feat: add psbt signing for v1 wallets

This commit adds optional PSBT signing to the v1 wallet API.

Call the `signTransaction` method with a `psbt: string` (hex-encoded) argument
to co-sign a PSBT.

The method will return the signed PSBT as a hex-encoded string.

Issue: BTC-1351

modules/sdk-api/src/v1/signPsbt.ts

@@ -0,0 +1,38 @@
+import * as utxolib from '@bitgo/utxo-lib';
+
+import * as buildDebug from 'debug';
+
+const debug = buildDebug('bitgo:v1:txb');
+
+/**
+ * Co-sign a PSBT.
+ * Simply a wrapper around `utxolib.bitgo.createPsbtFromBuffer` and `psbt.signAllInputsHD`.
+ * @param params
+ */
+export function signPsbtRequest(params: { psbt: string; keychain: { xprv: string } } | unknown): {
+  psbt: string;
+} {
+  if (typeof params !== 'object' || params === null) {
+    throw new Error(`invalid argument`);
+  }
+
+  if (!('psbt' in params) || typeof params.psbt !== 'string') {
+    throw new Error(`invalid params.psbt`);
+  }
+
+  if (!('keychain' in params) || typeof params.keychain !== 'object' || params.keychain === null) {
+    throw new Error(`invalid params.keychain`);
+  }
+
+  if (!('xprv' in params.keychain) || typeof params.keychain.xprv !== 'string') {
+    throw new Error(`invalid params.keychain.xprv`);
+  }
+
+  const psbt = utxolib.bitgo.createPsbtFromBuffer(Buffer.from(params.psbt, 'hex'), utxolib.networks.bitcoin);
+  const keypair = utxolib.bip32.fromBase58(params.keychain.xprv, utxolib.networks.bitcoin);
+  debug('signing PSBT with keychain %s', keypair.neutered().toBase58());
+  utxolib.bitgo.withUnsafeNonSegwit(psbt, () => psbt.signAllInputsHD(keypair));
+  return {
+    psbt: psbt.toBuffer().toString('hex'),
+  };
+}

modules/sdk-api/src/v1/wallet.ts

@@ -26,6 +26,7 @@ import {
 } from '@bitgo/sdk-core';
 import * as Bluebird from 'bluebird';
 import * as _ from 'lodash';
+import { signPsbtRequest } from './signPsbt';
 
 const TransactionBuilder = require('./transactionBuilder');
 const PendingApproval = require('./pendingapproval');
@@ -896,6 +897,15 @@ Wallet.prototype.createTransaction = function (params, callback) {
 //   callback(err, transaction)
 Wallet.prototype.signTransaction = function (params, callback) {
   params = _.extend({}, params);
+
+  if (params.psbt) {
+    try {
+      return callback(null, signPsbtRequest(params));
+    } catch (e) {
+      return callback(e);
+    }
+  }
+
   common.validateParams(params, ['transactionHex'], [], callback);
 
   if (!Array.isArray(params.unspents)) {

modules/sdk-api/test/unit/v1/signPsbt.ts

@@ -0,0 +1,27 @@
+import * as assert from 'assert';
+import * as utxolib from '@bitgo/utxo-lib';
+import { signPsbtRequest } from '../../../src/v1/signPsbt';
+
+describe('signPsbt', function () {
+  it('signs psbt', function () {
+    const keys = utxolib.testutil.getDefaultWalletKeys();
+    const psbt = utxolib.testutil.constructPsbt(
+      [{ scriptType: 'p2sh', value: BigInt(1e8) }],
+      [{ scriptType: 'p2sh', value: BigInt(1e8 - 1000) }],
+      utxolib.networks.bitcoin,
+      keys,
+      'unsigned'
+    );
+    const result = signPsbtRequest({
+      psbt: psbt.toHex(),
+      keychain: {
+        xprv: keys.triple[0].toBase58(),
+      },
+    });
+    const halfSignedPsbt = utxolib.bitgo.createPsbtFromBuffer(
+      Buffer.from(result.psbt, 'hex'),
+      utxolib.networks.bitcoin
+    );
+    assert.ok(halfSignedPsbt.validateSignaturesOfInputHD(0, keys.triple[0]));
+  });
+});