Q-Lock: Quantum-Resistant Spending via ECDSA + Hash-Based Secrets

This protocol integrates a hash-based secret layer atop ECDSA, employing SHA256 and Merkle trees, which are already proven cryptographic methods. The essence of Q-Lock lies in its ability to safeguard transactions through a two-phase commit-reveal scheme, ensuring that quantum computers cannot reverse-engineer SHA256 to compromise transactions. With transaction sizes approximately 3 KB—similar to those of FALCON signatures—and compatibility with soft forks and BIP-32 HD wallets, Q-Lock presents a promising alternative to current proposals like BIP-360 P2QRH.

Q-Lock's methodology is straightforward yet effective. Initially, users generate a standard ECDSA keypair along with a series of 64 random secrets. These secrets form the basis of a commitment array, which is then structured into a Merkle tree. The resulting Merkle root, alongside the public key hash, is stored on-chain, while the secrets and their corresponding tree are kept within the user's wallet. To execute a transaction, the first phase involves locking the transaction outputs without exposing the public key, thereby rendering it secure from quantum decryption attempts. The second phase requires revealing the public key and selected secrets based on the block hash at the time of the transaction, finalizing the spend securely.

Furthermore, Q-Lock addresses are to adopt a new format, incorporating witness versioning and Bech32m encoding for enhanced security and efficiency. The spending process, divided into the aforementioned commit and reveal phases, demands specific validation rules to ensure integrity and resistance against quantum attacks. Notably, the reveal phase's requirement for the original commit transaction's details ensures that only the rightful owner can complete the transaction, effectively nullifying any potential quantum threat.

Security analyses underscore Q-Lock's resilience against various theoretical quantum attack vectors, including those leveraging Shor's algorithm and Grover's algorithm. The protocol's design inherently binds the transaction to the user's unique secrets and transaction ID, significantly mitigating the risk posed by quantum computing advancements.

To facilitate Q-Lock's implementation, the introduction of one or two new opcodes is suggested, enabling seamless integration with existing Bitcoin infrastructure. This soft-fork-compatible approach ensures backward compatibility, allowing users to gradually transition to Q-Lock addresses without necessitating a network-wide mandate.

In summary, Q-Lock represents a forward-thinking solution to the quantum threat looming over blockchain technology, particularly Bitcoin. By maintaining the core cryptographic practices of Bitcoin while introducing an additional quantum-resistant layer, Q-Lock offers a balance between innovation and tradition, ensuring the longevity and security of Bitcoin transactions in the face of evolving quantum computational capabilities.