PQC output type discussion

The discussion revolves around the strategic implications of adopting the P2TRv2 protocol, emphasizing the potential for mempool congestion and fee rate spikes which businesses and projects must account for in their long-term planning. The introduction of CISA into the P2TRv2 mix is considered a possibly complex addition that might hinder rather than help the broad adoption of post-quantum cryptography (PQC) due to its increased specification and softfork requirements. However, it is noted that for certain implementations, such as single-signer wallets, CISA could simplify execution.

The debate extends to the impact on blockchain technology, particularly concerning witness data and block size in a post-quantum scenario. A significant block size increase might be necessary to accommodate hash-based signatures, potentially leading to substantial storage requirements. These changes are likely to affect archival nodes significantly, pushing the need for innovative solutions like sharded storage using Forward Error Correction techniques. Furthermore, there's an acknowledgment that while some blockchain properties can be mitigated through technological advances, the overall cost to network operators could become prohibitive, leading to a preference for pruning nodes.

Verification and storage costs are crucial considerations; the former being a one-time cost per node and the latter an ongoing expense. With advancements in technology, methods like multithreaded verification have been introduced, which Bitcoin has implemented since 2013. Despite these improvements, the increasing demands on bandwidth and CPU for validation remain pressing concerns underpinning the need for a balanced approach to blockchain resource limits.

Additionally, there’s a discussion about future-proofing blockchain technology against quantum threats by provisioning additional capacity specifically for post-quantum signatures. This strategy involves setting aside a larger "pqdata" area in anticipation of a quantum event ("Q-day"), which would adjust the ideal block size significantly from current standards to accommodate new cryptographic practices without overwhelming the system immediately if Q-day does not occur.

Overall, the conversation underscores a cautious yet proactive approach to evolving blockchain architecture, taking into account both current technological capabilities and future needs, to ensure scalability, security, and widespread adoption.