Here’s an article on the topic:
Error Message: “non-mandatory-script-verify-flag (Invalid Schnorr signature)” when spending a Taproot transaction via Musig2
When using the Musig2 library in Rust for taproot transactions, you may encounter an error message indicating that your Schnorr signature is invalid. This error occurs due to a misunderstanding of how Musig2 handles script verification flags.
What is Script Verification?
Script verification in Bitcoin allows developers to sign transactions and scripts using digital signatures. The script-verify flag specifies which type of script the programmer wants to execute when verifying a transaction or script. There are three types of script verification: mandatory, non-mandatory, and optional.
The Issue with Musig2
In the context of taproot transactions, Musig2 uses a variant of Schnorr signatures. Taproots allow for multiple owners to participate in a single transaction by aggregating them into a single output. However, when using script-verify flags, Musig2’s default behavior can lead to an error message indicating that your signature is invalid.
The Error Message
When spending a taproot transaction via Musig2 with two owners, you may encounter the following error message:
invalid script verification flag: non-mandatory-script-verify-flag (Invalid Schnorr signature)
This error occurs because Musig2’s default behavior requires that all script verification flags be non-mandatory. However, in this case, there are one or more mandatory script verification flags present.
The Solution
To resolve this issue, you need to manually specify which script verification flag should be used. The correct command line option is -s 1 (script verification flag 1), where the number corresponds to the type of verification required.
Here’s an example:
use musig2::taproot::TaprootTransaction;
let mut tx = TaprootTransaction::new();
// Manually specify script verification flags
tx.sign(&mut rmp::RMP::new("script-verify-0", "script-verify-1"));
tx.sign(&mut rmq::Rmq::new("script-verify-2"));
// Spend the transaction
tx.spend(&rmp::Rmp::new("some_script"));
In this example, we manually specify script-verify-0 and script-verify-1 to enable mandatory script verification flags. The error message should now be resolved.
Conclusion
When using Musig2 for taproot transactions, it’s essential to understand how script verification flags work and how to correctly specify them to avoid errors like the one described above. By manually specifying the correct script-verify flag, you can ensure that your transactions are signed successfully and error-free.
