feat(utxo-core): implement BIP322 to_spend transaction builder

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

@@ -1,4 +1,5 @@
 import { Hash } from 'fast-sha256';
+import { Psbt } from '@bitgo/utxo-lib';
 
 export const BIP322_TAG = 'BIP0322-signed-message';
 
@@ -22,3 +23,42 @@ export function hashMessageWithTag(message: string | Buffer, tag = BIP322_TAG):
   const messageHash = messageHasher.digest();
   return Buffer.from(messageHash);
 }
+
+/**
+ * Build a BIP322 "to spend" transaction
+ * Source: https://github.com/bitcoin/bips/blob/master/bip-0322.mediawiki#full
+ *
+ * @param {Buffer} scriptPubKey - The scriptPubKey to use for the output
+ * @param {string | Buffer} message - The message to include in the transaction
+ * @param {Buffer} [tag=BIP322_TAG] - The tag to use for hashing, defaults to BIP322_TAG.
+ * @returns {string} - The hex representation of the constructed transaction
+ */
+export function buildToSpendTransaction(scriptPubKey: Buffer, message: string | Buffer, tag = BIP322_TAG): string {
+  // Create PSBT object for constructing the transaction
+  const psbt = new Psbt();
+  // Set default value for nVersion and nLockTime
+  psbt.setVersion(0); // nVersion = 0
+  psbt.setLocktime(0); // nLockTime = 0
+  // Compute the message hash - SHA256(SHA256(tag) || SHA256(tag) || message)
+  const messageHash = hashMessageWithTag(message, tag);
+  // Construct the scriptSig - OP_0 PUSH32[ message_hash ]
+  const scriptSigPartOne = new Uint8Array([0x00, 0x20]); // OP_0 PUSH32
+  const scriptSig = new Uint8Array(scriptSigPartOne.length + messageHash.length);
+  scriptSig.set(scriptSigPartOne);
+  scriptSig.set(messageHash, scriptSigPartOne.length);
+  // Set the input
+  psbt.addInput({
+    hash: '0'.repeat(64), // vin[0].prevout.hash = 0000...000
+    index: 0xffffffff, // vin[0].prevout.n = 0xFFFFFFFF
+    sequence: 0, // vin[0].nSequence = 0
+    finalScriptSig: Buffer.from(scriptSig), // vin[0].scriptSig = OP_0 PUSH32[ message_hash ]
+    witnessScript: Buffer.from([]), // vin[0].scriptWitness = []
+  });
+  // Set the output
+  psbt.addOutput({
+    value: BigInt(0), // vout[0].nValue = 0
+    script: scriptPubKey, // vout[0].scriptPubKey = message_challenge
+  });
+  // Return transaction
+  return psbt.extractTransaction().toHex();
+}

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

@@ -1,6 +1,8 @@
 import assert from 'assert';
 
-import { hashMessageWithTag } from '../../src/bip322';
+import { payments, Transaction } from '@bitgo/utxo-lib';
+
+import { buildToSpendTransaction, hashMessageWithTag } from '../../src/bip322';
 
 describe('to_spend', function () {
   describe('Message hashing', function () {
@@ -27,4 +29,30 @@ describe('to_spend', function () {
       });
     });
   });
+
+  describe('build to_spend transaction', function () {
+    const scriptPubKey = payments.p2wpkh({
+      address: 'bc1q9vza2e8x573nczrlzms0wvx3gsqjx7vavgkx0l',
+    }).output as Buffer;
+
+    // Source: https://github.com/bitcoin/bips/blob/master/bip-0322.mediawiki#transaction-hashes
+    const fixtures = [
+      {
+        message: '',
+        txid: 'c5680aa69bb8d860bf82d4e9cd3504b55dde018de765a91bb566283c545a99a7',
+      },
+      {
+        message: 'Hello World',
+        txid: 'b79d196740ad5217771c1098fc4a4b51e0535c32236c71f1ea4d61a2d603352b',
+      },
+    ];
+
+    fixtures.forEach(({ message, txid }) => {
+      it(`should build a to_spend transaction for message "${message}"`, function () {
+        const result = buildToSpendTransaction(scriptPubKey, Buffer.from(message));
+        const computedTxid = Transaction.fromHex(result).getId();
+        assert.strictEqual(computedTxid, txid, `Transaction ID for message "${message}" does not match expected value`);
+      });
+    });
+  });
 });