pczt: Support applying external spendAuthSigs to Spends

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to Rust's notion of
 
 ## [Unreleased]
 
+### Added
+- `orchard::pczt::Action::apply_signature`
+
+### Changed
+- `orchard::pczt::SignerError` has added variants:
+  - `InvalidExternalSignature`
+
 ## [0.11.0] - 2025-02-20
 
 ### Added

src/pczt.rs

@@ -480,6 +480,10 @@ mod tests {
         for action in pczt_bundle.actions_mut() {
             if action.spend.zip32_derivation.as_ref() == Some(&zip32_derivation) {
                 action.sign(sighash, &ask, rng).unwrap();
+
+                // We can also apply the signature as an external signature.
+                let signature = action.spend().spend_auth_sig().clone().expect("signed");
+                action.apply_signature(sighash, signature).unwrap();
             }
         }
 

src/pczt/signer.rs

@@ -1,6 +1,9 @@
 use rand::{CryptoRng, RngCore};
 
-use crate::{keys::SpendAuthorizingKey, primitives::redpallas};
+use crate::{
+    keys::SpendAuthorizingKey,
+    primitives::redpallas::{self, SpendAuth},
+};
 
 impl super::Action {
     /// Signs the Orchard spend with the given spend authorizing key.
@@ -29,11 +32,31 @@ impl super::Action {
             Err(SignerError::WrongSpendAuthorizingKey)
         }
     }
+
+    /// Applies the given signature to the Orchard spend, if valid.
+    ///
+    /// It is the caller's responsibility to perform any semantic validity checks on the
+    /// PCZT (for example, comfirming that the change amounts are correct) before calling
+    /// this method.
+    pub fn apply_signature(
+        &mut self,
+        sighash: [u8; 32],
+        signature: redpallas::Signature<SpendAuth>,
+    ) -> Result<(), SignerError> {
+        if self.spend.rk.verify(&sighash, &signature).is_ok() {
+            self.spend.spend_auth_sig = Some(signature);
+            Ok(())
+        } else {
+            Err(SignerError::InvalidExternalSignature)
+        }
+    }
 }
 
 /// Errors that can occur while signing an Orchard action in a PCZT.
 #[derive(Debug)]
 pub enum SignerError {
+    /// A provided external signature was not valid for the action's spend.
+    InvalidExternalSignature,
     /// The Signer role requires `alpha` to be set.
     MissingSpendAuthRandomizer,
     /// The provided `ask` does not own the action's spent note.