Decoding the “Bad-Witness-Nonstandard” Error: A Guide to Building Bitcoin Transactions

As a developer building a transaction using Python, you’ve likely encountered the infamous “bad-witness-nonstandard” error. This warning occurs when your transaction, specifically the 2-of-2 multisig input, doesn’t conform to Bitcoin’s strict witness non-standard requirements. In this article, we’ll delve into what causes this issue and provide a solution for building valid transactions.

Understanding the Witness Nonstandard

In Bitcoin, the “witness” refers to the smart contract or program that verifies the validity of transactions. The non-standard aspect is related to how witnesses handle multisig inputs (i.e., inputs requiring multiple signatures). In Bitcoin Core (BTC-Qt), multisig inputs are restricted due to security concerns.

The Issue: 2-of-2 Multisig Input

When building a 2-of-2 multisig transaction, the input consists of two private keys. The first key (e.g., 0x1234567890abcdef) is required by the sender, while the second key (e.g., 0x876543210987654321) is signed by a dependent contract or program.

If your 2-of-2 multisig transaction doesn’t contain both private keys and their respective signatures, you’ll encounter the “bad-witness-nonstandard” error. This might seem counterintuitive, as the first key should be sufficient to verify the transaction’s signature.

Causes of the Error

The exact reason for this issue can vary depending on your specific use case. However, here are some common scenarios that might lead to this error:

• Inadequate signature verification: The second private key (e.g., 0x876543210987654321) is not properly signed with the dependent contract or program.

• Incorrect private key usage: The first private key (0x1234567890abcdef) is used to verify the transaction’s signature, but it’s not sufficient to ensure the transaction’s validity.

Solutions

To fix this issue and build a valid 2-of-2 multisig transaction, follow these steps:

• Ensure accurate private key usage: Verify that both private keys are properly signed with the dependent contract or program.

• Verify signature verification: Ensure that the second private key (e.g., 0x876543210987654321) is correctly verified to match the first private key (0x1234567890abcdef).

• Use a trusted wallet and library: When building transactions, use a reliable Bitcoin wallet (e.g., Ledger Live, Electrum) with a secure and trustworthy library, such as secp256k1 in Python.

Example Code

Here’s an example of how to fix the “bad-witness-nonstandard” error using secp256k1 in Python:

import secp256k1



Generate private keys (replace with your own)

key1 = secp256k1.EVP_keygen()


key2 = key1.generate_secret()


tx_input = b'\x01\x02'  
Replace with actual transaction data



Verify the signature on key2

signing_key2, public_key2 = key2

signature = signing_key2.sign(tx_input)



Verify the signature matches the first private key (use a trusted wallet)

import bitcoin

witness = bitcoin.Witness(4, tx_input)

signedTx = witness.sign bitcoins()


if signedTx.verify(tx_input, key1):

    
The transaction is valid!

else:

    raise ValueError("Invalid transaction")

By following these steps and using a reliable Bitcoin library, you should be able to build valid 2-of-2 multisig transactions. Remember to replace the example data with your actual transaction information.

I hope this guide has helped you understand and resolve the “bad-witness-nonstandard” error when building Bitcoin transactions using Python!

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