From 81b2b1b5545c6080b2050a06f5426fbe7c49e6b8 Mon Sep 17 00:00:00 2001
From: Jack Grigg <jack@z.cash>
Date: Mon, 15 Oct 2018 15:51:40 +0100
Subject: [PATCH] Wallet spending key derivation path

---
 Cargo.lock                                    |  3 +++
 zcash_client_backend/Cargo.toml               |  1 +
 zcash_client_backend/src/constants.rs         |  4 ++++
 zcash_client_backend/src/constants/mainnet.rs |  4 ++++
 zcash_client_backend/src/constants/testnet.rs |  4 ++++
 zcash_client_backend/src/keys.rs              | 24 +++++++++++++++++++
 zcash_client_backend/src/lib.rs               |  9 ++-----
 7 files changed, 42 insertions(+), 7 deletions(-)
 create mode 100644 zcash_client_backend/src/constants.rs
 create mode 100644 zcash_client_backend/src/constants/mainnet.rs
 create mode 100644 zcash_client_backend/src/constants/testnet.rs
 create mode 100644 zcash_client_backend/src/keys.rs

diff --git a/Cargo.lock b/Cargo.lock
index 222965138..d3b592d29 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -511,6 +511,9 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [[package]]
 name = "zcash_client_backend"
 version = "0.0.0"
+dependencies = [
+ "zcash_primitives 0.0.0",
+]
 
 [[package]]
 name = "zcash_primitives"
diff --git a/zcash_client_backend/Cargo.toml b/zcash_client_backend/Cargo.toml
index f3c6fd603..d1993baf2 100644
--- a/zcash_client_backend/Cargo.toml
+++ b/zcash_client_backend/Cargo.toml
@@ -7,3 +7,4 @@ authors = [
 edition = "2018"
 
 [dependencies]
+zcash_primitives = { path = "../zcash_primitives" }
diff --git a/zcash_client_backend/src/constants.rs b/zcash_client_backend/src/constants.rs
new file mode 100644
index 000000000..8e5727c89
--- /dev/null
+++ b/zcash_client_backend/src/constants.rs
@@ -0,0 +1,4 @@
+//! Zcash global and per-network constants.
+
+pub mod mainnet;
+pub mod testnet;
diff --git a/zcash_client_backend/src/constants/mainnet.rs b/zcash_client_backend/src/constants/mainnet.rs
new file mode 100644
index 000000000..753956e5f
--- /dev/null
+++ b/zcash_client_backend/src/constants/mainnet.rs
@@ -0,0 +1,4 @@
+/// The mainnet coin type for ZEC, as defined by [SLIP 44].
+///
+/// [SLIP 44]: https://github.com/satoshilabs/slips/blob/master/slip-0044.md
+pub const COIN_TYPE: u32 = 133;
diff --git a/zcash_client_backend/src/constants/testnet.rs b/zcash_client_backend/src/constants/testnet.rs
new file mode 100644
index 000000000..836340cf0
--- /dev/null
+++ b/zcash_client_backend/src/constants/testnet.rs
@@ -0,0 +1,4 @@
+/// The testnet coin type for ZEC, as defined by [SLIP 44].
+///
+/// [SLIP 44]: https://github.com/satoshilabs/slips/blob/master/slip-0044.md
+pub const COIN_TYPE: u32 = 1;
diff --git a/zcash_client_backend/src/keys.rs b/zcash_client_backend/src/keys.rs
new file mode 100644
index 000000000..4d4cd6f1a
--- /dev/null
+++ b/zcash_client_backend/src/keys.rs
@@ -0,0 +1,24 @@
+//! Helper functions for managing light client key material.
+
+use zcash_primitives::zip32::{ChildIndex, ExtendedSpendingKey};
+
+/// Derives the ZIP 32 [`ExtendedSpendingKey`] for a given coin type and account from the
+/// given seed.
+///
+/// # Examples
+///
+/// ```
+/// use zcash_client_backend::{constants::testnet::COIN_TYPE, keys::spending_key};
+///
+/// let extsk = spending_key(&[0; 32][..], COIN_TYPE, 0);
+/// ```
+pub fn spending_key(seed: &[u8], coin_type: u32, account: u32) -> ExtendedSpendingKey {
+    ExtendedSpendingKey::from_path(
+        &ExtendedSpendingKey::master(&seed),
+        &[
+            ChildIndex::Hardened(32),
+            ChildIndex::Hardened(coin_type),
+            ChildIndex::Hardened(account),
+        ],
+    )
+}
diff --git a/zcash_client_backend/src/lib.rs b/zcash_client_backend/src/lib.rs
index 056312640..de76975ba 100644
--- a/zcash_client_backend/src/lib.rs
+++ b/zcash_client_backend/src/lib.rs
@@ -3,10 +3,5 @@
 //! `zcash_client_backend` contains Rust structs and traits for creating shielded Zcash
 //! light clients.
 
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
-    }
-}
+pub mod constants;
+pub mod keys;