Understanding the “Non-mandatory-script-verify-flag (Invalid Schnorr signature)” Error in Bitcoin

When attempting to verify a 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 verification of the signature with a valid script flag.

The Issue: Invalid Signature

In Bitcoin, a Schnorr signature is used to verify that a sender intended to broadcast a particular 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 component of a Schnorr signature. It specifies how the user wants to spend the coins intended for the transaction. A non-mandatory-script-verify-flag error occurs when the script flag is present but not used correctly.

Possible Causes of the Error

There are several reasons why you may encounter this error despite successful verification of your Schnorr signature:

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

• Invalid Input Data: If the input data used to generate the signature does not match the expected value, it can also lead to an incorrect signature.

• Script Flag Mismatch: The script flag must be present and correctly specified for a valid Schnorr signature. However, if the flag is missing or has incorrect values, it can cause issues.

Workarounds and Solutions

To resolve this error, you may need to:

• Verify Your Script Flags: Make sure that the script flags in your sender’s wallet match the expected input data.

• Re-check Input Data: Ensure that the input data used to generate the signature matches the expected value.

• Check for Inconsistent Script Flags: Verify that the script flags do not conflict with each other.

Example Code

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

#!/bin/bash



Create a sender's public key

pk = "your_public_key_here"



Define the input data for the transaction

input_data = "your_input_data_here"



Create a Schnorr signature

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



Verify the signature with the 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 [ $? -ne 0 ]; then

    echo "Error: Invalid Schnorr signature"

else

    echo "Success: Verifying Schnorr signature with valid script flag ($verifier_script_flag)"

fi



Broadcast the transaction using the verified signature

tx_hash=$(bitcoin-cli send --txid <(echo "$schnorr_signature") -pubkey <(echo "$pk") --script-verify-flag 1)

Conclusion

The “non-mandatory-script-verify-flag (Invalid Schnorr signature)” error can occur despite successful verification of a Schnorr signature. By understanding the possible causes and working through potential solutions, you should be able to resolve this issue and successfully broadcast your transaction using the verified signature.

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