From d19d19c284355d4282abcd0a721d1e732da6f50e Mon Sep 17 00:00:00 2001
From: KonradStaniec <konrad.staniec@gmail.com>
Date: Mon, 3 Mar 2025 13:15:55 +0100
Subject: [PATCH] add sanity check for transactions standards (#146)

* add sanity check for transactions standards
---
 CHANGELOG.md               |  1 +
 staker/babylontypes.go     | 12 ++++++++++++
 staker/types.go            |  6 ++++++
 utils/btc_utils.go         | 38 ++++++++++++++++++++++++++++++++++++++
 walletcontroller/client.go |  9 +++++++++
 5 files changed, 66 insertions(+)
 create mode 100644 utils/btc_utils.go

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 700d34e..9bbf12f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -44,6 +44,7 @@ configuration to enabled it (disabled by default).
 * [#114](https://github.com/babylonlabs-io/btc-staker/pull/114) **Multi-staking support**.
 This PR contains a series of PRs on multi-staking support and BTC staking integration.
 * [#134](https://github.com/babylonlabs-io/btc-staker/pull/134) Removal of both the watch-staking endpoint and the post-approval flow, and reduction of state in the database.
+* [#146](https://github.com/babylonlabs-io/btc-staker/pull/146) Sanity check that all transactions are standard
 
 ## v0.15.2
 
diff --git a/staker/babylontypes.go b/staker/babylontypes.go
index ed194ef..a179e87 100644
--- a/staker/babylontypes.go
+++ b/staker/babylontypes.go
@@ -133,6 +133,18 @@ func (app *App) buildDelegation(
 		return nil, fmt.Errorf("failed to receive unbondingSlashingSig.Signature ")
 	}
 
+	// sanity check that all our transactions are standard
+	// if they are not this can mean bug either in Babylon parameters or in Staker code
+	if err := utils.CheckTransaction(stakingSlashingTx); err != nil {
+		return nil, fmt.Errorf("failed to build delegation data: failed to build staking slashing tx: %w", err)
+	}
+	if err := utils.CheckTransaction(undelegationDesc.UnbondingTransaction); err != nil {
+		return nil, fmt.Errorf("failed to build delegation data: failed to build unbonding tx: %w", err)
+	}
+	if err := utils.CheckTransaction(undelegationDesc.SlashUnbondingTransaction); err != nil {
+		return nil, fmt.Errorf("failed to build delegation data: failed to build unbondingslashing tx: %w", err)
+	}
+
 	dg := createDelegationData(
 		req,
 		externalData.stakerPublicKey,
diff --git a/staker/types.go b/staker/types.go
index 686c938..a8f174d 100644
--- a/staker/types.go
+++ b/staker/types.go
@@ -12,6 +12,7 @@ import (
 
 	cl "github.com/babylonlabs-io/btc-staker/babylonclient"
 	"github.com/babylonlabs-io/btc-staker/stakerdb"
+	"github.com/babylonlabs-io/btc-staker/utils"
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcec/v2/schnorr"
 	"github.com/btcsuite/btcd/btcutil"
@@ -211,6 +212,11 @@ func createSpendStakeTx(
 		return nil, nil, fmt.Errorf("too big fee rate for spend stake tx. calculated fee: %d. funding output value: %d", fee, fundingOutput.Value)
 	}
 
+	// sanity check that transaction is standard
+	if err := utils.CheckTransaction(spendTx); err != nil {
+		return nil, nil, fmt.Errorf("failed to build spend stake tx: %w", err)
+	}
+
 	return spendTx, &fee, nil
 }
 
diff --git a/utils/btc_utils.go b/utils/btc_utils.go
new file mode 100644
index 0000000..3949dcf
--- /dev/null
+++ b/utils/btc_utils.go
@@ -0,0 +1,38 @@
+package utils
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btcd/mempool"
+	"github.com/btcsuite/btcd/txscript"
+	"github.com/btcsuite/btcd/wire"
+)
+
+const (
+	minTransactionSize = 65
+)
+
+// Perform subset of transactions standard tests:
+// - whether transaction is not considered dust
+// - whether transactions is not too small
+func CheckTransaction(tx *wire.MsgTx) error {
+	if tx.SerializeSizeStripped() < minTransactionSize {
+		return fmt.Errorf("transaction is too small. Tx size: %d, min size: %d", tx.SerializeSizeStripped(), minTransactionSize)
+	}
+
+	numOpReturns := 0
+	for _, txOut := range tx.TxOut {
+		scriptClass := txscript.GetScriptClass(txOut.PkScript)
+		if scriptClass == txscript.NullDataTy {
+			numOpReturns++
+		} else if mempool.IsDust(txOut, mempool.DefaultMinRelayTxFee) {
+			return fmt.Errorf("transaction output is dust. Value: %d", txOut.Value)
+		}
+	}
+
+	if numOpReturns > 1 {
+		return fmt.Errorf("transaction has more than one op_return output")
+	}
+
+	return nil
+}
diff --git a/walletcontroller/client.go b/walletcontroller/client.go
index ae333d6..d47aea6 100644
--- a/walletcontroller/client.go
+++ b/walletcontroller/client.go
@@ -11,6 +11,7 @@ import (
 	"github.com/babylonlabs-io/babylon/crypto/bip322"
 	"github.com/babylonlabs-io/btc-staker/stakercfg"
 	"github.com/babylonlabs-io/btc-staker/types"
+	"github.com/babylonlabs-io/btc-staker/utils"
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcec/v2/schnorr"
 	"github.com/btcsuite/btcd/btcjson"
@@ -256,6 +257,14 @@ func (w *RPCWalletController) CreateTransaction(
 		return nil, err
 	}
 
+	err = utils.CheckTransaction(tx)
+
+	if err != nil {
+		// returning error here means our tx building code is buggy, but it will save
+		// user from submitting invalid transaction to the network
+		return nil, fmt.Errorf("transaction is not standard: %w", err)
+	}
+
 	return tx, err
 }