Merge rust-bitcoin/rust-bitcoin#4623: Improve `Xpriv::derive_xpriv` and `Xpub::derive_xpub` ergonomics slightly

352712257e psbt: Use new `derive_xpriv` flexibility in GetKey (Daniel Roberts)
bd3f4b6bf1 psbt: Add test for GetKey bip32 (Daniel Roberts)
c7bdec14fb Fix clippy lint and formatting for `Xpriv::derive_xpriv` and `Xpriv::derive_xpub` calls (Daniel Roberts)
9d4381c8fe Improve `Xpriv::derive_xpriv` and `Xpub::derive_xpub` ergonomics (Daniel Roberts)

Pull request description:

  This enables a couple more things to be passed to the bip32 derive methods, with (afaict) no downside (all existing call sites remain valid and work as before).
  
  Given
  
  ```
  let secp = Secp256k1::new();
  let path: DerivationPath = "42'/350'/0".parse().unwrap();
  let xpriv: Xpriv = "xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi".parse().unwrap();
  ```
  
  The following *new* ways to call derive are enabled:
  
  ```
  /// Derive using only part of the path
  xpriv.derive_xpriv(&secp, &path[1..])
  ```
  
  ```
  /// Derive moving into the method
  xpriv.derive_xpriv(&secp, path)
  ```
  
  The second case is probably of questionable usefulness, but I've ended up writing something like
  ```
  let path: DerivationPath = path.into_iter().skip(n).cloned().collect()
  ```
  enough that the first case would be welcome, and can be done with a minimal change.
  
  I'm reasonably confident this doesn't break anything (and it indeed doesn't break any tests that I can see) but this is definitely on the edge of my comfort with the rust type system, which is why I've created this as a draft.


ACKs for top commit:
  tcharding:
    ACK 352712257e
  apoelstra:
    ACK 352712257e66c7959b80a9f6375961295a292306; successfully ran local tests; nice!


Tree-SHA512: 0babdd9589d3312d4df54d77202a71dbb021e216167514c1a87f1f992f254f2b298b874411f8c7a3d56d7f730b22d104c3a78f9145715ea8df6bedad35c1ffd5

bitcoin/examples/bip32.rs

@@ -46,7 +46,7 @@ fn main() {
     // generate first receiving address at m/0/0
     // manually creating indexes this time
     let zero = ChildNumber::ZERO_NORMAL;
-    let public_key = xpub.derive_xpub(&secp, &[zero, zero]).unwrap().public_key;
+    let public_key = xpub.derive_xpub(&secp, [zero, zero]).unwrap().public_key;
     let address = Address::p2wpkh(CompressedPublicKey(public_key), KnownHrp::Mainnet);
     println!("First receiving address: {address}");
 }

bitcoin/examples/ecdsa-psbt-simple.rs

@@ -67,7 +67,7 @@ fn get_external_address_xpriv<C: Signing>(
     let external_index = ChildNumber::ZERO_NORMAL;
     let idx = ChildNumber::from_normal_idx(index).expect("valid index number");
 
-    child_xpriv.derive_xpriv(secp, &[external_index, idx]).expect("only deriving two more steps")
+    child_xpriv.derive_xpriv(secp, [external_index, idx]).expect("only deriving two more steps")
 }
 
 // Derive the internal address xpriv.
@@ -83,7 +83,7 @@ fn get_internal_address_xpriv<C: Signing>(
     let internal_index = ChildNumber::ONE_NORMAL;
     let idx = ChildNumber::from_normal_idx(index).expect("valid index number");
 
-    child_xpriv.derive_xpriv(secp, &[internal_index, idx]).expect("only deriving two more steps")
+    child_xpriv.derive_xpriv(secp, [internal_index, idx]).expect("only deriving two more steps")
 }
 
 // The address to send to.

bitcoin/examples/ecdsa-psbt.rs

@@ -260,7 +260,7 @@ impl WatchOnly {
         secp: &Secp256k1<C>,
     ) -> Result<(CompressedPublicKey, Address, DerivationPath)> {
         let path = [ChildNumber::ONE_NORMAL, ChildNumber::ZERO_NORMAL];
-        let derived = self.account_0_xpub.derive_xpub(secp, &path)?;
+        let derived = self.account_0_xpub.derive_xpub(secp, path)?;
 
         let pk = derived.to_public_key();
         let addr = Address::p2wpkh(pk, NETWORK);

bitcoin/examples/taproot-psbt-simple.rs

@@ -65,7 +65,7 @@ fn get_external_address_xpriv<C: Signing>(
     let external_index = ChildNumber::ZERO_NORMAL;
     let idx = ChildNumber::from_normal_idx(index).expect("valid index number");
 
-    child_xpriv.derive_xpriv(secp, &[external_index, idx]).expect("only deriving two more steps")
+    child_xpriv.derive_xpriv(secp, [external_index, idx]).expect("only deriving two more steps")
 }
 
 // Derive the internal address xpriv.
@@ -81,7 +81,7 @@ fn get_internal_address_xpriv<C: Signing>(
     let internal_index = ChildNumber::ONE_NORMAL;
     let idx = ChildNumber::from_normal_idx(index).expect("valid index number");
 
-    child_xpriv.derive_xpriv(secp, &[internal_index, idx]).expect("only deriving two more steps")
+    child_xpriv.derive_xpriv(secp, [internal_index, idx]).expect("only deriving two more steps")
 }
 
 // Get the Taproot Key Origin.

bitcoin/examples/taproot-psbt.rs

@@ -298,7 +298,7 @@ fn generate_bip86_key_spend_tx(
                 .ok_or("missing Taproot key origin")?;
 
             let secret_key =
-                master_xpriv.derive_xpriv(secp, &derivation_path)?.to_private_key().inner;
+                master_xpriv.derive_xpriv(secp, derivation_path)?.to_private_key().inner;
             sign_psbt_taproot(
                 secret_key,
                 input.tap_internal_key.unwrap(),
@@ -540,7 +540,7 @@ impl BenefactorWallet {
                     .ok_or("missing Taproot key origin")?;
                 let secret_key = self
                     .master_xpriv
-                    .derive_xpriv(&self.secp, &derivation_path)
+                    .derive_xpriv(&self.secp, derivation_path)
                     .expect("derivation path is short")
                     .to_private_key()
                     .inner;
@@ -664,11 +664,8 @@ impl BeneficiaryWallet {
         for (x_only_pubkey, (leaf_hashes, (_, derivation_path))) in
             &psbt.inputs[0].tap_key_origins.clone()
         {
-            let secret_key = self
+            let secret_key =
-                .master_xpriv
+                self.master_xpriv.derive_xpriv(&self.secp, derivation_path)?.to_private_key().inner;
-                .derive_xpriv(&self.secp, &derivation_path)?
-                .to_private_key()
-                .inner;
             for lh in leaf_hashes {
                 let sighash_type = TapSighashType::All;
                 let hash = SighashCache::new(&unsigned_tx).taproot_script_spend_signature_hash(

bitcoin/src/bip32.rs

@@ -756,7 +756,7 @@ impl Xpriv {
     pub fn derive_priv<C: secp256k1::Signing, P: AsRef<[ChildNumber]>>(
         &self,
         secp: &Secp256k1<C>,
-        path: &P,
+        path: P,
     ) -> Result<Xpriv, DerivationError> {
         self.derive_xpriv(secp, path)
     }
@@ -767,7 +767,7 @@ impl Xpriv {
     pub fn derive_xpriv<C: secp256k1::Signing, P: AsRef<[ChildNumber]>>(
         &self,
         secp: &Secp256k1<C>,
-        path: &P,
+        path: P,
     ) -> Result<Xpriv, DerivationError> {
         let mut sk: Xpriv = *self;
         for cnum in path.as_ref() {
@@ -910,7 +910,7 @@ impl Xpub {
     pub fn derive_pub<C: secp256k1::Verification, P: AsRef<[ChildNumber]>>(
         &self,
         secp: &Secp256k1<C>,
-        path: &P,
+        path: P,
     ) -> Result<Xpub, DerivationError> {
         self.derive_xpub(secp, path)
     }
@@ -921,7 +921,7 @@ impl Xpub {
     pub fn derive_xpub<C: secp256k1::Verification, P: AsRef<[ChildNumber]>>(
         &self,
         secp: &Secp256k1<C>,
-        path: &P,
+        path: P,
     ) -> Result<Xpub, DerivationError> {
         let mut pk: Xpub = *self;
         for cnum in path.as_ref() {

bitcoin/src/psbt/mod.rs

@@ -21,7 +21,7 @@ use std::collections::{HashMap, HashSet};
 use internals::write_err;
 use secp256k1::{Keypair, Message, Secp256k1, Signing, Verification};
 
-use crate::bip32::{self, DerivationPath, KeySource, Xpriv, Xpub};
+use crate::bip32::{self, KeySource, Xpriv, Xpub};
 use crate::crypto::key::{PrivateKey, PublicKey};
 use crate::crypto::{ecdsa, taproot};
 use crate::key::{TapTweak, XOnlyPublicKey};
@@ -814,14 +814,13 @@ impl GetKey for Xpriv {
             KeyRequest::XOnlyPubkey(_) => Err(GetKeyError::NotSupported),
             KeyRequest::Bip32((fingerprint, path)) => {
                 let key = if self.fingerprint(secp) == *fingerprint {
-                    let k = self.derive_xpriv(secp, &path).map_err(GetKeyError::Bip32)?;
+                    let k = self.derive_xpriv(secp, path).map_err(GetKeyError::Bip32)?;
                     Some(k.to_private_key())
                 } else if self.parent_fingerprint == *fingerprint
                     && !path.is_empty()
                     && path[0] == self.child_number
                 {
-                    let path = DerivationPath::from_iter(path.into_iter().skip(1).copied());
+                    let k = self.derive_xpriv(secp, &path[1..]).map_err(GetKeyError::Bip32)?;
-                    let k = self.derive_xpriv(secp, &path).map_err(GetKeyError::Bip32)?;
                     Some(k.to_private_key())
                 } else {
                     None
@@ -1330,7 +1329,7 @@ mod tests {
     #[cfg(feature = "rand-std")]
     use {
         crate::address::script_pubkey::ScriptBufExt as _,
-        crate::bip32::{DerivationPath, Fingerprint},
+        crate::bip32::Fingerprint,
         crate::locktime,
         crate::witness_version::WitnessVersion,
         crate::WitnessProgram,
@@ -1339,7 +1338,7 @@ mod tests {
 
     use super::*;
     use crate::address::script_pubkey::ScriptExt as _;
-    use crate::bip32::ChildNumber;
+    use crate::bip32::{ChildNumber, DerivationPath};
     use crate::locktime::absolute;
     use crate::network::NetworkKind;
     use crate::psbt::serialize::{Deserialize, Serialize};
@@ -2397,6 +2396,27 @@ mod tests {
         );
     }
 
+    #[test]
+    fn get_key_xpriv_bip32_parent() {
+        let secp = Secp256k1::new();
+
+        let seed = hex!("000102030405060708090a0b0c0d0e0f");
+        let parent_xpriv: Xpriv = Xpriv::new_master(NetworkKind::Main, &seed);
+        let path: DerivationPath = "m/1/2/3".parse().unwrap();
+        let path_prefix: DerivationPath = "m/1".parse().unwrap();
+
+        let expected_private_key =
+            parent_xpriv.derive_xpriv(&secp, &path).unwrap().to_private_key();
+
+        let derived_xpriv = parent_xpriv.derive_xpriv(&secp, &path_prefix).unwrap();
+
+        let derived_key = derived_xpriv
+            .get_key(&KeyRequest::Bip32((parent_xpriv.fingerprint(&secp), path.clone())), &secp)
+            .unwrap();
+
+        assert_eq!(derived_key, Some(expected_private_key));
+    }
+
     #[test]
     fn fee() {
         let output_0_val = Amount::from_sat_u32(99_999_699);