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pset: use the ECDH pubkey as the nonce of outputs not marked for blinding #306

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pset: use the ECDH pubkey as the nonce of outputs not marked for blinding #306

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02cce61
pset: use the ECDH pubkey as the nonce of outputs not marked for blin…
KyrylR Nov 3, 2026
db70ff5
pset: test the nonce of Output::to_txout
KyrylR Nov 3, 2026
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55 changes: 43 additions & 12 deletions src/pset/map/output.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,9 +23,9 @@ use crate::pset::map::Map;
use crate::pset::raw;
use crate::pset::Error;
use crate::{confidential, pset};
use crate::{encode, RangeProof, Script, TxOutWitness, SurjectionProof};
use crate::{encode, RangeProof, Script, SurjectionProof, TxOutWitness};
use bitcoin::bip32::KeySource;
use bitcoin::{PublicKey, key::XOnlyPublicKey};
use bitcoin::{key::XOnlyPublicKey, PublicKey};
use secp256k1_zkp::{self, Generator};

use crate::issuance;
Expand Down Expand Up @@ -247,14 +247,11 @@ impl Output {
(None, Some(x)) => confidential::Value::Explicit(x),
(None, None) => confidential::Value::Null,
},
nonce: if self.is_partially_blinded() {
self.ecdh_pubkey
.map(|pk| confidential::Nonce::from(pk.inner))
} else {
self.blinding_key
.map(|pk| confidential::Nonce::from(pk.inner))
}
.unwrap_or_default(),
nonce: self
.ecdh_pubkey
.or(self.blinding_key)
.map(|pk| confidential::Nonce::from(pk.inner))
.unwrap_or_default(),
Comment on lines -250 to +254

@KyrylR KyrylR Nov 4, 2026

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I am not sure why previous code had this if statement

After I added one more condition it started looking too complicated, so I decided to simplify it and it turned out that simple rule "use the ECDH pubkey if there is one, otherwise the receiver's blinding key" can be applied here

@KyrylR KyrylR Nov 4, 2026

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the ECDH pubkey becomes the nonce whenever it's set in elements core

PartiallySignedTransaction::GetUnsignedTx (
https://github.com/ElementsProject/elements/blob/2ac51cac13876bf14d3d709b07535b1868eee896/src/psbt.cpp#L183-L185) does:

if (output.m_ecdh_pubkey.IsValid() && !force_unblinded) {
    txout.nNonce.vchCommitment.insert(..., output.m_ecdh_pubkey.begin(), output.m_ecdh_pubkey.end());
}

script_pubkey: self.script_pubkey.clone(),
witness: TxOutWitness {
surjection_proof: self.asset_surjection_proof
Expand Down Expand Up @@ -587,8 +584,11 @@ impl Decodable for Output {

#[cfg(test)]
mod tests {
use crate::pset::{raw, Map, PartiallySignedTransaction};
use crate::{encode, Transaction};
use std::str::FromStr;

use crate::confidential::Nonce;
use crate::pset::{raw, Map, Output, PartiallySignedTransaction};
use crate::{encode, AssetId, Script, Transaction};

// Regression test from Claude that the next two commits do not cause
// `Pset::from_tx` to insert 0-length proofs when given unblinded outputs.
Expand Down Expand Up @@ -639,4 +639,35 @@ mod tests {
);
}
}

#[test]
fn to_txout_nonce() {
let blinding_key = bitcoin::PublicKey::from_str(
"0205d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470",
)
.unwrap();
let ecdh_pubkey = bitcoin::PublicKey::from_str(
"0212bf0ea45b733dfde8ecb5e896306c4165c666c99fc5d1ab887f71393a975cea",
)
.unwrap();
let asset = AssetId::from_byte_array([1; 32]);

let explicit = Output::new_explicit(Script::new(), 1000, asset, None);
assert_eq!(explicit.to_txout().nonce, Nonce::Null);

let marked = Output::new_explicit(Script::new(), 1000, asset, Some(blinding_key));
assert_eq!(marked.to_txout().nonce, Nonce::from(blinding_key.inner));

let blinded = Output {
ecdh_pubkey: Some(ecdh_pubkey),
..marked.clone()
};
assert_eq!(blinded.to_txout().nonce, Nonce::from(ecdh_pubkey.inner));

let unmarked = Output {
blinding_key: None,
..blinded
};
assert_eq!(unmarked.to_txout().nonce, Nonce::from(ecdh_pubkey.inner));
}
}
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