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Fix weight estimation for P2TR inputs #880

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Fix weight estimation for P2TR inputs #880

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155 changes: 153 additions & 2 deletions payjoin/src/core/psbt/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@ use std::fmt;
use bitcoin::address::FromScriptError;
use bitcoin::psbt::Psbt;
use bitcoin::transaction::InputWeightPrediction;
use bitcoin::{bip32, psbt, Address, AddressType, Network, TxIn, TxOut, Weight};
use bitcoin::{bip32, psbt, Address, AddressType, Network, TapSighashType, TxIn, TxOut, Weight};

#[derive(Debug, PartialEq)]
pub(crate) enum InconsistentPsbt {
Expand Down Expand Up @@ -230,7 +230,24 @@ impl InternalInputPair<'_> {
.ok_or(InputWeightError::NotSupported)?;
Ok(iwp)
},
P2tr => Ok(InputWeightPrediction::P2TR_KEY_DEFAULT_SIGHASH),
P2tr => {
// Script spends can't be predicted, so we don't support those
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@arminsabouri arminsabouri Jul 15, 2025

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We should be able to use the same strategy as #862 at least for the sender where final_script_witness should be provided for all inputs.

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@arminsabouri arminsabouri Jul 15, 2025

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One strategy is to first look at whats on the witness stack. We have two cases:

  1. You have one element per bip341 we assume its a key spend. If a non-default sighash is used then we need to account for an extra witness byte.
  2. greater than one witness elements. In which case you can do what we did in Derive weight from witness for p2wsh inputs #862.

If witness is not provided then we can't know how much the input will weight. We need to rely on the application to provide this information (#772).

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@shinghim shinghim Jul 19, 2025

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We should be able to use the same strategy as #862 at least for the sender where final_script_witness should be provided for all inputs.

IIUC, this would be for a finalized psbt, and the checks i currently have here would be for unfinalized psbts. Would we want to support both finalized and unfinalized psbts? If so, I should keep the existing checks and add the checks for final_script_witness, right?

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@arminsabouri arminsabouri Jul 21, 2025

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Would we want to support both finalized and unfinalized psbts? If so, I should keep the existing checks and add the checks for final_script_witness, right?

yes, exactly. We want to support finalized and unfinalized txs. For the final_script_witness & input.segwit checks we can probably extract what exists for p2wsh and create a common method.

if !self.psbtin.tap_scripts.is_empty()
|| !self.psbtin.tap_script_sigs.is_empty()
|| self.psbtin.tap_merkle_root.is_some()
{
return Err(InputWeightError::NotSupported);
}

match self.psbtin.tap_key_sig {
None => Err(InputWeightError::NotSupported),
Some(signature) => match signature.sighash_type {
TapSighashType::Default =>
Ok(InputWeightPrediction::P2TR_KEY_DEFAULT_SIGHASH),
_ => Ok(InputWeightPrediction::P2TR_KEY_NON_DEFAULT_SIGHASH),
},
}
}
_ => Err(AddressTypeError::UnknownAddressType.into()),
}?;

Expand Down Expand Up @@ -407,3 +424,137 @@ impl std::error::Error for InputWeightError {
impl From<AddressTypeError> for InputWeightError {
fn from(value: AddressTypeError) -> Self { Self::AddressType(value) }
}

#[cfg(test)]
mod tests {
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@arminsabouri arminsabouri Jul 15, 2025

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The other input pair test live in /src/core/psbt.rs. Any reason these new tests aren't added there?

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@shinghim shinghim Jul 19, 2025

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oh, i didn't see. I just had them here since that's where the method that's being tested is located, but i can move it

use bitcoin::key::Secp256k1;
use bitcoin::taproot::{ControlBlock, LeafVersion};
use bitcoin::{psbt, secp256k1, taproot, PublicKey, ScriptBuf, TapNodeHash, XOnlyPublicKey};

use super::*;
use crate::core::psbt::InternalInputPair;
use crate::receive::InputPair;

/// Lengths of txid, index and sequence: (32, 4, 4)
const TXID_INDEX_SEQUENCE_WEIGHT: Weight = Weight::from_non_witness_data_size(32 + 4 + 4);

#[test]
fn expected_weight_for_p2tr() {
let pubkey_string = "0347ff3dacd07a1f43805ec6808e801505a6e18245178609972a68afbc2777ff2b";
let pubkey = pubkey_string.parse::<PublicKey>().expect("valid pubkey");
let xonly_pubkey = XOnlyPublicKey::from(pubkey.inner);
let p2tr_utxo = TxOut {
value: Default::default(),
script_pubkey: ScriptBuf::new_p2tr(&Secp256k1::new(), xonly_pubkey, None),
};
let default_sighash_pair = InputPair {
txin: Default::default(),
psbtin: psbt::Input {
tap_key_sig: Some(
taproot::Signature::from_slice(
&[0; secp256k1::constants::SCHNORR_SIGNATURE_SIZE],
)
.unwrap(),
),
witness_utxo: Some(p2tr_utxo.clone()),
..Default::default()
},
expected_weight: Weight::from_wu(0),
};
assert_eq!(
InternalInputPair::from(&default_sighash_pair).expected_input_weight().unwrap(),
InputWeightPrediction::P2TR_KEY_DEFAULT_SIGHASH.weight() + TXID_INDEX_SEQUENCE_WEIGHT
);

// Add a sighash byte
let mut sig_bytes = [0; secp256k1::constants::SCHNORR_SIGNATURE_SIZE + 1];
sig_bytes[sig_bytes.len() - 1] = 1;
let non_default_sighash_pair = InputPair {
txin: Default::default(),
psbtin: psbt::Input {
tap_key_sig: Some(taproot::Signature::from_slice(&sig_bytes).unwrap()),
witness_utxo: Some(p2tr_utxo),
..Default::default()
},
expected_weight: Weight::from_wu(0),
};
assert_eq!(
InternalInputPair::from(&non_default_sighash_pair).expected_input_weight().unwrap(),
InputWeightPrediction::P2TR_KEY_NON_DEFAULT_SIGHASH.weight()
+ TXID_INDEX_SEQUENCE_WEIGHT
);
}

#[test]
fn not_supported_p2tr_expected_weights() {
let pubkey_string = "0347ff3dacd07a1f43805ec6808e801505a6e18245178609972a68afbc2777ff2b";
let pubkey = pubkey_string.parse::<PublicKey>().expect("valid pubkey");
let xonly_pubkey = XOnlyPublicKey::from(pubkey.inner);
let p2tr_script = ScriptBuf::new_p2tr(&Secp256k1::new(), xonly_pubkey.clone(), None);
let p2tr_utxo = TxOut { value: Default::default(), script_pubkey: p2tr_script.clone() };

let mut tap_scripts = BTreeMap::new();
let leaf_version: u8 = 0xC0;
let mut control_block_vec = Vec::with_capacity(33);
control_block_vec.push(leaf_version);
control_block_vec.extend_from_slice(&xonly_pubkey.serialize());
let control_block = ControlBlock::decode(control_block_vec.as_slice()).unwrap();
tap_scripts
.insert(control_block.clone(), (p2tr_script.clone(), control_block.leaf_version));

let pair_with_tapscripts = InputPair {
txin: Default::default(),
psbtin: psbt::Input {
tap_scripts,
witness_utxo: Some(p2tr_utxo.clone()),
..Default::default()
},
expected_weight: Weight::from_wu(0),
};
assert_eq!(
InternalInputPair::from(&pair_with_tapscripts).expected_input_weight().err().unwrap(),
InputWeightError::NotSupported
);

let mut tap_script_sigs = BTreeMap::new();
tap_script_sigs.insert(
(xonly_pubkey.clone(), p2tr_script.tapscript_leaf_hash()),
taproot::Signature::from_slice(&[0; secp256k1::constants::SCHNORR_SIGNATURE_SIZE])
.unwrap(),
);
let pair_with_tap_script_sigs = InputPair {
txin: Default::default(),
psbtin: psbt::Input {
tap_script_sigs,
witness_utxo: Some(p2tr_utxo.clone()),
..Default::default()
},
expected_weight: Weight::from_wu(0),
};
assert_eq!(
InternalInputPair::from(&pair_with_tap_script_sigs)
.expected_input_weight()
.err()
.unwrap(),
InputWeightError::NotSupported
);

let tap_merkle_root = TapNodeHash::from_script(&p2tr_script, LeafVersion::TapScript);
let pair_with_tap_merkle_root = InputPair {
txin: Default::default(),
psbtin: psbt::Input {
tap_merkle_root: Some(tap_merkle_root),
witness_utxo: Some(p2tr_utxo.clone()),
..Default::default()
},
expected_weight: Weight::from_wu(0),
};
assert_eq!(
InternalInputPair::from(&pair_with_tap_merkle_root)
.expected_input_weight()
.err()
.unwrap(),
InputWeightError::NotSupported
);
}
}
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