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Error message: “non-mandatory-script-verify-flag (invalid Schnorr signature)” when making a Taproot transaction using Musig2
When using the Musig2 library in Rust for master rooted transactions, you may receive an error message stating that the Schnorr signature is invalid. This error occurs due to a misunderstanding of how Musig2 handles script validation flags.
What is script validation?
Bitcoin script verification allows developers to sign transactions and scripts using digital signatures. The script-verify flag specifies what type of script the programmer wants to run when verifying a transaction or script. There are three types of script validation: mandatory, optional, and optional.
Problem with Musig2
In the context of master rooted transactions, Musig2 uses a variant of Schnorr signatures. Taproots allow multiple owners to participate in a single transaction by aggregating them into a single output. However, when using the script-verify flags, Musig2’s default behavior may result in an error message indicating that the signature is invalid.
Error Message
When passing a master transaction through Musig2 with two owners, you may receive the following error message:
Invalid script validation flag: Optional script validation flag (invalid Schnorr signature)
This error occurs because Musig2’s default behavior requires all script validation flags to be “optional”. However, in this case, there are one or more mandatory script validation flags.
Solution
To work around this issue, you need to manually specify which script validation flag to use. The correct command line option is -s 1 (script validation flag 1), where the number corresponds to the type of validation required.
Here is an example:
use musig2::taproot::TaprootTransaction;
let mut tx = TaprootTransaction::new();
// Manually specify the script validation flags
tx.sign(&mut rmp::RMP::new("script-verify-0", "script-verify-1"));
tx.sign(&mut rmq::Rmq::new("script-verify-2"));
// Elapse the transaction
tx.spend(&rmp::Rmp::new("some_script"));
In this example, we manually specify script-verify-0 and script-verify-1 to enable the required script verification flags. The error message should now be resolved.
Conclusion
When using Musig2 for root-master transactions, it is important to understand how script validation flags work and how to specify them correctly to avoid errors like the one described above. By manually specifying the correct script-verify flag, you can ensure that your transactions are signed correctly and without errors.
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