xmr-btc-swap/swap/src/bitcoin/refund.rs

use crate::bitcoin::wallet::Watchable;
use crate::bitcoin::{
    verify_sig, Address, Amount, EmptyWitnessStack, NoInputs, NotThreeWitnesses, PublicKey,
    TooManyInputs, Transaction, TxCancel,
};
use crate::{bitcoin, monero};
use ::bitcoin::util::bip143::SigHashCache;
use ::bitcoin::{Script, SigHash, SigHashType, Txid};
use anyhow::{bail, Context, Result};
use bdk::miniscript::{Descriptor, DescriptorTrait};
use ecdsa_fun::Signature;
use std::collections::HashMap;

#[derive(Debug)]
pub struct TxRefund {
    inner: Transaction,
    digest: SigHash,
    cancel_output_descriptor: Descriptor<::bitcoin::PublicKey>,
    watch_script: Script,
}

impl TxRefund {
    pub fn new(tx_cancel: &TxCancel, refund_address: &Address, spending_fee: Amount) -> Self {
        let tx_refund = tx_cancel.build_spend_transaction(refund_address, None, spending_fee);

        let digest = SigHashCache::new(&tx_refund).signature_hash(
            0, // Only one input: cancel transaction
            &tx_cancel.output_descriptor.script_code(),
            tx_cancel.amount().as_sat(),
            SigHashType::All,
        );

        Self {
            inner: tx_refund,
            digest,
            cancel_output_descriptor: tx_cancel.output_descriptor.clone(),
            watch_script: refund_address.script_pubkey(),
        }
    }

    pub fn txid(&self) -> Txid {
        self.inner.txid()
    }

    pub fn digest(&self) -> SigHash {
        self.digest
    }

    pub fn add_signatures(
        self,
        (A, sig_a): (PublicKey, Signature),
        (B, sig_b): (PublicKey, Signature),
    ) -> Result<Transaction> {
        let satisfier = {
            let mut satisfier = HashMap::with_capacity(2);

            let A = ::bitcoin::PublicKey {
                compressed: true,
                key: A.0.into(),
            };
            let B = ::bitcoin::PublicKey {
                compressed: true,
                key: B.0.into(),
            };

            // The order in which these are inserted doesn't matter
            satisfier.insert(A, (sig_a.into(), ::bitcoin::SigHashType::All));
            satisfier.insert(B, (sig_b.into(), ::bitcoin::SigHashType::All));

            satisfier
        };

        let mut tx_refund = self.inner;
        self.cancel_output_descriptor
            .satisfy(&mut tx_refund.input[0], satisfier)?;

        Ok(tx_refund)
    }

    pub fn extract_monero_private_key(
        &self,
        published_refund_tx: bitcoin::Transaction,
        s_a: monero::Scalar,
        a: bitcoin::SecretKey,
        S_b_bitcoin: bitcoin::PublicKey,
    ) -> Result<monero::PrivateKey> {
        let s_a = monero::PrivateKey { scalar: s_a };

        let tx_refund_sig = self
            .extract_signature_by_key(published_refund_tx, a.public())
            .context("Failed to extract signature from Bitcoin refund tx")?;
        let tx_refund_encsig = a.encsign(S_b_bitcoin, self.digest());

        let s_b = bitcoin::recover(S_b_bitcoin, tx_refund_sig, tx_refund_encsig)
            .context("Failed to recover Monero secret key from Bitcoin signature")?;

        let s_b = monero::private_key_from_secp256k1_scalar(s_b.into());

        let spend_key = s_a + s_b;

        Ok(spend_key)
    }

    fn extract_signature_by_key(
        &self,
        candidate_transaction: Transaction,
        B: PublicKey,
    ) -> Result<Signature> {
        let input = match candidate_transaction.input.as_slice() {
            [input] => input,
            [] => bail!(NoInputs),
            [inputs @ ..] => bail!(TooManyInputs(inputs.len())),
        };

        let sigs = match input
            .witness
            .iter()
            .map(|vec| vec.as_slice())
            .collect::<Vec<_>>()
            .as_slice()
        {
            [sig_1, sig_2, _script] => [sig_1, sig_2]
                .iter()
                .map(|sig| {
                    bitcoin::secp256k1::Signature::from_der(&sig[..sig.len() - 1])
                        .map(Signature::from)
                })
                .collect::<std::result::Result<Vec<_>, _>>(),
            [] => bail!(EmptyWitnessStack),
            [witnesses @ ..] => bail!(NotThreeWitnesses(witnesses.len())),
        }?;

        let sig = sigs
            .into_iter()
            .find(|sig| verify_sig(&B, &self.digest(), sig).is_ok())
            .context("Neither signature on witness stack verifies against B")?;

        Ok(sig)
    }

    pub fn weight() -> usize {
        548
    }
}

impl Watchable for TxRefund {
    fn id(&self) -> Txid {
        self.txid()
    }

    fn script(&self) -> Script {
        self.watch_script.clone()
    }
}