Against Allowing Quantum Recovery of Bitcoin

In a recent discussion on the Bitcoin Development Mailing List, Thomas Suau highlighted some technical considerations and ambiguities related to the implementation of UTXOs (Unspent Transaction Outputs) in blockchain nodes. The primary focus was on ensuring that nodes are already storing blocks to verify UTXO confirmations relative to timelock validity, which is deemed implementable. However, certain parts of the protocol's explanation were identified as unclear, necessitating further clarification.

One of the key points raised involved the potential risks associated with predictable cliffs in the protocol, which could potentially attract adversarial behavior. The exact nature of these risks wasn't specified, prompting a call for a more detailed explanation of how such scenarios could unfold. Additionally, there was a concern about the protocol's approach to handling old transactions, particularly in relation to phasing out insecure constructions without retroactively invalidating them. The method by which this balance is achieved remains somewhat vague, especially considering the implications for very old UTXOs if a relative max age less than the total age of Bitcoin is set.

Another significant area of ambiguity pertains to the differentiation between legitimate and quantum-recovered spends within the protocol. This distinction raises questions about the feasibility of identifying whether a transaction was executed by a quantum attacker, an issue that currently lacks a clear resolution mechanism within the discussed framework.

These points underscore the need for more precise definitions and explanations within the protocol to prevent misinterpretations and ensure robust security measures against potential threats, including those posed by advancements in quantum computing.