Add PSBT v2

Add an implementation of PSBT version 2 - BOOM!

Includes all test vectors from BIP-370. Also includes changes to `v0` to
explicitly exclude keys as required by PSBT v2 upgrade.

Cargo.toml

@@ -8,7 +8,8 @@ description = "Partially signed Bitcoin Transaction, v0 and v1"
 categories = ["cryptography::cryptocurrencies"]
 keywords = [ "crypto", "bitcoin" ]
 readme = "../README.md"
-edition = "2018"
+edition = "2021"
+rust-version = "1.56.1"
 exclude = ["tests", "contrib"]
 
 [package.metadata.docs.rs]
@@ -47,3 +48,10 @@ secp256k1 = { version = "0.28", features = ["rand-std", "global-context"] }
 [[example]]
 name = "v0"
 required-features = ["std"]
+
+[[example]]
+name = "v2"
+required-features = ["std"]
+
+[[example]]
+name = "v2-separate-creator-constructor"

contrib/test.sh

@@ -24,6 +24,8 @@ if [ "$DO_LINT" = true ]
 then
     cargo clippy --all-features --all-targets -- -D warnings
     cargo clippy --example v0 -- -D warnings
+    cargo clippy --example v2 -- -D warnings
+    cargo clippy --example v2-separate-creator-constructor -- -D warnings
 fi
 
 # Test without any features other than std first (same as default)
@@ -38,6 +40,8 @@ do
 done
 
 cargo run --example v0
+cargo run --example v2
+cargo clippy --example v2-separate-creator-constructor
 
 if [ "$DO_NO_STD" = true ]
 then

examples/v2-separate-creator-constructor.rs

@@ -0,0 +1,65 @@
+//! PSBT v2 - Creator a PSBT and hand it around to various different entities to add inputs and outputs.
+
+use psbt::bitcoin::hashes::Hash as _;
+use psbt::bitcoin::{Amount, OutPoint, ScriptBuf, TxOut, Txid};
+use psbt::v2::{
+    Constructor, Creator, InputBuilder, InputsOnlyModifiable, OutputBuilder, OutputsOnlyModifiable,
+    Psbt,
+};
+
+fn main() -> anyhow::Result<()> {
+    // Create the PSBT.
+    let created = Creator::new().inputs_modifiable().outputs_modifiable().psbt();
+
+    let ser = created.serialize();
+
+    // The first constructor entity receives the PSBT and adds an input.
+    let psbt = Psbt::deserialize(&ser)?;
+    let in_0 = dummy_out_point();
+    let ser = Constructor::<InputsOnlyModifiable>::new(psbt)?
+        .input(InputBuilder::new(in_0).build())
+        .psbt()
+        .expect("valid lock time combination")
+        .serialize();
+
+    // The second constructor entity receives the PSBT with one input and adds a second input.
+    let psbt = Psbt::deserialize(&ser)?;
+    let in_1 = dummy_out_point();
+    let ser = Constructor::<InputsOnlyModifiable>::new(psbt)?
+        .input(InputBuilder::new(in_1).build())
+        .no_more_inputs()
+        .psbt()
+        .expect("valid lock time combination")
+        .serialize();
+
+    // The third constructor entity receives the PSBT with inputs and adds an output.
+    let psbt = Psbt::deserialize(&ser)?;
+    let output = dummy_tx_out();
+    let ser = Constructor::<OutputsOnlyModifiable>::new(psbt)?
+        .output(OutputBuilder::new(output).build())
+        .no_more_outputs()
+        .psbt()
+        .expect("valid lock time combination")
+        .serialize();
+
+    // The PSBT is now ready for handling with the updater role.
+    let _updatable_psbt = Psbt::deserialize(&ser)?;
+
+    Ok(())
+}
+
+/// A dummy `OutPoint`, this would usually be the unspent transaction that we are spending.
+fn dummy_out_point() -> OutPoint {
+    let txid = Txid::hash(b"some arbitrary bytes");
+    let vout = 0x15;
+    OutPoint { txid, vout }
+}
+
+/// A dummy `TxOut`, this would usually be the output we are creating with this transaction.
+fn dummy_tx_out() -> TxOut {
+    // Arbitrary script, may not even be a valid scriptPubkey.
+    let script = ScriptBuf::from_hex("76a914162c5ea71c0b23f5b9022ef047c4a86470a5b07088ac")
+        .expect("failed to parse script form hex");
+    let value = Amount::from_sat(123_456_789);
+    TxOut { value, script_pubkey: script }
+}

examples/v2.rs

@@ -0,0 +1,288 @@
+//! PSBT v2 2 of 2 multisig example.
+//!
+//! An example of using PSBT v0 to create a 2 of 2 multisig by spending two native segwit v0 inputs
+//! to a native segwit v0 output (the multisig output).
+//!
+//! We sign invalid inputs, this code is not run against Bitcoin Core so everything here should be
+//! taken as NOT PROVEN CORRECT.
+//!
+//! This code is similar to `v0.rs` on purpose to show the differences between the APIs.
+
+use std::str::FromStr;
+
+use psbt::bitcoin::bip32::{DerivationPath, KeySource, Xpriv, Xpub};
+use psbt::bitcoin::hashes::Hash as _;
+use psbt::bitcoin::locktime::absolute;
+use psbt::bitcoin::opcodes::all::OP_CHECKMULTISIG;
+use psbt::bitcoin::secp256k1::{self, SECP256K1};
+use psbt::bitcoin::{
+    script, Address, Amount, Network, OutPoint, PublicKey, ScriptBuf, Sequence, TxOut, Txid,
+};
+use psbt::v2::{
+    self, Constructor, Input, InputBuilder, Modifiable, Output, OutputBuilder, Psbt, Signer,
+    Updater,
+};
+
+pub const DUMMY_UTXO_AMOUNT: Amount = Amount::from_sat(20_000_000);
+pub const SPEND_AMOUNT: Amount = Amount::from_sat(20_000_000);
+
+const MAINNET: Network = Network::Bitcoin; // Bitcoin mainnet network.
+const FEE: Amount = Amount::from_sat(1_000); // Usually this would be calculated.
+const DUMMY_CHANGE_AMOUNT: Amount = Amount::from_sat(100_000);
+
+fn main() -> anyhow::Result<()> {
+    // Mimic two people, Alice and Bob, who wish to create a 2-of-2 multisig output together.
+    let alice = Alice::new();
+    let bob = Bob::new();
+
+    // Each person provides the pubkey they want this multisig to be locked to.
+    let pk_a = alice.multisig_public_key()?;
+    let pk_b = bob.multisig_public_key()?;
+
+    // Use of a locktime is of course optional.
+    let min_required_height = absolute::Height::from_consensus(800_000).expect("valid height");
+
+    // Each party will be contributing 20,000,000 sats to the mulitsig output, as such each party
+    // provides an unspent input to create the multisig output (and any change details if needed).
+
+    // Alice has a UTXO that is too big, she needs change.
+    let (previous_output_a, change_address_a, change_value_a) = alice.contribute_to_multisig()?;
+
+    // Bob has a UTXO the right size so no change needed.
+    let previous_output_b = bob.contribute_to_multisig();
+
+    // In PSBT v1 the creator and constructor roles can be the same entity, for an example of having
+    // them separate see `./v2-separate-creator-constructor.rs`.
+
+    // The constructor role.
+
+    let constructor = Constructor::<Modifiable>::default();
+
+    let input_a = InputBuilder::new(previous_output_a)
+        .minimum_required_height_based_lock_time(min_required_height)
+        .build();
+
+    // If no lock time is required we can just create the `Input` directly.
+    let input_b = Input::new(previous_output_b);
+
+    // Build Alice's change output.
+    let change = TxOut { value: change_value_a, script_pubkey: change_address_a.script_pubkey() };
+
+    // Create the witness script, receive address, and the locking script.
+    let witness_script = multisig_witness_script(&pk_a, &pk_b);
+    let address = Address::p2wsh(&witness_script, MAINNET);
+    let value = SPEND_AMOUNT * 2 - FEE;
+    // The spend output is locked by the witness script.
+    let multi = TxOut { value, script_pubkey: address.script_pubkey() };
+
+    let psbt = constructor
+        .input(input_a)
+        .input(input_b)
+        .output(OutputBuilder::new(multi).build()) // Use of the `OutputBuilder` is identical
+        .output(Output::new(change)) // to just creating the `Output`.
+        .psbt()
+        .expect("valid lock time combination");
+
+    // The updater role.
+
+    // We can act as updater.
+    let psbt = Updater::new(psbt)?.set_sequence(Sequence::ENABLE_LOCKTIME_NO_RBF, 1)?.psbt();
+
+    // Or we can get Alice and Bob to act as updaters.
+    let updated_by_a = alice.update(psbt.clone())?;
+    let updated_by_b = bob.update(psbt)?;
+
+    let updated = v2::combine(updated_by_a, updated_by_b)?;
+
+    // The signer role.
+
+    // Each party then acts in the signer role.
+    let signed_by_a = alice.sign(updated.clone())?;
+    let signed_by_b = bob.sign(updated)?;
+
+    let _signed = v2::combine(signed_by_a, signed_by_b);
+
+    // At this stage we would usually finalize with miniscript and extract the transaction.
+
+    Ok(())
+}
+
+/// Creates a 2-of-2 multisig script locking to a and b's keys.
+fn multisig_witness_script(a: &PublicKey, b: &PublicKey) -> ScriptBuf {
+    script::Builder::new()
+        .push_int(2)
+        .push_key(a)
+        .push_key(b)
+        .push_int(2)
+        .push_opcode(OP_CHECKMULTISIG)
+        .into_script()
+}
+
+/// Party 1 in a 2-of-2 multisig.
+pub struct Alice(Entity);
+
+impl Alice {
+    /// The derivation path associated with the dummy utxo we are spending.
+    const PATH: &'static str = "m/84'/0'/0'/0/42";
+
+    /// Creates a new Alice.
+    pub fn new() -> Self {
+        let seed = [0x00; 32]; // Fake example with a fake seed :)
+        let xpriv = Xpriv::new_master(MAINNET, &seed).unwrap();
+
+        Self(Entity::new(xpriv))
+    }
+
+    /// Returns the public key for this entity.
+    pub fn multisig_public_key(&self) -> anyhow::Result<bitcoin::PublicKey> {
+        self.0.public_key("m/84'/0'/0'/123")
+    }
+
+    /// Alice provides an input to be used to create the multisig and the details required to get
+    /// some change back (change address and amount).
+    pub fn contribute_to_multisig(&self) -> anyhow::Result<(OutPoint, Address, Amount)> {
+        // An obviously invalid output, we just use all zeros then use the `vout` to differentiate
+        // Alice's output from Bob's.
+        let out = OutPoint { txid: Txid::all_zeros(), vout: 0 };
+
+        // The usual caveat about reusing addresses applies here, this is just an example.
+        let address = Address::p2wpkh(&self.multisig_public_key()?, Network::Bitcoin)
+            .expect("uncompressed key");
+
+        // This is a made up value, it is supposed to represent the outpoints value minus the value
+        // contributed to the multisig.
+        let amount = DUMMY_CHANGE_AMOUNT;
+
+        Ok((out, address, amount))
+    }
+
+    /// Signs `psbt`.
+    pub fn sign(&self, psbt: Psbt) -> anyhow::Result<Psbt> { self.0.sign_ecdsa(psbt, Self::PATH) }
+
+    /// Alice updates the PSBT, adding her utxo and key source.
+    pub fn update(&self, mut psbt: Psbt) -> anyhow::Result<Psbt> {
+        let input = &mut psbt.inputs[0];
+
+        // The dummy input utxo we are spending and the pubkey/keysource that will be used to sign it.
+        input.witness_utxo = Some(self.input_utxo()?);
+        let (pk, key_source) = self.bip32_derivation()?;
+        input.bip32_derivation.insert(pk, key_source);
+        Ok(psbt)
+    }
+
+    /// Provides the actual UTXO that Alice is contributing, this would usually come from the chain.
+    fn input_utxo(&self) -> anyhow::Result<TxOut> { self.0.input_utxo(Self::PATH) }
+
+    fn bip32_derivation(&self) -> anyhow::Result<(secp256k1::PublicKey, KeySource)> {
+        self.0.bip32_derivation(Self::PATH)
+    }
+}
+
+impl Default for Alice {
+    fn default() -> Self { Self::new() }
+}
+
+/// Party 2 in a 2-of-2 multisig.
+pub struct Bob(Entity);
+
+impl Bob {
+    /// The derivation path associated with the dummy utxo we are spending.
+    const PATH: &'static str = "m/84'/0'/0'/0/0";
+
+    /// Creates a new Bob.
+    pub fn new() -> Self {
+        let seed = [0x11; 32]; // Fake example with a fake seed :)
+        let xpriv = Xpriv::new_master(MAINNET, &seed).unwrap();
+
+        Self(Entity::new(xpriv))
+    }
+
+    /// Returns the public key for this entity.
+    pub fn multisig_public_key(&self) -> anyhow::Result<bitcoin::PublicKey> {
+        self.0.public_key("m/84'/0'/0'/20")
+    }
+
+    /// Bob provides an input to be used to create the multisig, its the right size so no change.
+    pub fn contribute_to_multisig(&self) -> OutPoint {
+        // An obviously invalid output, we just use all zeros then use the `vout` to differentiate
+        // Alice's output from Bob's.
+        OutPoint { txid: Txid::all_zeros(), vout: 1 }
+    }
+
+    /// Signs `psbt`.
+    pub fn sign(&self, psbt: Psbt) -> anyhow::Result<Psbt> { self.0.sign_ecdsa(psbt, Self::PATH) }
+
+    /// Alice updates the PSBT, adding her utxo and key source.
+    pub fn update(&self, mut psbt: Psbt) -> anyhow::Result<Psbt> {
+        let input = &mut psbt.inputs[1];
+
+        // The dummy input utxo we are spending and the pubkey/keysource that will be used to sign it.
+        input.witness_utxo = Some(self.input_utxo()?);
+        let (pk, key_source) = self.bip32_derivation()?;
+        input.bip32_derivation.insert(pk, key_source);
+        Ok(psbt)
+    }
+
+    /// Provides the actual UTXO that Alice is contributing, this would usually come from the chain.
+    fn input_utxo(&self) -> anyhow::Result<TxOut> { self.0.input_utxo(Self::PATH) }
+
+    fn bip32_derivation(&self) -> anyhow::Result<(secp256k1::PublicKey, KeySource)> {
+        self.0.bip32_derivation(Self::PATH)
+    }
+}
+
+impl Default for Bob {
+    fn default() -> Self { Self::new() }
+}
+
+/// An entity that can take on one of the PSBT roles.
+pub struct Entity {
+    master: Xpriv,
+}
+
+impl Entity {
+    /// Creates a new entity with random keys.
+    pub fn new(master: Xpriv) -> Self { Self { master } }
+
+    /// Returns the pubkey for this entity at `derivation_path`.
+    fn public_key(&self, derivation_path: &str) -> anyhow::Result<bitcoin::PublicKey> {
+        let path = DerivationPath::from_str(derivation_path)?;
+        let xpriv = self.master.derive_priv(SECP256K1, &path)?;
+        let pk = Xpub::from_priv(SECP256K1, &xpriv);
+        Ok(pk.to_pub())
+    }
+
+    /// Returns a dummy utxo that we can spend.
+    fn input_utxo(&self, derivation_path: &str) -> anyhow::Result<TxOut> {
+        // A dummy script_pubkey representing a UTXO that is locked to a pubkey that Alice controls.
+        let script_pubkey = ScriptBuf::new_p2wpkh(
+            &self.public_key(derivation_path)?.wpubkey_hash().expect("uncompressed key"),
+        );
+        Ok(TxOut { value: DUMMY_UTXO_AMOUNT, script_pubkey })
+    }
+
+    /// Returns the BOP-32 stuff needed to sign an ECDSA input using the [`v2::Psbt`] BIP-32 signing API.
+    fn bip32_derivation(
+        &self,
+        derivation_path: &str,
+    ) -> anyhow::Result<(secp256k1::PublicKey, KeySource)> {
+        let path = DerivationPath::from_str(derivation_path)?;
+        let xpriv = self.master.derive_priv(SECP256K1, &path).expect("failed to derive xpriv");
+        let fingerprint = xpriv.fingerprint(SECP256K1);
+        let sk = xpriv.to_priv();
+        Ok((sk.public_key(SECP256K1).inner, (fingerprint, path)))
+    }
+
+    /// Signs any ECDSA inputs for which we have keys.
+    pub fn sign_ecdsa(&self, psbt: Psbt, derivation_path: &str) -> anyhow::Result<Psbt> {
+        // Usually we'd have to check this was our input and provide the correct key.
+        let path = DerivationPath::from_str(derivation_path)?;
+        let xpriv = self.master.derive_priv(SECP256K1, &path)?;
+
+        let signer = Signer::new(psbt)?;
+        match signer.sign(&xpriv, SECP256K1) {
+            Ok((psbt, _signing_keys)) => Ok(psbt),
+            Err(e) => panic!("signing failed: {:?}", e),
+        }
+    }
+}