The game-theory problems of PQ sunsetting modes

The introduction of a "tripwire" scenario, involving a NUMS puzzle set within a honeypot UTXO offering Bitcoin rewards, is technically plausible but brings forth critical questions about the rationality behind a quantum computing entity's decision to solve such a visible puzzle. This would reveal its capabilities prematurely, forfeiting the chance for covert operations which might be more beneficial. Furthermore, this scenario hints at possible collusion between a quantum computing entity and a majority of miners, aiming to block the inclusion of post-quantum proofs on the blockchain. Such actions could facilitate the unauthorized acquisition of legacy P2Pk coins valued at approximately $215 billion, providing a substantial economic motive for miners.

The discussion extends into deeper strategic shifts where two competing quantum entities could lead miners to support the one offering higher reorganization fees, thus favoring overt actions over covert strategies. Also highlighted is the potential vulnerability of the Bitcoin network due to the elimination of checkpoints in its software along with its heavy reliance on miner consensus. This could expose the network to significant risks, especially targeting legacy coins not converted to quantum-resistant formats. The potential for blockchain rollback to states prior to migration poses a dire threat to both legacy and upgraded coins alike.

In another detailed examination, the viability and implications of executing a chain rollback are scrutinized. This involves miners opting to mine an older blockchain version (termed the "revisionist" chain) instead of continuing with the established ("authentic") chain. This action would necessitate miners to abandon immediate rewards from the authentic chain unless the cumulative proof-of-work of the revisionist chain surpasses it. The success of mining the revisionist chain represents a massive betrayal to the system’s integrity, akin to a large-scale double-spend attack, potentially causing significant disruption in the market and relationships with exchanges.

Lastly, the correspondence suggests difficulties in implementing preventive measures like a sunset fork in the presence of competitive quantum computing entities (CQRCs), where incentives might undermine the efforts entirely. The possibility of mining silent reorganizations as a strategic maneuver among miners is proposed, highlighting a complex interplay of resources and incentives. There’s also a speculative solution involving PQ-blessed periodic checkpoints which might cap the maximum unsafe coins used as reorg budgets by establishing consensus rules for checkpoint finalization by PQ-safe public keys. However, these ideas still require further conceptual development to address the nuanced challenges posed by quantum advancements in blockchain technologies.