feat(utxo-core): implement function to extract BIP322 message by index

Add getBip322ProofMessageAtIndex to retrieve the BIP322 message from a
specific input index in a PSBT. The function returns the message as a Buffer
or undefined if not found.

Issue: BTC-2375

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

Author: davidkaplanbitgo

modules/utxo-core/src/bip322/utils.ts

@@ -11,20 +11,25 @@ export function addBip322ProofMessage(psbt: utxolib.Psbt, inputIndex: number, me
   });
 }
 
-export function getBip322ProofInputIndex(psbt: utxolib.Psbt): number | undefined {
-  const res = psbt.data.inputs.flatMap((input, inputIndex) => {
-    const proprietaryKeyVals = utxolib.bitgo.getPsbtInputProprietaryKeyVals(input, {
-      identifier: utxolib.bitgo.PSBT_PROPRIETARY_IDENTIFIER,
-      subtype: utxolib.bitgo.ProprietaryKeySubtype.BIP322_MESSAGE,
-    });
-    if (proprietaryKeyVals.length > 1) {
-      throw new Error(`Multiple BIP322 messages found at input index ${inputIndex}`);
-    }
-    return proprietaryKeyVals.length === 0 ? [] : [inputIndex];
+/**
+ * Get the BIP322 proof message at a specific input index of the PSBT
+ * @param psbt
+ * @param inputIndex
+ * @returns The BIP322 proof message as a Buffer, or undefined if not found
+ */
+export function getBip322ProofMessageAtIndex(psbt: utxolib.Psbt, inputIndex: number): Buffer | undefined {
+  if (psbt.data.inputs.length <= inputIndex) {
+    throw new Error(`Input index ${inputIndex} is out of bounds for the PSBT`);
+  }
+  const input = psbt.data.inputs[inputIndex];
+  const proprietaryKeyVals = utxolib.bitgo.getPsbtInputProprietaryKeyVals(input, {
+    identifier: utxolib.bitgo.PSBT_PROPRIETARY_IDENTIFIER,
+    subtype: utxolib.bitgo.ProprietaryKeySubtype.BIP322_MESSAGE,
   });
-  return res.length === 0 ? undefined : res[0];
-}
-
-export function psbtIsBip322Proof(psbt: utxolib.Psbt): boolean {
-  return getBip322ProofInputIndex(psbt) !== undefined;
+  if (proprietaryKeyVals.length === 0) {
+    return undefined;
+  } else if (proprietaryKeyVals.length > 1) {
+    throw new Error(`Multiple BIP322 messages found at input index ${inputIndex}`);
+  }
+  return Buffer.from(proprietaryKeyVals[0].value);
 }

modules/utxo-core/test/bip322/utils.ts

@@ -2,7 +2,7 @@ import assert from 'assert';
 
 import * as utxolib from '@bitgo/utxo-lib';
 
-import { getBip322ProofInputIndex, psbtIsBip322Proof } from '../../src/bip322';
+import { getBip322ProofMessageAtIndex } from '../../src/bip322';
 
 import { BIP322_FIXTURE_HELLO_WORLD_TOSIGN_PSBT } from './bip322.utils';
 
@@ -31,8 +31,8 @@ describe('BIP322 Proof utils', function () {
       utxolib.networks.bitcoin
     );
 
-    const resultIndex = getBip322ProofInputIndex(psbt);
-    assert.strictEqual(resultIndex, 0);
-    assert.ok(psbtIsBip322Proof(psbt));
+    const messageBuffer = getBip322ProofMessageAtIndex(psbt, 0);
+    assert.ok(messageBuffer, 'Message buffer should not be undefined');
+    assert.deepStrictEqual(messageBuffer.toString('utf-8'), 'Hello World', 'Message does not match expected value');
   });
 });