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descriptors: add a method to get a PSBT's change outputs #922

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Jan 24, 2024
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descriptors: add a method to get a PSBT's change outputs #922

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descriptors: unit test we do take into account derivation steps befor…
darosior Jan 15, 2024
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descriptors: add a method to get a PSBT's change outputs
darosior Jan 15, 2024
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137 changes: 137 additions & 0 deletions src/descriptors/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -116,6 +116,25 @@ impl PartialEq<descriptor::Descriptor<descriptor::DescriptorPublicKey>> for Sing
}
}

/// The index of a change output in a transaction's outputs list, differentiating between a change
/// output which uses a change address and one which uses a deposit address.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum ChangeOutput {
ChangeAddress { index: usize },
DepositAddress { index: usize },
}

impl ChangeOutput {
/// Get the index of the change output in the transaction's list of outputs regardless of its
/// type.
pub fn index(&self) -> usize {
match self {
Self::ChangeAddress { index } => *index,
Self::DepositAddress { index } => *index,
}
}
}

impl LianaDescriptor {
pub fn new(spending_policy: LianaPolicy) -> LianaDescriptor {
// Get the descriptor from the chosen spending policy.
Expand Down Expand Up @@ -299,6 +318,57 @@ impl LianaDescriptor {
Ok(spend_info)
}

/// List the indexes of the change outputs in this PSBT. It relies on the PSBT to be
/// well-formed: sane BIP32 derivations must be set for every change output, the inner
/// transaction must have the same number of outputs as the PSBT.
/// Will detect change outputs paying to either the change keychain or the deposit one.
pub fn change_indexes(
&self,
psbt: &Psbt,
secp: &secp256k1::Secp256k1<impl secp256k1::Verification>,
) -> Vec<ChangeOutput> {
let mut indexes = Vec::new();

// We iterate through all the BIP32 derivations of each output, but note we only ever set
// the BIP32 derivations for PSBT outputs which pay to ourselves.
for (index, psbt_out) in psbt.outputs.iter().enumerate() {
// We can only ever detect change on well-formed PSBTs. On such PSBTs, all keys in the
// BIP32 derivations belong to us. And they all use the same last derivation index,
// since that's where the wildcard is in the descriptor. So just pick the first one and
// infer the derivation index to use to derive the spks below from it.
let der_index = if let Some(i) = psbt_out
.bip32_derivation
.values()
.next()
.and_then(|(_, der_path)| der_path.into_iter().last())
{
i
} else {
continue;
};

// If any of the change and deposit addresses at this derivation index match, count it
// as a change output.
if let Some(txo) = psbt.unsigned_tx.output.get(index) {
let change_desc = self.change_desc.derive(*der_index, secp);
if change_desc.script_pubkey() == txo.script_pubkey {
indexes.push(ChangeOutput::ChangeAddress { index });
continue;
}
let receive_desc = self.receive_desc.derive(*der_index, secp);
if receive_desc.script_pubkey() == txo.script_pubkey {
indexes.push(ChangeOutput::DepositAddress { index });
}
} else {
log::error!(
"Provided a PSBT with non-matching tx outputs count and PSBT outputs count."
);
}
}

indexes
}

/// Prune the BIP32 derivations in all the PSBT inputs for all the spending paths but the given
/// one.
pub fn prune_bip32_derivs(&self, mut psbt: Psbt, spending_path: &PathInfo) -> Psbt {
Expand Down Expand Up @@ -540,6 +610,17 @@ mod tests {
.unwrap();
assert_eq!(LianaDescriptor::new(policy).to_string(), "wsh(or_d(multi(3,[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/0/<0;1>/*,[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/0/<0;1>/*,[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/0/<0;1>/*),and_v(v:thresh(2,pkh([aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/1/<0;1>/*),a:pkh([aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/1/<0;1>/*),a:pkh([aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/1/<0;1>/*)),older(26352))))#prj7nktq");

// Another derivation step before the wildcard is taken into account.
// desc_b is the very same descriptor as desc_a, except the very first xpub's derivation
// path is `/<0;1>/*` instead of `/0/<0;1>/*`.
let secp = secp256k1::Secp256k1::verification_only();
let desc_a = LianaDescriptor::from_str("wsh(or_d(multi(3,[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/0/<0;1>/*,[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/0/<0;1>/*,[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/0/<0;1>/*),and_v(v:thresh(2,pkh([aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/1/<0;1>/*),a:pkh([aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/1/<0;1>/*),a:pkh([aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/1/<0;1>/*)),older(26352))))").unwrap();
let desc_b = LianaDescriptor::from_str("wsh(or_d(multi(3,[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/<0;1>/*,[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/0/<0;1>/*,[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/0/<0;1>/*),and_v(v:thresh(2,pkh([aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/1/<0;1>/*),a:pkh([aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/1/<0;1>/*),a:pkh([aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/1/<0;1>/*)),older(26352))))").unwrap();
let a = desc_a.receive_descriptor().derive(0.into(), &secp);
let b = desc_b.receive_descriptor().derive(0.into(), &secp);
assert_ne!(a, b);
assert_ne!(a.script_pubkey(), b.script_pubkey());

// The same pseudo-realistic situation as above, but instead of using another derivation
// depth to derive from the same xpub, we reuse the multipath step.
// Note: this is required by Ledger and the wallet policies BIP
Expand Down Expand Up @@ -1268,5 +1349,61 @@ mod tests {
assert_eq!(psbt, pruned_psbt);
}

#[test]
fn change_detection() {
let secp = secp256k1::Secp256k1::verification_only();

// Reuse a desc from above desciptor_creation unit test and a psbt from unrelated above
// bip32_derivs_pruning unit test.
let desc = LianaDescriptor::from_str("wsh(or_d(multi(3,[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/0/<0;1>/*,[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/0/<0;1>/*,[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/0/<0;1>/*),and_v(v:thresh(2,pkh([aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/1/<0;1>/*),a:pkh([aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/1/<0;1>/*),a:pkh([aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/1/<0;1>/*)),older(26352))))").unwrap();
let mut psbt = Psbt::from_str("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").unwrap();

// The PSBT has unrelated outputs. Those aren't detected as change.
assert!(!psbt.outputs.is_empty());
assert_eq!(desc.change_indexes(&psbt, &secp).len(), 0);

// Add a change output, it's correctly detected as such.
let der_desc = desc.change_descriptor().derive(999.into(), &secp);
let txo = bitcoin::TxOut {
script_pubkey: der_desc.script_pubkey(),
value: bitcoin::Amount::MAX_MONEY,
};
let psbt_out = bitcoin::psbt::Output {
bip32_derivation: der_desc.bip32_derivations(),
..Default::default()
};
psbt.unsigned_tx.output.push(txo);
psbt.outputs.push(psbt_out);
let indexes = desc.change_indexes(&psbt, &secp);
assert_eq!(indexes.len(), 1);
assert!(matches!(
indexes[0],
ChangeOutput::ChangeAddress { index: 1 }
));

// Add another change output, but to a deposit address. Both change outputs are detected.
let der_desc = desc.receive_descriptor().derive(424242.into(), &secp);
let txo = bitcoin::TxOut {
script_pubkey: der_desc.script_pubkey(),
value: bitcoin::Amount::MAX_MONEY,
};
let psbt_out = bitcoin::psbt::Output {
bip32_derivation: der_desc.bip32_derivations(),
..Default::default()
};
psbt.unsigned_tx.output.push(txo);
psbt.outputs.push(psbt_out);
let indexes = desc.change_indexes(&psbt, &secp);
assert_eq!(indexes.len(), 2);
assert!(matches!(
indexes[0],
ChangeOutput::ChangeAddress { index: 1 }
));
assert!(matches!(
indexes[1],
ChangeOutput::DepositAddress { index: 2 }
));
}

// TODO: test error conditions of deserialization.
}
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