feat(abstract-utxo): verify the mint amount with external change amount for sbtc mint txs

modules/abstract-utxo/src/abstractUtxoCoin.ts

@@ -27,6 +27,7 @@ import {
   ParseTransactionOptions as BaseParseTransactionOptions,
   PrecreateBitGoOptions,
   PresignTransactionOptions,
+  BridgingParams,
   RequestTracer,
   SignedTransaction,
   TxIntentMismatchError,
@@ -276,6 +277,8 @@ export interface TransactionParams extends BaseTransactionParams {
   allowExternalChangeAddress?: boolean;
   changeAddress?: string;
   rbfTxIds?: string[];
+  /** Parameters for bridging intents (e.g. BTC -> sBTC peg-in), present when `type === 'bridging'`. */
+  bridgingParams?: BridgingParams;
 }
 
 export interface ParseTransactionOptions<TNumber extends number | bigint = number> extends BaseParseTransactionOptions {

modules/abstract-utxo/src/transaction/fixedScript/verifyTransaction.ts

@@ -65,6 +65,7 @@ export async function verifyTransaction<TNumber extends bigint | number>(
     throw new Error('should not have unspents in txInfo for psbt');
   }
   const disableNetworking = !!verification.disableNetworking;
+  const isBridging = txParams.type === 'bridging';
   const parsedTransaction: ParsedTransaction<TNumber> = await coin.parseTransaction<TNumber>({
     txParams,
     txPrebuild,
@@ -160,7 +161,21 @@ export async function verifyTransaction<TNumber extends bigint | number>(
 
   // There are two instances where we will get into this point here
   if (nonChangeAmount.gt(payAsYouGoLimit)) {
-    if (isPsbt && parsedTransaction.customChange) {
+    if (isBridging) {
+      // The implicit external output is the bridge deposit address (see note above); it has no
+      // recipient to match against, so instead verify the total implicit external spend equals the
+      // intended bridge amount from bridgingParams.
+      const bridgeAmount = txParams.bridgingParams?.sbtc?.amount;
+      if (bridgeAmount === undefined) {
+        throwTxMismatch('bridging transaction is missing bridgingParams.sbtc.amount');
+      } else if (!nonChangeAmount.eq(bridgeAmount.toString())) {
+        throwTxMismatch(
+          `bridging output amount (${nonChangeAmount.toString()}) does not match intended bridge amount (${bridgeAmount})`
+        );
+      } else {
+        debug('verified bridging output amount matches bridgingParams.sbtc.amount');
+      }
+    } else if (isPsbt && parsedTransaction.customChange) {
       // In the case that we have a custom change address on a wallet and we are building the transaction
       // with a PSBT, we do not have the metadata to verify the address from the custom change wallet, nor
       // can we fetch that information from the other wallet because we may not have the credentials. Therefore,

modules/abstract-utxo/test/unit/verifyTransaction.ts

@@ -305,6 +305,166 @@ describe('Verify Transaction', function () {
     bitcoinMock.restore();
   });
 
+  it('should verify a bridging transaction whose implicit external output matches the bridge amount', async () => {
+    // Bridging intents (e.g. BTC -> sBTC peg-in) carry no recipients; the single external output
+    // is the bridge deposit address computed server-side. With explicitExternalSpendAmount 0 the
+    // paygo limit is 0, so any implicit external output would normally be rejected as an
+    // "unintended external recipient". For type: 'bridging' we instead verify that the implicit
+    // external spend equals bridgingParams.sbtc.amount.
+    const coinMock = sinon.stub(coin, 'parseTransaction').resolves({
+      keychains: {} as any,
+      keySignatures: {},
+      outputs: [],
+      missingOutputs: [],
+      explicitExternalOutputs: [],
+      implicitExternalOutputs: [
+        {
+          address: 'sbtc_deposit_address',
+          amount: '22000',
+        },
+      ],
+      changeOutputs: [],
+      explicitExternalSpendAmount: 0,
+      implicitExternalSpendAmount: 22000,
+      needsCustomChangeKeySignatureVerification: false,
+    });
+
+    const bitcoinMock = sinon
+      .stub(coin, 'createTransactionFromHex')
+      .returns({ ins: [] } as unknown as utxolib.bitgo.UtxoTransaction);
+
+    const result = await coin.verifyTransaction({
+      txParams: {
+        walletPassphrase: passphrase,
+        type: 'bridging',
+        bridgingParams: { sbtc: { amount: '22000', stacksRecipient: 'SM1X', maxFee: '1000', lockTime: 100 } },
+      },
+      txPrebuild: {
+        txHex: '00',
+      },
+      wallet: unsignedSendingWallet as any,
+      verification: {},
+    });
+
+    assert.strictEqual(result, true);
+
+    coinMock.restore();
+    bitcoinMock.restore();
+  });
+
+  it('should reject a bridging transaction whose implicit external output does not match the bridge amount', async () => {
+    const coinMock = sinon.stub(coin, 'parseTransaction').resolves({
+      keychains: {} as any,
+      keySignatures: {},
+      outputs: [],
+      missingOutputs: [],
+      explicitExternalOutputs: [],
+      implicitExternalOutputs: [
+        {
+          address: 'sbtc_deposit_address',
+          amount: '50000',
+        },
+      ],
+      changeOutputs: [],
+      explicitExternalSpendAmount: 0,
+      implicitExternalSpendAmount: 50000,
+      needsCustomChangeKeySignatureVerification: false,
+    });
+
+    await assert.rejects(
+      coin.verifyTransaction({
+        txParams: {
+          walletPassphrase: passphrase,
+          type: 'bridging',
+          bridgingParams: { sbtc: { amount: '22000', stacksRecipient: 'SM1X', maxFee: '1000', lockTime: 100 } },
+        },
+        txPrebuild: {
+          txHex: '00',
+        },
+        wallet: unsignedSendingWallet as any,
+        verification: {},
+      }),
+      /bridging output amount \(50000\) does not match intended bridge amount \(22000\)/
+    );
+
+    coinMock.restore();
+  });
+
+  it('should reject a bridging transaction that is missing bridgingParams.sbtc.amount', async () => {
+    const coinMock = sinon.stub(coin, 'parseTransaction').resolves({
+      keychains: {} as any,
+      keySignatures: {},
+      outputs: [],
+      missingOutputs: [],
+      explicitExternalOutputs: [],
+      implicitExternalOutputs: [
+        {
+          address: 'sbtc_deposit_address',
+          amount: '22000',
+        },
+      ],
+      changeOutputs: [],
+      explicitExternalSpendAmount: 0,
+      implicitExternalSpendAmount: 22000,
+      needsCustomChangeKeySignatureVerification: false,
+    });
+
+    await assert.rejects(
+      coin.verifyTransaction({
+        txParams: {
+          walletPassphrase: passphrase,
+          type: 'bridging',
+        },
+        txPrebuild: {
+          txHex: '00',
+        },
+        wallet: unsignedSendingWallet as any,
+        verification: {},
+      }),
+      /bridging transaction is missing bridgingParams.sbtc.amount/
+    );
+
+    coinMock.restore();
+  });
+
+  it('should still reject the same implicit external output when the intent is not bridging', async () => {
+    // Same shape as the bridging test above, but without type: 'bridging' the implicit external
+    // output is treated as an unintended external recipient and rejected.
+    const coinMock = sinon.stub(coin, 'parseTransaction').resolves({
+      keychains: {} as any,
+      keySignatures: {},
+      outputs: [],
+      missingOutputs: [],
+      explicitExternalOutputs: [],
+      implicitExternalOutputs: [
+        {
+          address: 'sbtc_deposit_address',
+          amount: '22000',
+        },
+      ],
+      changeOutputs: [],
+      explicitExternalSpendAmount: 0,
+      implicitExternalSpendAmount: 22000,
+      needsCustomChangeKeySignatureVerification: false,
+    });
+
+    await assert.rejects(
+      coin.verifyTransaction({
+        txParams: {
+          walletPassphrase: passphrase,
+        },
+        txPrebuild: {
+          txHex: '00',
+        },
+        wallet: unsignedSendingWallet as any,
+        verification: {},
+      }),
+      /prebuild attempts to spend to unintended external recipients/
+    );
+
+    coinMock.restore();
+  });
+
   it('should work with bigint amounts', async () => {
     // need a coin that uses bigint
     const bigintCoin = getUtxoCoin('tdoge');