Multisig Digital Bearer Instruments - peer to peer electronic cash

Traditionally, the issuer holds significant control over the spending capabilities within multi-signature schemes. However, this new protocol, as outlined in the updated paper available here full paper and protocol specification, shifts the power balance by ensuring that the issuer remains permanently below the spending threshold while the bearer retains full control over the keys necessary for transactions. This setup inherently disables the issuer from interfering with or facilitating transactions, which is enforced not by policy but by the construct of Bitcoin's consensus rules themselves.

Significant contributions of this protocol include an enhanced buyer-generated key issuance process that effectively mitigates risks associated with malicious issuers. This is achieved through a procedure where the buyer autonomously generates their keys and shares only the public counterparts with the issuer, thereby eliminating any opportunity for the issuer to access the private keys. Moreover, the integration of a Nostr receipt mechanism provides an additional layer of security by requiring cryptographic proof of possession before the sender deletes their keys, ensuring the receiver's ownership is established beyond doubt prior to deletion.

Practical implementation of these concepts has been demonstrated on the Bitcoin Signet with successful batch funding and peer-to-peer transfers verified, showcasing the protocol's operational viability. While certain features like NFC bindings and offline verification remain under development, the core functionalities including dual-node UTXO verification and e-commerce merchant integration via a headless Key Management Server (KMS) are operational. This KMS setup facilitates secure transaction processes by verifying and sweeping UTXOs on-chain, further supporting immediate finality through webhook notifications to merchant platforms.

Open questions regarding the protocol focus on the robustness of the NC1 security gate, which combines cryptographic receipt verification with UTXO confirmation to safeguard against premature key deletion. Feedback is sought to explore potential vulnerabilities within this mechanism, especially in scenarios involving unfunded addresses or forged receipts. Finally, the protocol's adaptability to physical instruments using TEE attestation as an alternative method highlights ongoing efforts to refine and enhance the security features surrounding digital and potentially physical bearer instruments. These developments represent a significant shift towards more secure, decentralized financial transactions in the Bitcoin ecosystem.