Understanding the “Optional script verify flag (invalid Schnorr signature)” error in Bitcoin

When attempting to verify the Schnorr signature for a Bitcoin transaction, some users may encounter an error known as “non-mandatory-script-verify-flag (invalid Schnorr signature)”. This issue can occur despite successful signature verification with a valid script flag.

Issue: Invalid signature

In the Bitcoin system, a Schnorr signature is used to verify that the sender intended to submit a specific transaction. When a user creates a Schnorr signature, they specify which public key in the sender’s wallet should be used for the signature. The signature itself contains additional information, such as the input data and the expected output (i.e., the transaction hash).

The script flag is an important part of a Schnorr signature. This specifies how the user wants to spend the coins allocated for the transaction. The optional script verify flag error occurs when the script flag is present but not used correctly.

Possible causes of error

There are several reasons why this error can occur despite a successful Schnorr signature verification:

• Inconsistent script flags: If the script flags in the sender’s wallet do not match the expected input data, this can result in an invalid signature.

• Invalid input: If the input data used to create the signature does not match the expected value, this can also result in a bad signature.

• Script flag mismatch: For a Schnorr signature to be valid, a script flag must be present and specified correctly. However, if the flag is missing or has incorrect values, this can cause problems.

Solutions and Workarounds

To resolve this error, you may need to do the following:

• Check your script flags

  : Make sure the script flags in your sender wallet match the expected input.

• Recheck input: Make sure the input used to generate the signature matches the expected value.

• Check for consistent script flags: Check that the script flags do not conflict with each other.

Code example

Here is an example of how to create a Schnorr signature and verify it using the bitcoin-schnorr command line tool.

#!/bin/bash

Create sender's public key
pk = "your public key here"

Define transaction input data
input_data = "your_input_data_here"

Create Schnorr signature
schnorr_signature=$(bitcoin-schnorr -txin <(echo "$pk") --txin <<< "$input_data")

Verify signature with expected script flag (1)
verifier_script_flag=1
signature = $(bitcoin-schnorr -txin <(echo "$pk") --txin <<< "$input_data" --script-verify-flag $verifier_script_flag)

Check for errors
if [ $? -not 0 ]; Then
echo "Error: invalid Schnorr signature"
other
echo "Success: Schnorr signature verification with valid script flag ($verifier_script_flag)"
fi

Submit the transaction with a verified signature
tx_hash = $(bitcoin-cli send --txid <(echo "$schnorr_signature") -pubkey <(echo "$pk") --script-verify-flag 1)

Completion

The “non-mandatory-script-verify-flag (invalid Schnorr signature)” error can occur despite successful Schnorr signature verification. By understanding the possible causes and developing possible solutions, you should be able to resolve this issue and successfully submit the transaction with the verified signature.