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.
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