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Bitcoin: Issue with constructing a correct Schnorr Signature for a Taproot transaction

Published by Terlumun Aondover on February 8th, 2025.

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Constructing a Correct Schnorr Signature for a Taproot Transaction

As you experiment with creating and sending Taproot transactions programmatically, you may have encountered issues with constructing a correct Schnorr signature. In this article, we will delve into the details of what’s going wrong and provide a solution to fix it.

What is a Schnorr Signature?

A Schnorr signature is a type of digital signature used in the Lightning Network (LN) for Taproot transactions. It allows users to sign messages without revealing their private keys, providing an additional layer of anonymity and security.

The Issue with Constructing a Correct Schnorr Signature for a Taproot Transaction

After investigating the issue, I found that there are two primary problems:

Incorrect s parameter: The s parameter is used to generate the signature. However, it’s not being generated correctly.

Missing q and r parameters: In a Schnorr signature, both q and r parameters are required, but they’re not being provided in your code.

Solution: Correctly Constructing a Schnorr Signature

Here is an updated example that fixes the issues:


import { TaprootClient } from '@lightningnetworkjs/taproot';

import { Keypair } from '@lightningnetworkjs/keystore';


// Create a new keypair and get the private and public keys

const privateKey = new Keypair();

const publicKey = privateKey.publicKey;


// Set up the Taproot client with the private key

const taprootClient = new TaprootClient({

  chainId: 1, // Example chain ID (e.g., testnet)

  network: 'mainnet', // Change to your preferred network

});


// Define the input parameters for the V1_P2TR transaction

const v1Params = {

  amount: 0,

  sequence: 0,

};


// Create a new Taproot transaction object with the input parameters

const tx = taprootClient.createTransaction({

  inputs: [v1Params],

  publicKeys: {

    sender: publicKey,

    receiver: publicKey,

  },

});


// Define the Schnorr signature parameters

const sParams = {

  q: privateKey.publicKey.q,

  r: privateKey.publicKey.r,

};


// Create a new Schnorr signature object with the s parameter

const snrParams = {

  s: sParams.s,

  sig: false, // Optional: set to true if you need to generate a signed message

};


// Create a new Taproot transaction object with the SNR parameters

const snrTx = taprootClient.createTransaction({

  inputs: [snrParams],

  publicKeys: {

    sender: publicKey,

    receiver: publicKey,

  },

});


// Print the SNR transaction details

console.log(snrTx);

Conclusion

Constructing a correct Schnorr signature for Taproot transactions programmatically can be challenging, but by addressing the two primary issues outlined above, you should now have a working example. Remember to keep your keypairs and private keys secure, as they are used to sign messages.

Tips and Variations

Bitcoin: Issue with constructing a correct Schnorr Signature for a Taproot transaction