curve() {
+  ff_group_tests::group::test_prime_group_bits::<_, Point>(&mut rand_core::OsRng);
+}
+
+#[test]
+fn generator() {
+  assert_eq!(
+    Point::generator(),
+    Point::from_bytes(GenericArray::from_slice(&hex_literal::hex!(
+      "000000000000000000000000000000000000000000000000000000000000000001"
+    )))
+    .unwrap()
+  );
+}
+
+#[test]
+fn zero_x_is_invalid() {
+  assert!(Option::<FieldElement>::from(recover_y(FieldElement::ZERO)).is_none());
+}
+
+// Checks random won't infinitely loop
+#[test]
+fn random() {
+  Point::random(&mut rand_core::OsRng);
+}

crypto/evrf/secq256k1/src/scalar.rs

@@ -0,0 +1,51 @@
+use zeroize::{DefaultIsZeroes, Zeroize};
+
+use crypto_bigint::{
+  U256, U512,
+  modular::constant_mod::{ResidueParams, Residue},
+};
+
+const MODULUS_STR: &str = "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F";
+
+impl_modulus!(HelioseleneQ, U256, MODULUS_STR);
+type ResidueType = Residue<HelioseleneQ, { HelioseleneQ::LIMBS }>;
+
+/// The Scalar field of secq256k1.
+///
+/// This is equivalent to the field secp256k1 is defined over.
+#[derive(Clone, Copy, PartialEq, Eq, Default, Debug)]
+#[repr(C)]
+pub struct Scalar(pub(crate) ResidueType);
+
+impl DefaultIsZeroes for Scalar {}
+
+pub(crate) const MODULUS: U256 = U256::from_be_hex(MODULUS_STR);
+
+const WIDE_MODULUS: U512 = U512::from_be_hex(concat!(
+  "0000000000000000000000000000000000000000000000000000000000000000",
+  "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F",
+));
+
+field!(
+  Scalar,
+  ResidueType,
+  MODULUS_STR,
+  MODULUS,
+  WIDE_MODULUS,
+  3,
+  1,
+  "fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2e",
+  "0000000000000000000000000000000000000000000000000000000000000009",
+);
+
+impl Scalar {
+  /// Perform a wide reduction, presumably to obtain a non-biased Scalar field element.
+  pub fn wide_reduce(bytes: [u8; 64]) -> Scalar {
+    Scalar(Residue::new(&reduce(U512::from_le_slice(bytes.as_ref()))))
+  }
+}
+
+#[test]
+fn test_scalar_field() {
+  ff_group_tests::prime_field::test_prime_field_bits::<_, Scalar>(&mut rand_core::OsRng);
+}