serai/crypto/frost/src/tests/schnorr.rs

use rand_core::{RngCore, CryptoRng};

use ff::Field;

use crate::{Curve, schnorr, algorithm::SchnorrSignature};

pub(crate) fn sign<R: RngCore + CryptoRng, C: Curve>(rng: &mut R) {
  let private_key = C::F::random(&mut *rng);
  let nonce = C::F::random(&mut *rng);
  let challenge = C::F::random(rng); // Doesn't bother to craft an HRAM
  assert!(
    schnorr::verify::<C>(
      C::generator_table() * private_key,
      challenge,
      &schnorr::sign(private_key, nonce, challenge)
    )
  );
}

// The above sign function verifies signing works
// This verifies invalid signatures don't pass, using zero signatures, which should effectively be
// random
pub(crate) fn verify<R: RngCore + CryptoRng, C: Curve>(rng: &mut R) {
  assert!(
    !schnorr::verify::<C>(
      C::generator_table() * C::F::random(&mut *rng),
      C::F::random(rng),
      &SchnorrSignature { R: C::generator_table() * C::F::zero(), s: C::F::zero() }
    )
  );
}

pub(crate) fn batch_verify<R: RngCore + CryptoRng, C: Curve>(rng: &mut R) {
  // Create 5 signatures
  let mut keys = vec![];
  let mut challenges = vec![];
  let mut sigs = vec![];
  for i in 0 .. 5 {
    keys.push(C::F::random(&mut *rng));
    challenges.push(C::F::random(&mut *rng));
    sigs.push(schnorr::sign::<C>(keys[i], C::F::random(&mut *rng), challenges[i]));
  }

  // Batch verify
  let triplets = (0 .. 5).map(
    |i| (u16::try_from(i + 1).unwrap(), C::generator_table() * keys[i], challenges[i], sigs[i])
  ).collect::<Vec<_>>();
  schnorr::batch_verify(rng, &triplets).unwrap();

  // Shift 1 from s from one to another and verify it fails
  // This test will fail if unique factors aren't used per-signature, hence its inclusion
  {
    let mut triplets = triplets.clone();
    triplets[1].3.s += C::F::one();
    triplets[2].3.s -= C::F::one();
    if let Err(blame) = schnorr::batch_verify(rng, &triplets) {
      assert_eq!(blame, 2);
    } else {
      assert!(false);
    }
  }

  // Make sure a completely invalid signature fails when included
  for i in 0 .. 5 {
    let mut triplets = triplets.clone();
    triplets[i].3.s = C::F::random(&mut *rng);
    if let Err(blame) = schnorr::batch_verify(rng, &triplets) {
      assert_eq!(blame, u16::try_from(i + 1).unwrap());
    } else {
      assert!(false);
    }
  }
}