Changes to BIP-360 - Pay to Quantum Resistant Hash (P2QRH)

By transitioning Pay to Quantum Resistant Hash (P2QRH) to a script-only version of Taproot (P2TR), the proposal eliminates the quantum-vulnerable key-spend pathway. This ensures that P2QRH outputs are directly committed to the tapleaf merkle root, enhancing security against quantum attacks. The scriptPubKey for P2QRH outputs adopts a concise format, demonstrating these changes while leveraging the existing taproot infrastructure for smooth integration and familiarity among users.

The strategic removal of taptweak steps in favor of direct script and tapleaf support simplifies the implementation process and significantly bolsters protection against long-exposure quantum attacks, which are likely to precede short-exposure ones. This approach provides a viable alternative to the drastic measure of disabling key-spends in P2TR in the event of a quantum breakthrough, thus offering the Bitcoin ecosystem a buffer period to adjust without the looming threat of emergency consensus changes.

Moreover, BIP-360's updated framework lays the groundwork for future developments concerning Post-Quantum (PQ) signatures by separating their specification from the main proposal. This allows for the independent evaluation and adoption of PQ signature algorithms, ensuring that progress on P2QRH is not hindered by the ongoing debate over which PQ signatures to adopt. The proposal hints at introducing tapscript PQ signature verification opcodes for ML-DSA (CRYSTALS-Dilithium) and SLH-DSA (SPHINCS+) through OP_SUCCESSx opcodes, offering a flexible and forward-looking mechanism for integrating new signature algorithms as they become available. This approach not only facilitates the eventual activation of PQ signatures but also establishes a protocol for incorporating emerging cryptographic technologies into the Bitcoin network, thereby reinforcing its quantum resistance and overall security posture.