serai/coins/monero/src/ringct/clsag/mod.rs

#![allow(non_snake_case)]

use lazy_static::lazy_static;
use thiserror::Error;
use rand_core::{RngCore, CryptoRng};

use curve25519_dalek::{
  constants::ED25519_BASEPOINT_TABLE,
  scalar::Scalar,
  traits::VartimePrecomputedMultiscalarMul,
  edwards::{EdwardsPoint, VartimeEdwardsPrecomputation}
};

use crate::{
  Commitment,
  wallet::decoys::Decoys,
  random_scalar, hash_to_scalar, hash_to_point,
  serialize::*
};

#[cfg(feature = "multisig")]
mod multisig;
#[cfg(feature = "multisig")]
pub use multisig::{ClsagDetails, ClsagMultisig};

lazy_static! {
  static ref INV_EIGHT: Scalar = Scalar::from(8 as u8).invert();
}

#[derive(Error, Debug)]
pub enum ClsagError {
  #[error("internal error ({0})")]
  InternalError(String),
  #[error("invalid ring member (member {0}, ring size {1})")]
  InvalidRingMember(u8, u8),
  #[error("invalid commitment")]
  InvalidCommitment,
  #[error("invalid D")]
  InvalidD,
  #[error("invalid s")]
  InvalidS,
  #[error("invalid c1")]
  InvalidC1
}

#[derive(Clone, PartialEq, Debug)]
pub struct ClsagInput {
  // The actual commitment for the true spend
  pub commitment: Commitment,
  // True spend index, offsets, and ring
  pub decoys: Decoys
}

impl ClsagInput {
  pub fn new(
    commitment: Commitment,
    decoys: Decoys
  ) -> Result<ClsagInput, ClsagError> {
    let n = decoys.len();
    if n > u8::MAX.into() {
      Err(ClsagError::InternalError("max ring size in this library is u8 max".to_string()))?;
    }
    if decoys.i >= (n as u8) {
      Err(ClsagError::InvalidRingMember(decoys.i, n as u8))?;
    }

    // Validate the commitment matches
    if decoys.ring[usize::from(decoys.i)][1] != commitment.calculate() {
      Err(ClsagError::InvalidCommitment)?;
    }

    Ok(ClsagInput { commitment, decoys })
  }
}

enum Mode {
  Sign(usize, EdwardsPoint, EdwardsPoint),
  #[cfg(feature = "experimental")]
  Verify(Scalar)
}

// Core of the CLSAG algorithm, applicable to both sign and verify with minimal differences
// Said differences are covered via the above Mode
fn core(
  ring: &[[EdwardsPoint; 2]],
  I: &EdwardsPoint,
  pseudo_out: &EdwardsPoint,
  msg: &[u8; 32],
  D: &EdwardsPoint,
  s: &[Scalar],
  A_c1: Mode
) -> ((EdwardsPoint, Scalar, Scalar), Scalar) {
  let n = ring.len();

  let images_precomp = VartimeEdwardsPrecomputation::new([I, D]);
  let D = D * *INV_EIGHT;

  // Generate the transcript
  // Instead of generating multiple, a single transcript is created and then edited as needed
  let mut to_hash = vec![];
  to_hash.reserve_exact(((2 * n) + 5) * 32);
  const PREFIX: &[u8] = "CLSAG_".as_bytes();
  const AGG_0:  &[u8] = "CLSAG_agg_0".as_bytes();
  const ROUND:  &[u8] =       "round".as_bytes();
  to_hash.extend(AGG_0);
  to_hash.extend([0; 32 - AGG_0.len()]);

  let mut P = Vec::with_capacity(n);
  for member in ring {
    P.push(member[0]);
    to_hash.extend(member[0].compress().to_bytes());
  }

  let mut C = Vec::with_capacity(n);
  for member in ring {
    C.push(member[1] - pseudo_out);
    to_hash.extend(member[1].compress().to_bytes());
  }

  to_hash.extend(I.compress().to_bytes());
  to_hash.extend(D.compress().to_bytes());
  to_hash.extend(pseudo_out.compress().to_bytes());
  // mu_P with agg_0
  let mu_P = hash_to_scalar(&to_hash);
  // mu_C with agg_1
  to_hash[AGG_0.len() - 1] = '1' as u8;
  let mu_C = hash_to_scalar(&to_hash);

  // Truncate it for the round transcript, altering the DST as needed
  to_hash.truncate(((2 * n) + 1) * 32);
  for i in 0 .. ROUND.len() {
    to_hash[PREFIX.len() + i] = ROUND[i] as u8;
  }
  // Unfortunately, it's I D pseudo_out instead of pseudo_out I D, meaning this needs to be
  // truncated just to add it back
  to_hash.extend(pseudo_out.compress().to_bytes());
  to_hash.extend(msg);

  // Configure the loop based on if we're signing or verifying
  let start;
  let end;
  let mut c;
  match A_c1 {
    Mode::Sign(r, A, AH) => {
      start = r + 1;
      end = r + n;
      to_hash.extend(A.compress().to_bytes());
      to_hash.extend(AH.compress().to_bytes());
      c = hash_to_scalar(&to_hash);
    },

    #[cfg(feature = "experimental")]
    Mode::Verify(c1) => {
      start = 0;
      end = n;
      c = c1;
    }
  }

  // Perform the core loop
  let mut c1 = None;
  for i in (start .. end).map(|i| i % n) {
    if i == 0 {
      c1 = Some(c);
    }

    let c_p = mu_P * c;
    let c_c = mu_C * c;

    let L = (&s[i] * &ED25519_BASEPOINT_TABLE) + (c_p * P[i]) + (c_c * C[i]);
    let PH = hash_to_point(&P[i]);
    // Shouldn't be an issue as all of the variables in this vartime statement are public
    let R = (s[i] * PH) + images_precomp.vartime_multiscalar_mul(&[c_p, c_c]);

    to_hash.truncate(((2 * n) + 3) * 32);
    to_hash.extend(L.compress().to_bytes());
    to_hash.extend(R.compress().to_bytes());
    c = hash_to_scalar(&to_hash);
  }

  // This first tuple is needed to continue signing, the latter is the c to be tested/worked with
  ((D, c * mu_P, c * mu_C), c1.unwrap_or(c))
}

#[derive(Clone, PartialEq, Debug)]
pub struct Clsag {
  pub D: EdwardsPoint,
  pub s: Vec<Scalar>,
  pub c1: Scalar
}

impl Clsag {
  // Sign core is the extension of core as needed for signing, yet is shared between single signer
  // and multisig, hence why it's still core
  pub(crate) fn sign_core<R: RngCore + CryptoRng>(
    rng: &mut R,
    I: &EdwardsPoint,
    input: &ClsagInput,
    mask: Scalar,
    msg: &[u8; 32],
    A: EdwardsPoint,
    AH: EdwardsPoint
  ) -> (Clsag, EdwardsPoint, Scalar, Scalar) {
    let r: usize = input.decoys.i.into();

    let pseudo_out = Commitment::new(mask, input.commitment.amount).calculate();
    let z = input.commitment.mask - mask;

    let H = hash_to_point(&input.decoys.ring[r][0]);
    let D = H * z;
    let mut s = Vec::with_capacity(input.decoys.ring.len());
    for _ in 0 .. input.decoys.ring.len() {
      s.push(random_scalar(rng));
    }
    let ((D, p, c), c1) = core(&input.decoys.ring, I, &pseudo_out, msg, &D, &s, Mode::Sign(r, A, AH));

    (
      Clsag { D, s, c1 },
      pseudo_out,
      p,
      c * z
    )
  }

  // Single signer CLSAG
  pub fn sign<R: RngCore + CryptoRng>(
    rng: &mut R,
    inputs: &[(Scalar, EdwardsPoint, ClsagInput)],
    sum_outputs: Scalar,
    msg: [u8; 32]
  ) -> Vec<(Clsag, EdwardsPoint)> {
    let nonce = random_scalar(rng);
    let mut rand_source = [0; 64];
    rng.fill_bytes(&mut rand_source);

    let mut res = Vec::with_capacity(inputs.len());
    let mut sum_pseudo_outs = Scalar::zero();
    for i in 0 .. inputs.len() {
      let mut mask = random_scalar(rng);
      if i == (inputs.len() - 1) {
        mask = sum_outputs - sum_pseudo_outs;
      } else {
        sum_pseudo_outs += mask;
      }

      let mut rand_source = [0; 64];
      rng.fill_bytes(&mut rand_source);
      let (mut clsag, pseudo_out, p, c) = Clsag::sign_core(
        rng,
        &inputs[i].1,
        &inputs[i].2,
        mask,
        &msg,
        &nonce * &ED25519_BASEPOINT_TABLE,
        nonce * hash_to_point(&inputs[i].2.decoys.ring[usize::from(inputs[i].2.decoys.i)][0])
      );
      clsag.s[inputs[i].2.decoys.i as usize] = nonce - ((p * inputs[i].0) + c);

      res.push((clsag, pseudo_out));
    }

    res
  }

  // Not extensively tested nor guaranteed to have expected parity with Monero
  #[cfg(feature = "experimental")]
  pub fn rust_verify(
    &self,
    ring: &[[EdwardsPoint; 2]],
    I: &EdwardsPoint,
    pseudo_out: &EdwardsPoint,
    msg: &[u8; 32]
  ) -> Result<(), ClsagError> {
    let (_, c1) = core(
      ring,
      I,
      pseudo_out,
      msg,
      &self.D.mul_by_cofactor(),
      &self.s,
      Mode::Verify(self.c1)
    );
    if c1 != self.c1 {
      Err(ClsagError::InvalidC1)?;
    }
    Ok(())
  }

  pub fn serialize<W: std::io::Write>(&self, w: &mut W) -> std::io::Result<()> {
    write_raw_vec(write_scalar, &self.s, w)?;
    w.write_all(&self.c1.to_bytes())?;
    write_point(&self.D, w)
  }

  pub fn deserialize<R: std::io::Read>(decoys: usize, r: &mut R) -> std::io::Result<Clsag> {
    Ok(
      Clsag {
        s: read_raw_vec(read_scalar, decoys, r)?,
        c1: read_scalar(r)?,
        D: read_point(r)?
      }
    )
  }

  pub fn verify(
    &self,
    ring: &[[EdwardsPoint; 2]],
    I: &EdwardsPoint,
    pseudo_out: &EdwardsPoint,
    msg: &[u8; 32]
  ) -> Result<(), ClsagError> {
    // Serialize it to pass the struct to Monero without extensive FFI
    let mut serialized = Vec::with_capacity(1 + ((self.s.len() + 2) * 32));
    write_varint(&self.s.len().try_into().unwrap(), &mut serialized).unwrap();
    self.serialize(&mut serialized).unwrap();

    let I_bytes = I.compress().to_bytes();

    let mut ring_bytes = vec![];
    for member in ring {
      ring_bytes.extend(&member[0].compress().to_bytes());
      ring_bytes.extend(&member[1].compress().to_bytes());
    }

    let pseudo_out_bytes = pseudo_out.compress().to_bytes();

    unsafe {
      // Uses Monero's C verification function to ensure compatibility with Monero
      #[link(name = "wrapper")]
      extern "C" {
        pub(crate) fn c_verify_clsag(
          serialized_len: usize,
          serialized: *const u8,
          ring_size: u8,
          ring: *const u8,
          I: *const u8,
          pseudo_out: *const u8,
          msg: *const u8
        ) -> bool;
      }

      if c_verify_clsag(
        serialized.len(), serialized.as_ptr(),
        ring.len() as u8, ring_bytes.as_ptr(),
        I_bytes.as_ptr(), pseudo_out_bytes.as_ptr(), msg.as_ptr()
      ) {
        Ok(())
      } else {
        Err(ClsagError::InvalidC1)
      }
    }
  }
}