Bitcoin Unlimited, a notable fork of the Bitcoin Core software, has
garnered attention in recent months for its proposal to hand control of
Bitcoin's block size limit to users and miners. While this idea offers
increased flexibility, it also introduces several challenges and potential
risks to the network's stability and consensus mechanism.
If not all significant Bitcoin implementations adopt Bitcoin Unlimited's proposal, the blockchain could split into two incompatible networks and currencies. In such a scenario, users on the one-megabyte chain would experience slower block confirmations and face the risk of replay attacks when attempting to use the "Bitcoin Unlimited chain." Conversely, Bitcoin Unlimited users would face uncertainty about the longevity of their chain, as it could be discarded if the one-megabyte chain surpasses it in length.
Even if all users switch to Bitcoin Unlimited, meaningful convergence on a single chain is not guaranteed. Users can configure their nodes to remain out of consensus with the rest of the network, leading to different chains coexisting. This lack of convergence raises security concerns, as transactions with fewer confirmations become less secure, increasing the risk of double-spending attacks.
To achieve a higher degree of convergence, Bitcoin Unlimited users can set a low Excessive Acceptance Depth (AD) level. However, this compromises the security of transactions with fewer confirmations and effectively gives a majority of miners the ability to override block size settings. This undermines the purpose of Bitcoin Unlimited's proposal to empower users and miners with control over block sizes.
Under adversarial conditions, the security assumptions of Bitcoin Unlimited further degrade. Node signaling can be easily spoofed, leading miners to fork a chain with less user support than perceived. Malicious miners can intentionally mine blocks that fall between the block size limits accepted by other miners, causing chain splits. An adversary with a small percentage of global hash power could repeatedly cause network turmoil and exploit the inconsistent acceptance of block sizes.
Bitcoin Unlimited proponents rely on the concept of "Emergent Consensus," suggesting that participants in the Bitcoin ecosystem have sufficient economic incentives to find consensus on a single blockchain, even if their software does not ensure automatic convergence.
While Bitcoin Unlimited offers greater personal autonomy, it poses challenges to achieving technical blockchain convergence and introduces potential risks to network reliability and security.