serai/coordinator/tributary-sdk/src/tendermint/mod.rs

use core::{ops::Deref, future::Future};
use std::{sync::Arc, collections::HashMap};

use subtle::ConstantTimeEq;
use zeroize::{Zeroize, Zeroizing};

use rand::{SeedableRng, seq::SliceRandom};
use rand_chacha::ChaCha12Rng;

use transcript::{Transcript, RecommendedTranscript};

use ciphersuite::{
  group::{
    GroupEncoding,
    ff::{Field, PrimeField},
  },
  Ciphersuite, Ristretto,
};
use schnorr::{
  SchnorrSignature,
  aggregate::{SchnorrAggregator, SchnorrAggregate},
};

use serai_db::Db;

use scale::{Encode, Decode};
use tendermint::{
  SignedMessageFor,
  ext::{
    BlockNumber, RoundNumber, Signer as SignerTrait, SignatureScheme, Weights, Block as BlockTrait,
    BlockError as TendermintBlockError, Commit, Network,
  },
  SlashEvent,
};

use tokio::sync::RwLock;

use crate::{
  TENDERMINT_MESSAGE, TRANSACTION_MESSAGE, ReadWrite, transaction::Transaction as TransactionTrait,
  Transaction, BlockHeader, Block, BlockError, Blockchain, P2p,
};

pub mod tx;
use tx::TendermintTx;

const DST: &[u8] = b"Tributary Tendermint Commit Aggregator";

fn challenge(
  genesis: [u8; 32],
  key: [u8; 32],
  nonce: &[u8],
  msg: &[u8],
) -> <Ristretto as Ciphersuite>::F {
  let mut transcript = RecommendedTranscript::new(b"Tributary Chain Tendermint Message");
  transcript.append_message(b"genesis", genesis);
  transcript.append_message(b"key", key);
  transcript.append_message(b"nonce", nonce);
  transcript.append_message(b"message", msg);

  <Ristretto as Ciphersuite>::F::from_bytes_mod_order_wide(&transcript.challenge(b"schnorr").into())
}

#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Signer {
  genesis: [u8; 32],
  key: Zeroizing<<Ristretto as Ciphersuite>::F>,
}

impl Signer {
  pub(crate) fn new(genesis: [u8; 32], key: Zeroizing<<Ristretto as Ciphersuite>::F>) -> Signer {
    Signer { genesis, key }
  }
}

impl SignerTrait for Signer {
  type ValidatorId = [u8; 32];
  type Signature = [u8; 64];

  /// Returns the validator's current ID. Returns None if they aren't a current validator.
  fn validator_id(&self) -> impl Send + Future<Output = Option<Self::ValidatorId>> {
    async move { Some((Ristretto::generator() * self.key.deref()).to_bytes()) }
  }

  /// Sign a signature with the current validator's private key.
  fn sign(&self, msg: &[u8]) -> impl Send + Future<Output = Self::Signature> {
    async move {
      let mut nonce =
        Zeroizing::new(RecommendedTranscript::new(b"Tributary Chain Tendermint Nonce"));
      nonce.append_message(b"genesis", self.genesis);
      nonce.append_message(b"key", Zeroizing::new(self.key.deref().to_repr()).as_ref());
      nonce.append_message(b"message", msg);
      let mut nonce = nonce.challenge(b"nonce");

      let mut nonce_arr = [0; 64];
      nonce_arr.copy_from_slice(nonce.as_ref());

      let nonce_ref: &mut [u8] = nonce.as_mut();
      nonce_ref.zeroize();
      let nonce_ref: &[u8] = nonce.as_ref();
      assert_eq!(nonce_ref, [0; 64].as_ref());

      let nonce =
        Zeroizing::new(<Ristretto as Ciphersuite>::F::from_bytes_mod_order_wide(&nonce_arr));
      nonce_arr.zeroize();

      assert!(!bool::from(nonce.ct_eq(&<Ristretto as Ciphersuite>::F::ZERO)));

      let challenge = challenge(
        self.genesis,
        (Ristretto::generator() * self.key.deref()).to_bytes(),
        (Ristretto::generator() * nonce.deref()).to_bytes().as_ref(),
        msg,
      );

      let sig = SchnorrSignature::<Ristretto>::sign(&self.key, nonce, challenge).serialize();

      let mut res = [0; 64];
      res.copy_from_slice(&sig);
      res
    }
  }
}

#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Validators {
  genesis: [u8; 32],
  total_weight: u64,
  weights: HashMap<[u8; 32], u64>,
  robin: Vec<[u8; 32]>,
}

impl Validators {
  pub(crate) fn new(
    genesis: [u8; 32],
    validators: Vec<(<Ristretto as Ciphersuite>::G, u64)>,
  ) -> Option<Validators> {
    let mut total_weight = 0;
    let mut weights = HashMap::new();

    let mut transcript = RecommendedTranscript::new(b"Round Robin Randomization");
    let mut robin = vec![];
    for (validator, weight) in validators {
      let validator = validator.to_bytes();
      if weight == 0 {
        return None;
      }
      total_weight += weight;
      weights.insert(validator, weight);

      transcript.append_message(b"validator", validator);
      transcript.append_message(b"weight", weight.to_le_bytes());
      robin.extend(vec![validator; usize::try_from(weight).unwrap()]);
    }
    robin.shuffle(&mut ChaCha12Rng::from_seed(transcript.rng_seed(b"robin")));

    Some(Validators { genesis, total_weight, weights, robin })
  }
}

impl SignatureScheme for Validators {
  type ValidatorId = [u8; 32];
  type Signature = [u8; 64];
  type AggregateSignature = Vec<u8>;
  type Signer = Arc<Signer>;

  #[must_use]
  fn verify(&self, validator: Self::ValidatorId, msg: &[u8], sig: &Self::Signature) -> bool {
    if !self.weights.contains_key(&validator) {
      return false;
    }
    let Ok(validator_point) = Ristretto::read_G::<&[u8]>(&mut validator.as_ref()) else {
      return false;
    };
    let Ok(actual_sig) = SchnorrSignature::<Ristretto>::read::<&[u8]>(&mut sig.as_ref()) else {
      return false;
    };
    actual_sig.verify(validator_point, challenge(self.genesis, validator, &sig[.. 32], msg))
  }

  fn aggregate(
    &self,
    validators: &[Self::ValidatorId],
    msg: &[u8],
    sigs: &[Self::Signature],
  ) -> Self::AggregateSignature {
    assert_eq!(validators.len(), sigs.len());

    let mut aggregator = SchnorrAggregator::<Ristretto>::new(DST);
    for (key, sig) in validators.iter().zip(sigs) {
      let actual_sig = SchnorrSignature::<Ristretto>::read::<&[u8]>(&mut sig.as_ref()).unwrap();
      let challenge = challenge(self.genesis, *key, actual_sig.R.to_bytes().as_ref(), msg);
      aggregator.aggregate(challenge, actual_sig);
    }

    let aggregate = aggregator.complete().unwrap();
    aggregate.serialize()
  }

  #[must_use]
  fn verify_aggregate(
    &self,
    signers: &[Self::ValidatorId],
    msg: &[u8],
    sig: &Self::AggregateSignature,
  ) -> bool {
    let Ok(aggregate) = SchnorrAggregate::<Ristretto>::read::<&[u8]>(&mut sig.as_slice()) else {
      return false;
    };

    if signers.len() != aggregate.Rs().len() {
      return false;
    }

    let mut challenges = vec![];
    for (key, nonce) in signers.iter().zip(aggregate.Rs()) {
      challenges.push(challenge(self.genesis, *key, nonce.to_bytes().as_ref(), msg));
    }

    aggregate.verify(
      DST,
      signers
        .iter()
        .zip(challenges)
        .map(|(s, c)| (<Ristretto as Ciphersuite>::read_G(&mut s.as_slice()).unwrap(), c))
        .collect::<Vec<_>>()
        .as_slice(),
    )
  }
}

impl Weights for Validators {
  type ValidatorId = [u8; 32];

  fn total_weight(&self) -> u64 {
    self.total_weight
  }
  fn weight(&self, validator: Self::ValidatorId) -> u64 {
    self.weights[&validator]
  }
  fn proposer(&self, block: BlockNumber, round: RoundNumber) -> Self::ValidatorId {
    let block = usize::try_from(block.0).unwrap();
    let round = usize::try_from(round.0).unwrap();
    // If multiple rounds are used, a naive block + round would cause the same index to be chosen
    // in quick succession.
    // Accordingly, if we use additional rounds, jump halfway around.
    // While this is still game-able, it's not explicitly reusing indexes immediately after each
    // other.
    self.robin
      [(block + (if round == 0 { 0 } else { round + (self.robin.len() / 2) })) % self.robin.len()]
  }
}

#[derive(Clone, PartialEq, Eq, Debug, Encode, Decode)]
pub struct TendermintBlock(pub Vec<u8>);
impl BlockTrait for TendermintBlock {
  type Id = [u8; 32];
  fn id(&self) -> Self::Id {
    BlockHeader::read::<&[u8]>(&mut self.0.as_ref()).unwrap().hash()
  }
}

#[derive(Clone, Debug)]
pub struct TendermintNetwork<D: Db, T: TransactionTrait, P: P2p> {
  pub(crate) genesis: [u8; 32],

  pub(crate) signer: Arc<Signer>,
  pub(crate) validators: Arc<Validators>,
  pub(crate) blockchain: Arc<RwLock<Blockchain<D, T>>>,

  pub(crate) p2p: P,
}

pub const BLOCK_PROCESSING_TIME: u32 = 999;
pub const LATENCY_TIME: u32 = 1667;
pub const TARGET_BLOCK_TIME: u32 = BLOCK_PROCESSING_TIME + (3 * LATENCY_TIME);

impl<D: Db, T: TransactionTrait, P: P2p> Network for TendermintNetwork<D, T, P> {
  type Db = D;

  type ValidatorId = [u8; 32];
  type SignatureScheme = Arc<Validators>;
  type Weights = Arc<Validators>;
  type Block = TendermintBlock;

  // These are in milliseconds and create a six-second block time.
  // The block time is the latency on message delivery (where a message is some piece of data
  // embedded in a transaction) times three plus the block processing time, hence why it should be
  // kept low.
  const BLOCK_PROCESSING_TIME: u32 = BLOCK_PROCESSING_TIME;
  const LATENCY_TIME: u32 = LATENCY_TIME;

  fn signer(&self) -> Arc<Signer> {
    self.signer.clone()
  }
  fn signature_scheme(&self) -> Arc<Validators> {
    self.validators.clone()
  }
  fn weights(&self) -> Arc<Validators> {
    self.validators.clone()
  }

  fn broadcast(&mut self, msg: SignedMessageFor<Self>) -> impl Send + Future<Output = ()> {
    async move {
      let mut to_broadcast = vec![TENDERMINT_MESSAGE];
      to_broadcast.extend(msg.encode());
      self.p2p.broadcast(self.genesis, to_broadcast).await
    }
  }

  fn slash(
    &mut self,
    validator: Self::ValidatorId,
    slash_event: SlashEvent,
  ) -> impl Send + Future<Output = ()> {
    async move {
      log::error!(
        "validator {} triggered a slash event on tributary {} (with evidence: {})",
        hex::encode(validator),
        hex::encode(self.genesis),
        matches!(slash_event, SlashEvent::WithEvidence(_)),
      );

      let signer = self.signer();
      let Some(tx) = (match slash_event {
        SlashEvent::WithEvidence(evidence) => {
          // create an unsigned evidence tx
          Some(TendermintTx::SlashEvidence(evidence))
        }
        SlashEvent::Id(_reason, _block, _round) => {
          // TODO: Increase locally observed slash points
          None
        }
      }) else {
        return;
      };

      // add tx to blockchain and broadcast to peers
      let mut to_broadcast = vec![TRANSACTION_MESSAGE];
      tx.write(&mut to_broadcast).unwrap();
      if self.blockchain.write().await.add_transaction::<Self>(
        true,
        Transaction::Tendermint(tx),
        &self.signature_scheme(),
      ) == Ok(true)
      {
        self.p2p.broadcast(signer.genesis, to_broadcast).await;
      }
    }
  }

  fn validate(
    &self,
    block: &Self::Block,
  ) -> impl Send + Future<Output = Result<(), TendermintBlockError>> {
    async move {
      let block =
        Block::read::<&[u8]>(&mut block.0.as_ref()).map_err(|_| TendermintBlockError::Fatal)?;
      self
        .blockchain
        .read()
        .await
        .verify_block::<Self>(&block, &self.signature_scheme(), false)
        .map_err(|e| match e {
          BlockError::NonLocalProvided(_) => TendermintBlockError::Temporal,
          _ => {
            log::warn!("Tributary Tendermint validate returning BlockError::Fatal due to {e:?}");
            TendermintBlockError::Fatal
          }
        })
    }
  }

  fn add_block(
    &mut self,
    serialized_block: Self::Block,
    commit: Commit<Self::SignatureScheme>,
  ) -> impl Send + Future<Output = Option<Self::Block>> {
    async move {
      let invalid_block = || {
        // There's a fatal flaw in the code, it's behind a hard fork, or the validators turned
        // malicious
        // All justify a halt to then achieve social consensus from
        // TODO: Under multiple validator sets, a small validator set turning malicious knocks
        // off the entire network. That's an unacceptable DoS.
        panic!("validators added invalid block to tributary {}", hex::encode(self.genesis));
      };

      // Tendermint should only produce valid commits
      assert!(self.verify_commit(serialized_block.id(), &commit));

      let Ok(block) = Block::read::<&[u8]>(&mut serialized_block.0.as_ref()) else {
        return invalid_block();
      };

      let encoded_commit = commit.encode();
      loop {
        let block_res = self.blockchain.write().await.add_block::<Self>(
          &block,
          encoded_commit.clone(),
          &self.signature_scheme(),
        );
        match block_res {
          Ok(()) => {
            // If we successfully added this block, break
            break;
          }
          Err(BlockError::NonLocalProvided(hash)) => {
            log::error!(
              "missing provided transaction {} which other validators on tributary {} had",
              hex::encode(hash),
              hex::encode(self.genesis)
            );
            tokio::time::sleep(core::time::Duration::from_secs(5)).await;
          }
          _ => return invalid_block(),
        }
      }

      Some(TendermintBlock(
        self.blockchain.write().await.build_block::<Self>(&self.signature_scheme()).serialize(),
      ))
    }
  }
}