feat(utxo-core): add PSBT signing utilities

Add helper functions for signing PSBTs with Schnorr and ECDSA signatures.
Include utilities to count new signatures and sign with different key
types.

Issue: BTC-1966
Co-authored-by: llm-git <[email protected]>

Author: OttoAllmendinger

modules/utxo-core/src/descriptor/psbt/index.ts

@@ -3,3 +3,4 @@ export * from './createPsbt';
 export * from './parse';
 export * from './findDescriptors';
 export * from './wrap';
+export * from './sign';

modules/utxo-core/src/descriptor/psbt/sign.ts

@@ -0,0 +1,47 @@
+import assert from 'assert';
+
+import * as utxolib from '@bitgo/utxo-lib';
+import { Psbt as WasmPsbt } from '@bitgo/wasm-miniscript';
+
+/** These can be replaced when @bitgo/wasm-miniscript is updated */
+export type SignPsbtInputResult = { Schnorr: string[] } | { Ecdsa: string[] };
+export type SignPsbtResult = {
+  [inputIndex: number]: SignPsbtInputResult;
+};
+
+/**
+ * @param signResult
+ * @return the number of new signatures created by the signResult for a single input
+ */
+export function getNewSignatureCountForInput(signResult: SignPsbtInputResult): number {
+  if ('Schnorr' in signResult) {
+    return signResult.Schnorr.length;
+  }
+  if ('Ecdsa' in signResult) {
+    return signResult.Ecdsa.length;
+  }
+  throw new Error(`Unknown signature type ${Object.keys(signResult).join(', ')}`);
+}
+
+/**
+ * @param signResult
+ * @return the number of new signatures created by the signResult
+ */
+export function getNewSignatureCount(signResult: SignPsbtResult): number {
+  return Object.values(signResult).reduce((sum, signatures) => sum + getNewSignatureCountForInput(signatures), 0);
+}
+
+type Key = Buffer | utxolib.BIP32Interface | utxolib.ECPairInterface;
+
+/** Convenience function to sign a PSBT with a key */
+export function signWithKey(psbt: WasmPsbt, key: Key): SignPsbtResult {
+  // we need to do casting here because the type definitions in wasm-miniscript are a little bit buggy
+  if (Buffer.isBuffer(key)) {
+    return psbt.signWithPrv(key) as unknown as SignPsbtResult;
+  }
+  if ('toBase58' in key) {
+    return psbt.signWithXprv(key.toBase58()) as unknown as SignPsbtResult;
+  }
+  assert(key.privateKey);
+  return psbt.signWithPrv(key.privateKey) as unknown as SignPsbtResult;
+}

modules/utxo-core/test/descriptor/psbt/psbt.ts

@@ -15,6 +15,7 @@ import {
   toPlainObjectFromPsbt,
   toPlainObjectFromTx,
 } from '../../../src/testutil/descriptor';
+import { getNewSignatureCount, signWithKey } from '../../../src/descriptor/psbt/sign';
 
 function normalize(v: unknown): unknown {
   if (typeof v === 'bigint') {
@@ -55,12 +56,6 @@ function toPlain(k: BIP32Interface): ECPairInterface {
   return ECPair.fromPrivateKey(k.privateKey);
 }
 
-function isBIP32Interface(k: BIP32Interface | ECPairInterface): k is BIP32Interface {
-  const { name } = k.constructor;
-  assert(name === 'BIP32' || name === 'ECPair');
-  return k.constructor.name === 'BIP32';
-}
-
 type PsbtStage = {
   name: string;
   keys: (BIP32Interface | ECPairInterface)[];
@@ -82,12 +77,7 @@ function getStages(
     stages.map((stage) => {
       const psbtStageWrapped = toWrappedPsbt(psbt);
       for (const key of stage.keys) {
-        if (isBIP32Interface(key)) {
-          psbtStageWrapped.signWithXprv(key.toBase58());
-        } else {
-          assert(key.privateKey);
-          psbtStageWrapped.signWithPrv(key.privateKey);
-        }
+        assert(getNewSignatureCount(signWithKey(psbtStageWrapped, key)) > 0, 'No new signatures were created');
       }
       const psbtStage = toUtxoPsbt(psbtStageWrapped, utxolib.networks.bitcoin);
       let psbtFinal: utxolib.bitgo.UtxoPsbt | undefined;