Merge bitcoindevkit/bdk#682: Add a custom signer for hardware wallets

138acc3b7d Change `populate_test_db` to not return empty input (wszdexdrf)
d6e1dd1040 Change CI to add test using ledger emulator (wszdexdrf)
76034772cb Add a custom signer for hardware wallets (wszdexdrf)

Pull request description:

  Also adds a new test in CI for building and testing on a virtual
  hardware wallet.

  ### Description

  This PR would enable BDK users to sign transactions using a hardware wallet. It is just the beginning hence there are no complex features, but I hope not for long.
  I have added a test in CI for building a ledger emulator and running the new test on it. The test is similar to the one on bitcoindevkit/rust-hwi.

  ### Notes to the reviewers
  The PR is incomplete (and wouldn't work, as the rust-hwi in `cargo.toml` is pointing to a local crate, temporarily) as a small change is required in rust-hwi (https://github.com/bitcoindevkit/rust-hwi/pull/42).

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  afilini:
    ACK 138acc3b7d

Tree-SHA512: 54337f06247829242b4dc60f733346173d957de8e9f8b80beb91304d679cfb4e0e4db722c967469265a5b6ede2bd641ba5c089760391c671995dc30de37897de

src/database/memory.rs

@@ -491,11 +491,10 @@ macro_rules! populate_test_db {
         let mut db = $db;
         let tx_meta = $tx_meta;
         let current_height: Option<u32> = $current_height;
-        let input = if $is_coinbase {
-            vec![$crate::bitcoin::TxIn::default()]
-        } else {
-            vec![]
-        };
+        let mut input = vec![$crate::bitcoin::TxIn::default()];
+        if !$is_coinbase {
+            input[0].previous_output.vout = 0;
+        }
         let tx = $crate::bitcoin::Transaction {
             version: 1,
             lock_time: 0,

src/wallet/hardwaresigner.rs

@@ -0,0 +1,64 @@
+// Bitcoin Dev Kit
+// Written in 2020 by Alekos Filini <alekos.filini@gmail.com>
+//
+// Copyright (c) 2020-2021 Bitcoin Dev Kit Developers
+//
+// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
+// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
+// You may not use this file except in accordance with one or both of these
+// licenses.
+
+//! HWI Signer
+//!
+//! This module contains a simple implementation of a Custom signer for rust-hwi
+
+use bitcoin::psbt::PartiallySignedTransaction;
+use bitcoin::secp256k1::{All, Secp256k1};
+use bitcoin::util::bip32::Fingerprint;
+
+use hwi::error::Error;
+use hwi::types::{HWIChain, HWIDevice};
+use hwi::HWIClient;
+
+use crate::signer::{SignerCommon, SignerError, SignerId, TransactionSigner};
+
+#[derive(Debug)]
+/// Custom signer for Hardware Wallets
+///
+/// This ignores `sign_options` and leaves the decisions up to the hardware wallet.
+pub struct HWISigner {
+    fingerprint: Fingerprint,
+    client: HWIClient,
+}
+
+impl HWISigner {
+    /// Create a instance from the specified device and chain
+    pub fn from_device(device: &HWIDevice, chain: HWIChain) -> Result<HWISigner, Error> {
+        let client = HWIClient::get_client(device, false, chain)?;
+        Ok(HWISigner {
+            fingerprint: device.fingerprint,
+            client,
+        })
+    }
+}
+
+impl SignerCommon for HWISigner {
+    fn id(&self, _secp: &Secp256k1<All>) -> SignerId {
+        SignerId::Fingerprint(self.fingerprint)
+    }
+}
+
+/// This implementation ignores `sign_options`
+impl TransactionSigner for HWISigner {
+    fn sign_transaction(
+        &self,
+        psbt: &mut PartiallySignedTransaction,
+        _sign_options: &crate::SignOptions,
+        _secp: &crate::wallet::utils::SecpCtx,
+    ) -> Result<(), SignerError> {
+        psbt.combine(self.client.sign_tx(psbt)?.psbt)
+            .expect("Failed to combine HW signed psbt with passed PSBT");
+        Ok(())
+    }
+}

src/wallet/mod.rs

@@ -48,6 +48,9 @@ pub(crate) mod utils;
 #[cfg_attr(docsrs, doc(cfg(feature = "verify")))]
 pub mod verify;
 
+#[cfg(feature = "hardware-signer")]
+pub mod hardwaresigner;
+
 pub use utils::IsDust;
 
 #[allow(deprecated)]
@@ -5466,4 +5469,33 @@ pub(crate) mod test {
         // ...and checking that everything is fine
         assert_fee_rate!(psbt, details.fee.unwrap_or(0), fee_rate);
     }
+
+    #[cfg(feature = "test-hardware-signer")]
+    #[test]
+    fn test_create_signer() {
+        use crate::wallet::hardwaresigner::HWISigner;
+        use hwi::types::HWIChain;
+        use hwi::HWIClient;
+
+        let devices = HWIClient::enumerate().unwrap();
+        let device = devices.first().expect("No devices found");
+        let client = HWIClient::get_client(device, true, HWIChain::Regtest).unwrap();
+        let descriptors = client.get_descriptors(None).unwrap();
+        let custom_signer = HWISigner::from_device(device, HWIChain::Regtest).unwrap();
+
+        let (mut wallet, _, _) = get_funded_wallet(&descriptors.internal[0]);
+        wallet.add_signer(
+            KeychainKind::External,
+            SignerOrdering(200),
+            Arc::new(custom_signer),
+        );
+
+        let addr = wallet.get_address(LastUnused).unwrap();
+        let mut builder = wallet.build_tx();
+        builder.drain_to(addr.script_pubkey()).drain_wallet();
+        let (mut psbt, _) = builder.finish().unwrap();
+
+        let finalized = wallet.sign(&mut psbt, Default::default()).unwrap();
+        assert!(finalized);
+    }
 }

src/wallet/signer.rs

@@ -159,6 +159,16 @@ pub enum SignerError {
     InvalidSighash,
     /// Error while computing the hash to sign
     SighashError(sighash::Error),
+    /// Error while signing using hardware wallets
+    #[cfg(feature = "hardware-signer")]
+    HWIError(hwi::error::Error),
+}
+
+#[cfg(feature = "hardware-signer")]
+impl From<hwi::error::Error> for SignerError {
+    fn from(e: hwi::error::Error) -> Self {
+        SignerError::HWIError(e)
+    }
 }
 
 impl From<sighash::Error> for SignerError {