QR-based signing flow payloads in Miniscript context

The recent discussions and developments surrounding QR-based signing flows, particularly with the integration of Miniscript, have highlighted several challenges and innovative approaches to enhancing transaction security and user experience in cryptocurrency transactions. Traditional multisig protocols facilitate the transfer of descriptors, xpubs, and PSBTs between software wallets and air-gapped devices effectively using a common template. However, Miniscript introduces complexities such as customizable spending policies that may incorporate features like timelocked recovery or decaying thresholds, which are not well-supported by existing QR protocols.

To address these complexities, a proposal has been made to define a comprehensive set of payloads to support the entire Miniscript flow over QR. These payloads would potentially include JSON for readability, though a bytes/binary encoding might be preferred to economize space and minimize QR frames, enhancing efficiency in data transfer without committing to a specific wire format at this stage.

One of the key additions with Miniscript is the flexible retrieval of extended public keys (xpubs). The software wallet could be enhanced to request multiple xpubs from various derivation paths in one go. This would not only simplify user interaction by reducing the need for multiple QR scans but also support advanced backup encryption schemes, such as those discussed here. The device's response to these requests would include the xpubs along with optional information about the device model and firmware version, which could be crucial for ensuring compatibility and functionality, particularly with newer cryptographic methods like tapminiscript.

Another significant proposal is the registration and verification of descriptors on the signing devices. This process would allow the device to display and let users verify new or previously unregistered descriptors, addressing potential issues with devices that have limited storage or are stateless. The registration request from the software might include an alias for the descriptor and optional parameters related to different BIPs, while the device’s response could confirm the registration or return an error.

Furthermore, enhancing the signing process itself, where the device needs precise information about which descriptor a transaction relates to, is critical. This ensures the inputs and outputs can be accurately labeled, improving both the security and user experience on the device. The proposed method involves the software sending detailed information about each transaction, including descriptors and proofs of registration if necessary, and the device returning either the full PSBT or just the signatures, depending on the setup preferences and QR bandwidth considerations.

This structured approach aims to refine the communication and operational efficiency between software wallets and hardware signing devices, ultimately leading to more secure and user-friendly cryptocurrency transactions. This proposal is open for feedback, particularly from developers who are already working with or planning to integrate Miniscript into their solutions.