BIP Proposal: Stateless PSBT Coordination (Blind Relay)

The draft BIP for Stateless PSBT Coordination aims to address the challenges in coordinating multisig transactions across different hardware and software wallets, which currently require either manual file transfers or reliance on a centralized server. The proposed solution introduces a "Blind Relay" model utilizing an ephemeral, end-to-end encrypted WebSocket architecture, designed to enhance security and privacy. In this model, the relay server does not have access to the Partially Signed Bitcoin Transactions (PSBT), extended public keys (xpubs), or signatures. This ensures zero-knowledge operation, where all sensitive data remains unseen by the server.

Furthermore, the protocol is designed to be stateless; all data related to the transaction room is deleted immediately when the room is closed or after 24 hours. This approach addresses concerns about data persistence and privacy leakage. The operational security (OpSec) of the setup is bolstered by requiring that the room URL and decryption key (a key fragment) are shared out-of-band, meaning they are transported separately by the users themselves, outside of the online platform.

To demonstrate the feasibility and practical application of this BIP, an open-source reference implementation called "Signing Room" has been developed. The latest release, v1.8.0, focuses on improving operational security by ensuring that the transport of the room link and decryption key are handled separately within the user interface. This split-key approach is pivotal in balancing usability with security, aiming to mitigate risks associated with key handling and transportation.

Community feedback is sought particularly on the draft BIP available at (bip-stateless-psbt-coordination) and thoughts on the user experience and operational security tradeoffs implemented in the new version of Signing Room. Insights on potential features or improvements for the reference implementation are also highly encouraged to refine and enhance the proposed standard.