transparent: Fix bugs in AccountPubKey::derive_pubkey_at_bip32_path

- A typo in a panic guard condition instead exposed the panic.
- The match logic wasn't correctly handling the first element of the
  path.

zcash_transparent/CHANGELOG.md

@@ -7,6 +7,10 @@ and this library adheres to Rust's notion of
 
 ## [Unreleased]
 
+### Fixed
+- `zcash_transparent::keys::AccountPubKey::derive_pubkey_at_bip32_path` now
+  returns the correct result for valid paths instead of an error or panic.
+
 ## [0.1.0] - 2024-12-16
 
 The entries below are relative to the `zcash_primitives` crate as of the tag

zcash_transparent/src/keys.rs

@@ -273,17 +273,17 @@ impl AccountPubKey {
         expected_account_index: AccountId,
         path: &[ChildNumber],
     ) -> Result<secp256k1::PublicKey, bip32::Error> {
-        if path.len() > 3 {
+        if path.len() < 3 {
             Err(bip32::Error::ChildNumber)
         } else {
             match path.split_at(3) {
-                (
-                    [ChildNumber(44 | ChildNumber::HARDENED_FLAG), coin_type, account_index],
-                    sub_path,
-                ) if coin_type.is_hardened()
-                    && coin_type.index() == params.network_type().coin_type()
-                    && account_index.is_hardened()
-                    && account_index.index() == expected_account_index.into() =>
+                ([purpose, coin_type, account_index], sub_path)
+                    if purpose.is_hardened()
+                        && purpose.index() == 44
+                        && coin_type.is_hardened()
+                        && coin_type.index() == params.network_type().coin_type()
+                        && account_index.is_hardened()
+                        && account_index.index() == expected_account_index.into() =>
                 {
                     sub_path
                         .iter()