From 23cbee842f6534960bfdf21c8d85f692b140040a Mon Sep 17 00:00:00 2001
From: rishflab <rishflab@hotmail.com>
Date: Tue, 9 Feb 2021 17:23:13 +1100
Subject: [PATCH] Derive bitcoin private key from seed

---
 swap/src/bitcoin/wallet.rs |  4 +---
 swap/src/main.rs           |  9 +++++++++
 swap/src/seed.rs           | 27 +++++++++++++++++++++++++++
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/swap/src/bitcoin/wallet.rs b/swap/src/bitcoin/wallet.rs
index 7873b7ac..42f45a01 100644
--- a/swap/src/bitcoin/wallet.rs
+++ b/swap/src/bitcoin/wallet.rs
@@ -13,7 +13,6 @@ use backoff::{backoff::Constant as ConstantBackoff, tokio::retry};
 use bdk::{
     blockchain::{noop_progress, Blockchain, ElectrumBlockchain},
     electrum_client::{Client, ElectrumApi},
-    keys::GeneratableDefaultOptions,
     FeeRate,
 };
 use reqwest::{Method, Url};
@@ -36,6 +35,7 @@ impl Wallet {
         electrum_http_url: Url,
         network: bitcoin::Network,
         datadir: &Path,
+        p_key: bitcoin::PrivateKey,
     ) -> Result<Self> {
         // todo: Implement conversion to anyhow::error so we can use ?
         let client =
@@ -43,8 +43,6 @@ impl Wallet {
 
         let db = bdk::sled::open(datadir)?.open_tree(SLED_TREE_NAME)?;
 
-        // todo: make key generation configurable using a descriptor
-        let p_key = ::bitcoin::PrivateKey::generate_default()?;
         let bdk_wallet = bdk::Wallet::new(
             bdk::template::P2WPKH(p_key),
             None,
diff --git a/swap/src/main.rs b/swap/src/main.rs
index e81790a8..af67dc98 100644
--- a/swap/src/main.rs
+++ b/swap/src/main.rs
@@ -19,6 +19,7 @@ use crate::{
     },
     execution_params::GetExecutionParams,
     protocol::bob::cancel::CancelError,
+    seed::Seed,
 };
 use anyhow::{Context, Result};
 use database::Database;
@@ -97,6 +98,7 @@ async fn main() -> Result<()> {
                 bitcoin_network,
                 &wallet_data_dir,
                 monero_network,
+                seed,
             )
             .await?;
 
@@ -139,6 +141,7 @@ async fn main() -> Result<()> {
                 bitcoin_network,
                 &wallet_data_dir,
                 monero_network,
+                seed,
             )
             .await?;
 
@@ -188,6 +191,7 @@ async fn main() -> Result<()> {
                 bitcoin_network,
                 &wallet_data_dir,
                 monero_network,
+                seed,
             )
             .await?;
 
@@ -216,6 +220,7 @@ async fn main() -> Result<()> {
                 bitcoin_network,
                 &wallet_data_dir,
                 monero_network,
+                seed,
             )
             .await?;
 
@@ -247,6 +252,7 @@ async fn main() -> Result<()> {
                 bitcoin_network,
                 &wallet_data_dir,
                 monero_network,
+                seed,
             )
             .await?;
 
@@ -297,6 +303,7 @@ async fn main() -> Result<()> {
                 bitcoin_network,
                 &wallet_data_dir,
                 monero_network,
+                seed,
             )
             .await?;
 
@@ -334,6 +341,7 @@ async fn init_wallets(
     bitcoin_network: bitcoin::Network,
     bitcoin_wallet_data_dir: &Path,
     monero_network: monero::Network,
+    seed: Seed,
 ) -> Result<(bitcoin::Wallet, monero::Wallet)> {
     let config_path = if let Some(config_path) = config_path {
         config_path
@@ -354,6 +362,7 @@ async fn init_wallets(
         config.bitcoin.electrum_http_url,
         bitcoin_network,
         bitcoin_wallet_data_dir,
+        seed.root_private_key(bitcoin_network),
     )
     .await?;
     let bitcoin_balance = bitcoin_wallet.balance().await?;
diff --git a/swap/src/seed.rs b/swap/src/seed.rs
index 382f64da..5dbfa8cb 100644
--- a/swap/src/seed.rs
+++ b/swap/src/seed.rs
@@ -1,4 +1,8 @@
 use ::bitcoin::secp256k1::{self, constants::SECRET_KEY_SIZE, SecretKey};
+use bitcoin::{
+    hashes::{sha512, Hash, HashEngine, Hmac, HmacEngine},
+    PrivateKey,
+};
 use rand::prelude::*;
 use std::fmt;
 
@@ -18,6 +22,29 @@ impl Seed {
         Ok(Seed(bytes))
     }
 
+    /// Return the private key and chain code to be used as root extended
+    /// private key for a BIP32 wallet.
+    pub fn root_private_key(&self, network: bitcoin::Network) -> PrivateKey {
+        let bytes = self.bytes();
+
+        // Yes, this is as per BIP32 and used in both Bitcoin and Ethereum ecosystems
+        let hash_key = b"Bitcoin seed";
+
+        let mut engine = HmacEngine::<sha512::Hash>::new(hash_key);
+        engine.input(&bytes);
+        let hash = Hmac::<sha512::Hash>::from_engine(engine);
+        let output = &hash.into_inner()[..];
+        let key = &output[..32];
+
+        let secret_key = SecretKey::from_slice(key).expect("32 bytes array should be fine");
+
+        PrivateKey {
+            compressed: true,
+            network,
+            key: secret_key,
+        }
+    }
+
     pub fn bytes(&self) -> [u8; SEED_LENGTH] {
         self.0
     }