Add close_transaction API to cfd_protocol lib

cfd_protocol/src/lib.rs

@@ -5,8 +5,9 @@ pub mod interval;
 
 pub use oracle::{attest, nonce};
 pub use protocol::{
-    commit_descriptor, compute_adaptor_point, create_cfd_transactions, finalize_spend_transaction,
-    lock_descriptor, punish_transaction, renew_cfd_transactions, spending_tx_sighash,
-    CfdTransactions, PartyParams, Payout, PunishParams, TransactionExt, WalletExt,
+    close_transaction, commit_descriptor, compute_adaptor_point, create_cfd_transactions,
+    finalize_spend_transaction, lock_descriptor, punish_transaction, renew_cfd_transactions,
+    spending_tx_sighash, CfdTransactions, PartyParams, Payout, PunishParams, TransactionExt,
+    WalletExt,
 };
 pub use secp256k1_zkp;

cfd_protocol/src/protocol.rs

@@ -1,5 +1,5 @@
 pub use transaction_ext::TransactionExt;
-pub use transactions::punish_transaction;
+pub use transactions::{close_transaction, punish_transaction};
 
 use crate::interval;
 use crate::protocol::sighash_ext::SigHashExt;

cfd_protocol/src/protocol/transactions.rs

@@ -345,6 +345,61 @@ impl RefundTransaction {
     }
 }
 
+/// Build a transaction which closes the CFD contract.
+///
+/// This transaction spends directly from the lock transaction. Both
+/// parties must agree on the split of coins between `maker_amount`
+/// and `taker_amount`.
+pub fn close_transaction(
+    lock_descriptor: &Descriptor<PublicKey>,
+    lock_outpoint: OutPoint,
+    lock_amount: Amount,
+    (maker_address, maker_amount): (&Address, Amount),
+    (taker_address, taker_amount): (&Address, Amount),
+) -> Result<(Transaction, secp256k1_zkp::Message)> {
+    /// Expected size of signed transaction in virtual bytes, plus a
+    /// buffer to account for different signature lengths.
+    const SIGNED_VBYTES: f64 = 167.5 + (3.0 * 2.0) / 4.0;
+
+    let lock_input = TxIn {
+        previous_output: lock_outpoint,
+        ..Default::default()
+    };
+
+    // TODO: The fee could take into account the network state in this
+    // case, since this transaction is to be broadcast immediately
+    // after building and signing it
+    let fee = SIGNED_VBYTES * SATS_PER_VBYTE as f64;
+    let fee_per_party = (fee / 2.0) as u64;
+
+    let maker_output = TxOut {
+        value: maker_amount.as_sat() - fee_per_party,
+        script_pubkey: maker_address.script_pubkey(),
+    };
+    let taker_output = TxOut {
+        value: taker_amount.as_sat() - fee_per_party,
+        script_pubkey: taker_address.script_pubkey(),
+    };
+
+    let tx = Transaction {
+        version: 2,
+        lock_time: 0,
+        input: vec![lock_input],
+        output: vec![maker_output, taker_output],
+    };
+
+    let sighash = SigHashCache::new(&tx)
+        .signature_hash(
+            0,
+            &lock_descriptor.script_code(),
+            lock_amount.as_sat(),
+            SigHashType::All,
+        )
+        .to_message();
+
+    Ok((tx, sighash))
+}
+
 pub fn punish_transaction(
     commit_descriptor: &Descriptor<PublicKey>,
     address: &Address,

cfd_protocol/tests/cfds.rs

@@ -8,7 +8,7 @@ use bdk::SignOptions;
 use bit_vec::BitVec;
 use bitcoin::util::psbt::PartiallySignedTransaction;
 use cfd_protocol::{
-    attest, commit_descriptor, compute_adaptor_point, create_cfd_transactions,
+    attest, close_transaction, commit_descriptor, compute_adaptor_point, create_cfd_transactions,
     finalize_spend_transaction, interval, lock_descriptor, nonce, punish_transaction,
     renew_cfd_transactions, spending_tx_sighash, CfdTransactions, Payout, PunishParams,
     TransactionExt, WalletExt,
@@ -262,6 +262,74 @@ fn renew_cfd() {
     )
 }
 
+#[test]
+fn collaboratively_close_cfd() {
+    let mut rng = thread_rng();
+
+    let maker_lock_amount = Amount::ONE_BTC;
+    let taker_lock_amount = Amount::ONE_BTC;
+
+    let maker_wallet = build_wallet(&mut rng, Amount::from_btc(0.4).unwrap(), 5).unwrap();
+    let taker_wallet = build_wallet(&mut rng, Amount::from_btc(0.4).unwrap(), 5).unwrap();
+
+    let oracle = Oracle::new(&mut rng);
+    let (_, announcement) = announce(&mut rng);
+
+    let payouts = vec![Payout::new(
+        0..=10_000,
+        Amount::from_btc(1.5).unwrap(),
+        Amount::from_btc(0.5).unwrap(),
+    )
+    .unwrap()]
+    .concat();
+
+    let refund_timelock = 0;
+
+    let (maker_cfd_txs, _, maker, taker, maker_addr, taker_addr) = create_cfd_txs(
+        &mut rng,
+        (&maker_wallet, maker_lock_amount),
+        (&taker_wallet, taker_lock_amount),
+        (oracle.public_key(), &announcement.nonce_pks()),
+        payouts,
+        refund_timelock,
+    );
+
+    let lock_tx = maker_cfd_txs.lock.extract_tx();
+    let lock_desc = lock_descriptor(maker.pk, taker.pk);
+    let (lock_outpoint, lock_amount) = {
+        let outpoint = lock_tx
+            .outpoint(&lock_desc.script_pubkey())
+            .expect("lock script to be in lock tx");
+        let amount = Amount::from_sat(lock_tx.output[outpoint.vout as usize].value);
+
+        (outpoint, amount)
+    };
+
+    let maker_amount = Amount::ONE_BTC;
+    let taker_amount = Amount::ONE_BTC;
+
+    let (close_tx, close_sighash) = close_transaction(
+        &lock_desc,
+        lock_outpoint,
+        lock_amount,
+        (&maker_addr, maker_amount),
+        (&taker_addr, taker_amount),
+    )
+    .expect("to build close tx");
+
+    let sig_maker = SECP256K1.sign(&close_sighash, &maker.sk);
+    let sig_taker = SECP256K1.sign(&close_sighash, &taker.sk);
+    let signed_close_tx = finalize_spend_transaction(
+        close_tx,
+        &lock_desc,
+        (maker.pk, sig_maker),
+        (taker.pk, sig_taker),
+    )
+    .expect("to sign close tx");
+
+    check_tx(&lock_tx, &signed_close_tx, &lock_desc).expect("valid close tx");
+}
+
 fn create_cfd_txs(
     rng: &mut (impl RngCore + CryptoRng),
     (maker_wallet, maker_lock_amount): (&bdk::Wallet<(), bdk::database::MemoryDatabase>, Amount),