Post Quantum Signatures and Scaling Bitcoin

In a recent discussion within the Bitcoin Development Mailing List, two significant points regarding scalability and transaction aggregation were raised, shedding light on the nuanced challenges facing blockchain technology. The conversation began with an examination of the practicality of post-quantum (PQ) cryptographic schemes in the context of blockchain scalability. It was noted that while PQ schemes might increase the on-chain size due to their larger cryptographic keys, they could potentially offer lower per-byte computation costs. This distinction between "size" (bandwidth/storage requirements) and "computation" (CPU/IO demands) aspects of scalability suggests that a higher discount factor for PQ data could be justified, despite the increased size. However, this point does not address concerns related to JPEG resistance, indicating a need for further exploration of how size and computation trade-offs affect blockchain resilience against quantum computing threats.

The discussion then turned to the concept of pre-aggregation of transactions as a means to enhance scalability. Pre-aggregation, beyond a single transaction wide, was critiqued for its potential to disrupt in-mempool transaction replacement. This process involves bundling a group of dependent transactions together for relay as an atomic package, which must be accepted or rejected in its entirety. This approach poses a problem when different nodes relay partially overlapping sets of transactions due to not all transactions being visible to all nodes at the same time. As a result, this could force the network or miners to choose between competing transaction sets unless individual transactions are also available independently. Such a scenario is seen as detrimental to the decentralized consensus mechanism fundamental to blockchain technology. It raises concerns that mining could become permissioned and less competitive without direct-to-miner transaction submissions, challenging the core principles of decentralization and open access that underlie Bitcoin and similar cryptocurrencies.