Merge rust-bitcoin#4440: Add support for de/serializing PSBT_{IN,OUT}_MUSIG2_PARTICIPANT_PUBKEYS

2481695 Add tests for BIP-373 PSBT_{IN,OUT}_MUSIG2_PARTICIPANT_PUBKEYS serialization and deserialization (Daniel Roberts)
3e8e6d9 Add BIP-373 PSBT_{IN,OUT}_MUSIG2_PARTICIPANT_PUBKEYS serialization and deserialization (Daniel Roberts)

Pull request description:

  This change adds support for serializing and deserializing two PSBT keys from BIP-373: `PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS` and `PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS`

  This is a part of rust-bitcoin#4207 that can be implemented independently of the rest (which depends on rust-bitcoin/rust-secp256k1#716). I believe this satisfactorily avoids changing things multiple times on end users, *however* it's not *completely* transparent to end users, since any code that currently accesses these fields through `unknown` will need to be updated. Later, when `PSBT_IN_MUSIG2_PUB_NONCE` and `PSBT_IN_MUSIG2_PARTIAL_SIG` are supported, code will need to be updated a second time to retrieve them from the correct place instead of `unknown`. I'm of the opinion that this imposes a very minor maintenance burden, only consisting of *removing* deserialization code.

  ### Notes/Requests for feedback

  - For the most part I used my judgement rather than `cargo fmt` since `cargo fmt` already had a lot of other complaints, but of course I'll update if I need to.
  - To satisfy the requirement that every commit pass tests, the commit updating the psbt serde regression test should probably be squashed into the first commit, but I just wanted to confirm that before I did it. I suppose similarly, the test commit could be squashed as well?
  - I waffled between `musig2_participants` and `musig2_participant_pubkeys`, but I've decided to go with `musig2_participant_pubkeys` because that is consistent with Bitcoin Core

ACKs for top commit:
  tcharding:
    ACK 2481695
  apoelstra:
    ACK 2481695; successfully ran local tests

Tree-SHA512: af884923593c9cbb24ff3f1f08219458538592fabde85d5d65bc2d9bc7bf0b1a73dac38d2c56303b4f3162088db129ea7e879c3d4b324e965933c121ef939a07

Author: apoelstra

bitcoin/src/psbt/map/input.rs

@@ -59,6 +59,8 @@ const PSBT_IN_TAP_BIP32_DERIVATION: u64 = 0x16;
 const PSBT_IN_TAP_INTERNAL_KEY: u64 = 0x17;
 /// Type: Taproot Merkle Root PSBT_IN_TAP_MERKLE_ROOT = 0x18
 const PSBT_IN_TAP_MERKLE_ROOT: u64 = 0x18;
+/// Type: MuSig2 Public Keys Participating in Aggregate Input PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS = 0x1a
+const PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS: u64 = 0x1a;
 /// Type: Proprietary Use Type PSBT_IN_PROPRIETARY = 0xFC
 const PSBT_IN_PROPRIETARY: u64 = 0xFC;
 
@@ -113,6 +115,8 @@ pub struct Input {
     pub tap_internal_key: Option<XOnlyPublicKey>,
     /// Taproot Merkle root.
     pub tap_merkle_root: Option<TapNodeHash>,
+    /// Mapping from MuSig2 aggregate keys to the participant keys from which they were aggregated.
+    pub musig2_participant_pubkeys: BTreeMap<secp256k1::PublicKey, Vec<secp256k1::PublicKey>>,
     /// Proprietary key-value pairs for this input.
     pub proprietary: BTreeMap<raw::ProprietaryKey, Vec<u8>>,
     /// Unknown key-value pairs for this input.
@@ -352,6 +356,11 @@ impl Input {
                     self.tap_merkle_root <= <raw_key: _>|< raw_value: TapNodeHash>
                 }
             }
+            PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS => {
+                impl_psbt_insert_pair! {
+                    self.musig2_participant_pubkeys <= <raw_key: secp256k1::PublicKey>|< raw_value: Vec<secp256k1::PublicKey> >
+                }
+            }
             PSBT_IN_PROPRIETARY => {
                 let key = raw::ProprietaryKey::try_from(raw_key.clone())?;
                 match self.proprietary.entry(key) {
@@ -390,6 +399,7 @@ impl Input {
         self.tap_script_sigs.extend(other.tap_script_sigs);
         self.tap_scripts.extend(other.tap_scripts);
         self.tap_key_origins.extend(other.tap_key_origins);
+        self.musig2_participant_pubkeys.extend(other.musig2_participant_pubkeys);
         self.proprietary.extend(other.proprietary);
         self.unknown.extend(other.unknown);
 
@@ -482,6 +492,11 @@ impl Map for Input {
         impl_psbt_get_pair! {
             rv.push(self.tap_merkle_root, PSBT_IN_TAP_MERKLE_ROOT)
         }
+
+        impl_psbt_get_pair! {
+            rv.push_map(self.musig2_participant_pubkeys, PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS)
+        }
+
         for (key, value) in self.proprietary.iter() {
             rv.push(raw::Pair { key: key.to_key(), value: value.clone() });
         }

bitcoin/src/psbt/map/output.rs

@@ -20,6 +20,8 @@ const PSBT_OUT_TAP_INTERNAL_KEY: u64 = 0x05;
 const PSBT_OUT_TAP_TREE: u64 = 0x06;
 /// Type: Taproot Key BIP 32 Derivation Path PSBT_OUT_TAP_BIP32_DERIVATION = 0x07
 const PSBT_OUT_TAP_BIP32_DERIVATION: u64 = 0x07;
+/// Type: MuSig2 Public Keys Participating in Aggregate Output PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS = 0x08
+const PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS: u64 = 0x08;
 /// Type: Proprietary Use Type PSBT_IN_PROPRIETARY = 0xFC
 const PSBT_OUT_PROPRIETARY: u64 = 0xFC;
 
@@ -40,6 +42,8 @@ pub struct Output {
     pub tap_tree: Option<TapTree>,
     /// Map of tap root x only keys to origin info and leaf hashes contained in it.
     pub tap_key_origins: BTreeMap<XOnlyPublicKey, (Vec<TapLeafHash>, KeySource)>,
+    /// Mapping from MuSig2 aggregate keys to the participant keys from which they were aggregated.
+    pub musig2_participant_pubkeys: BTreeMap<secp256k1::PublicKey, Vec<secp256k1::PublicKey>>,
     /// Proprietary key-value pairs for this output.
     pub proprietary: BTreeMap<raw::ProprietaryKey, Vec<u8>>,
     /// Unknown key-value pairs for this output.
@@ -90,6 +94,11 @@ impl Output {
                     self.tap_key_origins <= <raw_key: XOnlyPublicKey>|< raw_value: (Vec<TapLeafHash>, KeySource)>
                 }
             }
+            PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS => {
+                impl_psbt_insert_pair! {
+                    self.musig2_participant_pubkeys <= <raw_key: secp256k1::PublicKey>|< raw_value: Vec<secp256k1::PublicKey> >
+                }
+            }
             _ => match self.unknown.entry(raw_key) {
                 btree_map::Entry::Vacant(empty_key) => {
                     empty_key.insert(raw_value);
@@ -107,6 +116,7 @@ impl Output {
         self.proprietary.extend(other.proprietary);
         self.unknown.extend(other.unknown);
         self.tap_key_origins.extend(other.tap_key_origins);
+        self.musig2_participant_pubkeys.extend(other.musig2_participant_pubkeys);
 
         combine!(redeem_script, self, other);
         combine!(witness_script, self, other);
@@ -143,6 +153,10 @@ impl Map for Output {
             rv.push_map(self.tap_key_origins, PSBT_OUT_TAP_BIP32_DERIVATION)
         }
 
+        impl_psbt_get_pair! {
+            rv.push_map(self.musig2_participant_pubkeys, PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS)
+        }
+
         for (key, value) in self.proprietary.iter() {
             rv.push(raw::Pair { key: key.to_key(), value: value.clone() });
         }

bitcoin/src/psbt/mod.rs

@@ -1339,6 +1339,8 @@ mod tests {
         secp256k1::{All, SecretKey},
     };
 
+    use std::str::FromStr;
+
     use super::*;
     use crate::address::script_pubkey::ScriptExt as _;
     use crate::bip32::ChildNumber;
@@ -2233,6 +2235,49 @@ mod tests {
         assert!(!rtt.proprietary.is_empty());
     }
 
+    // Deserialize MuSig2 PSBT participant keys according to BIP-373
+    #[test]
+    fn serialize_and_deserialize_musig2_participants() {
+        // XXX: Does not cover PSBT_IN_MUSIG2_PUB_NONCE, PSBT_IN_MUSIG2_PARTIAL_SIG (yet)
+
+        let expected_in_agg_pk  = secp256k1::PublicKey::from_str("021401301810a46a4e3f39e4603ec228ed301d9f2079767fda758dee7224b32e00").unwrap();
+        let expected_in_pubkeys = vec![
+            secp256k1::PublicKey::from_str("02bebd7a1cef20283444b96e9ce78137e951ce48705390933896311a9abc75736a").unwrap(),
+            secp256k1::PublicKey::from_str("0355212dff7b3d7e8126687a62fd0435a3fb4de56d9af9ae23a1c9ca05b349c8e2").unwrap(),
+        ];
+
+        let expected_out_agg_pk = secp256k1::PublicKey::from_str("0364934a64831bd917a2667b886671650846f021e1c025e4b2bb65e49ab3e7cba5").unwrap();
+
+        let expected_out_pubkeys = vec![
+            secp256k1::PublicKey::from_str("02841d69a8b80ae23a8090e6f3765540ea5efd8c287b1307c983a6e2a3a171b525").unwrap(),
+            secp256k1::PublicKey::from_str("02bad833849a98cdfb0a0749609ddccab16ad54485ecc67f828df4bdc4f2b90d4c").unwrap(),
+        ];
+
+        const PSBT_HEX: &str = "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";
+
+        let psbt = hex_psbt(PSBT_HEX).unwrap();
+
+        assert_eq!(psbt.inputs[0].musig2_participant_pubkeys.len(), 1);
+        assert_eq!(
+            psbt.inputs[0].musig2_participant_pubkeys.iter().next().unwrap(),
+            (&expected_in_agg_pk, &expected_in_pubkeys)
+        );
+
+        assert_eq!(psbt.outputs[0].musig2_participant_pubkeys.len(), 1);
+        assert_eq!(
+            psbt.outputs[0].musig2_participant_pubkeys.iter().next().unwrap(),
+            (&expected_out_agg_pk, &expected_out_pubkeys)
+        );
+
+        // Check round trip de/serialization
+        assert_eq!(psbt.serialize_hex(), PSBT_HEX);
+
+        const PSBT_TRUNCATED_MUSIG_PARTICIPANTS_HEX: &str = "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";
+
+        hex_psbt(PSBT_TRUNCATED_MUSIG_PARTICIPANTS_HEX)
+            .expect_err("Deserializing PSBT with truncated musig participants should error");
+    }
+
     // PSBTs taken from BIP 174 test vectors.
     #[test]
     fn combine_psbts() {

bitcoin/src/psbt/serialize.rs

@@ -172,6 +172,28 @@ impl Deserialize for secp256k1::PublicKey {
     }
 }
 
+impl Serialize for Vec<secp256k1::PublicKey> {
+    fn serialize(&self) -> Vec<u8> {
+        let mut result: Vec<u8> = Vec::with_capacity(secp256k1::constants::PUBLIC_KEY_SIZE * self.len());
+
+        for pubkey in self.iter() {
+            result.extend(Serialize::serialize(pubkey));
+        }
+
+        result
+    }
+}
+
+impl Deserialize for Vec<secp256k1::PublicKey> {
+    fn deserialize(bytes: &[u8]) -> Result<Self, Error> {
+        bytes.chunks(secp256k1::constants::PUBLIC_KEY_SIZE)
+            .map(|pubkey_bytes| {
+                secp256k1::PublicKey::deserialize(pubkey_bytes)
+            })
+            .collect()
+    }
+}
+
 impl Serialize for ecdsa::Signature {
     fn serialize(&self) -> Vec<u8> { self.to_vec() }
 }