serai/crypto/frost/src/curves/kp256.rs

use core::{marker::PhantomData, convert::TryInto};

use rand_core::{RngCore, CryptoRng};

use sha2::{digest::Update, Digest, Sha256};

use ff::{Field, PrimeField};
use group::{Group, GroupEncoding};

use elliptic_curve::{bigint::{Encoding, U384}, hash2curve::{Expander, ExpandMsg, ExpandMsgXmd}};

use crate::{CurveError, Curve};
#[cfg(any(test, feature = "p256"))]
use crate::algorithm::Hram;

#[allow(non_snake_case)]
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub struct KP256<G: Group> {
  _G: PhantomData<G>
}

pub(crate) trait KP256Instance<G> {
  const CONTEXT: &'static [u8];
  const ID: &'static [u8];
  const GENERATOR: G;
}

#[cfg(any(test, feature = "p256"))]
pub type P256 = KP256<p256::ProjectivePoint>;
#[cfg(any(test, feature = "p256"))]
impl KP256Instance<p256::ProjectivePoint> for P256 {
  const CONTEXT: &'static [u8] = b"FROST-P256-SHA256-v5";
  const ID: &'static [u8] = b"P-256";
  const GENERATOR: p256::ProjectivePoint = p256::ProjectivePoint::GENERATOR;
}

#[cfg(feature = "k256")]
pub type K256 = KP256<k256::ProjectivePoint>;
#[cfg(feature = "k256")]
impl KP256Instance<k256::ProjectivePoint> for K256 {
  const CONTEXT: &'static [u8] = b"FROST-secp256k1-SHA256-v5";
  const ID: &'static [u8] = b"secp256k1";
  const GENERATOR: k256::ProjectivePoint = k256::ProjectivePoint::GENERATOR;
}

impl<G: Group + GroupEncoding> Curve for KP256<G> where
  KP256<G>: KP256Instance<G>,
  G::Scalar: PrimeField,
  <G::Scalar as PrimeField>::Repr: From<[u8; 32]> + AsRef<[u8]>,
  G::Repr: From<[u8; 33]> + AsRef<[u8]> {
  type F = G::Scalar;
  type G = G;
  type T = G;

  const ID: &'static [u8] = <Self as KP256Instance<G>>::ID;

  const GENERATOR: Self::G = <Self as KP256Instance<G>>::GENERATOR;
  const GENERATOR_TABLE: Self::G = <Self as KP256Instance<G>>::GENERATOR;

  const LITTLE_ENDIAN: bool = false;

  fn random_nonce<R: RngCore + CryptoRng>(secret: Self::F, rng: &mut R) -> Self::F {
    let mut seed = vec![0; 32];
    rng.fill_bytes(&mut seed);
    seed.extend(secret.to_repr().as_ref());
    Self::hash_to_F(&[Self::CONTEXT, b"nonce"].concat(), &seed)
  }

  fn hash_msg(msg: &[u8]) -> Vec<u8> {
    (&Sha256::new()
      .chain(Self::CONTEXT)
      .chain(b"digest")
      .chain(msg)
      .finalize()
    ).to_vec()
  }

  fn hash_binding_factor(binding: &[u8]) -> Self::F {
    Self::hash_to_F(&[Self::CONTEXT, b"rho"].concat(), binding)
  }

  fn hash_to_F(dst: &[u8], msg: &[u8]) -> Self::F {
    let mut dst = dst;
    let oversize = Sha256::digest([b"H2C-OVERSIZE-DST-", dst].concat());
    if dst.len() > 255 {
      dst = &oversize;
    }

    let mut modulus = vec![0; 16];
    modulus.extend((Self::F::zero() - Self::F::one()).to_repr().as_ref());
    let modulus = U384::from_be_slice(&modulus).wrapping_add(&U384::ONE);
    Self::F_from_slice(
      &U384::from_be_slice(&{
        let mut bytes = [0; 48];
        ExpandMsgXmd::<Sha256>::expand_message(&[msg], dst, 48).unwrap().fill_bytes(&mut bytes);
        bytes
      }).reduce(&modulus).unwrap().to_be_bytes()[16 ..]
    ).unwrap()
  }

  fn F_len() -> usize {
    32
  }

  fn G_len() -> usize {
    33
  }

  fn F_from_slice(slice: &[u8]) -> Result<Self::F, CurveError> {
    let bytes: [u8; 32] = slice.try_into()
      .map_err(|_| CurveError::InvalidLength(32, slice.len()))?;

    let scalar = Self::F::from_repr(bytes.into());
    if scalar.is_none().into() {
      Err(CurveError::InvalidScalar)?;
    }

    Ok(scalar.unwrap())
  }

  fn G_from_slice(slice: &[u8]) -> Result<Self::G, CurveError> {
    let bytes: [u8; 33] = slice.try_into()
      .map_err(|_| CurveError::InvalidLength(33, slice.len()))?;

    let point = Self::G::from_bytes(&bytes.into());
    if point.is_none().into() || point.unwrap().is_identity().into() {
      Err(CurveError::InvalidPoint)?;
    }

    Ok(point.unwrap())
  }

  fn F_to_bytes(f: &Self::F) -> Vec<u8> {
    f.to_repr().as_ref().to_vec()
  }

  fn G_to_bytes(g: &Self::G) -> Vec<u8> {
    g.to_bytes().as_ref().to_vec()
  }
}

#[cfg(any(test, feature = "p256"))]
#[derive(Clone)]
pub struct IetfP256Hram;
#[cfg(any(test, feature = "p256"))]
impl Hram<P256> for IetfP256Hram {
  #[allow(non_snake_case)]
  fn hram(R: &p256::ProjectivePoint, A: &p256::ProjectivePoint, m: &[u8]) -> p256::Scalar {
    P256::hash_to_F(
      &[P256::CONTEXT, b"chal"].concat(),
      &[&P256::G_to_bytes(R), &P256::G_to_bytes(A), m].concat()
    )
  }
}