Understanding the “Invalid-Schnorr-signature” error in Bitcoin

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

Problem: Invalid signature

In Bitcoin, a Schnorr signature is used to verify that the sender intended to send 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 part of a Schnorr signature. It specifies how the user wants to spend the coins intended for the transaction. The optional script-verify-flag error occurs when a script flag exists but is not used correctly.

Possible causes of the error

There are several reasons why you might encounter this error despite successfully verifying your Schnorr signature:

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

• Invalid input data: If the input data used to create the signature does not match the expected value, it can also result in an invalid signature.

• Script flag mismatch: The script flag must exist and be configured correctly for a valid Schnorr signature. However, if the flag is missing or has incorrect values, it can cause problems.

Workarounds and Solutions

To resolve this error, follow these steps:

• Check script flags: Make sure the script flags in the sender’s wallet match the expected input.

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

• Check for inconsistent script flags: Make sure the script flags do not conflict with each other.

Sample Code

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

#!/bin/bash





Generate the sender's public key
pk = "your_public_key_here"

Specify the transaction input data
input_data = "your_input_data_here"

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

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

Check for errors
if [ $? -and 0]; then
echo "Error: Invalid Schnorr signature"
else
echo "Success: Verifying Schnorr signature with valid script flag ($verifier_script_flag)"
steal

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

Conclusion

The “No-mandatory-script-verify-flag (Invalid Schnorr signature)” error can occur despite successful Schnorr signature verification. By understanding the possible causes and applying the possible solutions, you should be able to resolve this issue and successfully send your transaction using a verified signature.

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