From 4b3d2bbe5e586414ad6da1704919f8939fa749b0 Mon Sep 17 00:00:00 2001
From: Jack Grigg <jack@electriccoin.co>
Date: Tue, 17 Dec 2024 17:33:47 +0000
Subject: [PATCH] transparent: Add `Bip32Derivation::extract_bip_44_fields`

---
 zcash_transparent/CHANGELOG.md |  3 +++
 zcash_transparent/src/pczt.rs  | 46 +++++++++++++++++++++++++++++++++-
 2 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/zcash_transparent/CHANGELOG.md b/zcash_transparent/CHANGELOG.md
index 35cc8874e..46dc2a245 100644
--- a/zcash_transparent/CHANGELOG.md
+++ b/zcash_transparent/CHANGELOG.md
@@ -11,6 +11,9 @@ and this library adheres to Rust's notion of
 - `zcash_transparent::keys::AccountPubKey::derive_pubkey_at_bip32_path` now
   returns the correct result for valid paths instead of an error or panic.
 
+### Added
+- `zcash_transparent::pczt::Bip32Derivation::extract_bip_44_fields`
+
 ## [0.1.0] - 2024-12-16
 
 The entries below are relative to the `zcash_primitives` crate as of the tag
diff --git a/zcash_transparent/src/pczt.rs b/zcash_transparent/src/pczt.rs
index faf053031..5f6837613 100644
--- a/zcash_transparent/src/pczt.rs
+++ b/zcash_transparent/src/pczt.rs
@@ -8,7 +8,11 @@ use bip32::ChildNumber;
 use getset::Getters;
 use zcash_protocol::{value::Zatoshis, TxId};
 
-use crate::{address::Script, sighash::SighashType};
+use crate::{
+    address::Script,
+    keys::{NonHardenedChildIndex, TransparentKeyScope},
+    sighash::SighashType,
+};
 
 mod parse;
 pub use parse::ParseError;
@@ -230,3 +234,43 @@ pub struct Bip32Derivation {
     /// The sequence of indices corresponding to the HD path.
     derivation_path: Vec<ChildNumber>,
 }
+
+impl Bip32Derivation {
+    /// Extracts the BIP 44 account index, scope, and address index from this derivation
+    /// path.
+    ///
+    /// Returns `None` if the seed fingerprints don't match, or if this is a non-standard
+    /// derivation path.
+    pub fn extract_bip_44_fields(
+        &self,
+        seed_fp: &zip32::fingerprint::SeedFingerprint,
+        expected_coin_type: ChildNumber,
+    ) -> Option<(zip32::AccountId, TransparentKeyScope, NonHardenedChildIndex)> {
+        if self.seed_fingerprint == seed_fp.to_bytes() {
+            match &self.derivation_path[..] {
+                [purpose, coin_type, account_index, scope, address_index]
+                    if purpose == &ChildNumber(44 | ChildNumber::HARDENED_FLAG)
+                        && coin_type.is_hardened()
+                        && coin_type == &expected_coin_type
+                        && account_index.is_hardened()
+                        && !scope.is_hardened()
+                        && !address_index.is_hardened() =>
+                {
+                    let account_index = zip32::AccountId::try_from(account_index.index())
+                        .expect("account_index is hardened");
+
+                    let scope =
+                        TransparentKeyScope::custom(scope.index()).expect("scope is not hardened");
+
+                    let address_index = NonHardenedChildIndex::from_index(address_index.index())
+                        .expect("address_index is not hardened");
+
+                    Some((account_index, scope, address_index))
+                }
+                _ => None,
+            }
+        } else {
+            None
+        }
+    }
+}