serai/crypto/frost/src/schnorr.rs

use rand_core::{RngCore, CryptoRng};

use ff::Field;
use group::Group;

use multiexp::BatchVerifier;

use crate::Curve;

#[allow(non_snake_case)]
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub struct SchnorrSignature<C: Curve> {
  pub R: C::G,
  pub s: C::F,
}

impl<C: Curve> SchnorrSignature<C> {
  pub fn serialize(&self) -> Vec<u8> {
    let mut res = Vec::with_capacity(C::G_len() + C::F_len());
    res.extend(C::G_to_bytes(&self.R));
    res.extend(C::F_to_bytes(&self.s));
    res
  }
}

pub(crate) fn sign<C: Curve>(
  private_key: C::F,
  nonce: C::F,
  challenge: C::F
) -> SchnorrSignature<C> {
  SchnorrSignature {
    R: C::generator_table() * nonce,
    s: nonce + (private_key * challenge)
  }
}

pub(crate) fn verify<C: Curve>(
  public_key: C::G,
  challenge: C::F,
  signature: &SchnorrSignature<C>
) -> bool {
  (C::generator_table() * signature.s) == (signature.R + (public_key * challenge))
}

pub(crate) fn batch_verify<C: Curve, R: RngCore + CryptoRng>(
  rng: &mut R,
  triplets: &[(u16, C::G, C::F, SchnorrSignature<C>)]
) -> Result<(), u16> {
  let mut values = [(C::F::one(), C::G::generator()); 3];
  let mut batch = BatchVerifier::new(triplets.len() * 3, C::little_endian());
  for triple in triplets {
    // R
    values[0].1 = triple.3.R;
    // cA
    values[1] = (triple.2, triple.1);
    // -sG
    values[2].0 = -triple.3.s;

    batch.queue(rng, triple.0, values);
  }

  // s = r + ca
  // sG == R + cA
  // R + cA - sG == 0
  if batch.verify_vartime() {
    Ok(())
  } else {
    Err(batch.blame_vartime().unwrap())
  }
}