Merged Bitcoin Achieves Maximum Attack Cost with Multiple Hash Types and 51% Protection

The security of proof-of-work blockchains is constantly under scrutiny, prompting researchers to explore innovative approaches to enhance resilience against attacks. Christopher Blake, Chen Feng, and Xuachao Wang, alongside Qianyu Yu, present a novel protocol termed ‘Merged Bitcoin’ which allows block connections to utilise multiple distinct hash types. Their work demonstrates that simply combining hash types does not inherently guarantee increased security, as a 51% attack across all types remains a potential vulnerability. However, the researchers prove that Merged Bitcoin maximises the cost of such an attack, offering a significant improvement in blockchain security, and introduce a difficulty adjustment method to mitigate asymmetric advantages gained through advances in hashing algorithms or hardware. This research provides a crucial step towards developing more robust and adaptable blockchain systems capable of withstanding increasingly sophisticated threats.

Bitcoin Security Against Hashing Power Asymmetries

Research demonstrates firm bounds on the security region within the Delta-bounded delay network model, with these theoretical bounds subsequently compared against simulation results. The protocol detailed is proven to maximise the cost of attack when considered within the linear cost-per-hash model. A novel difficulty adjustment method is introduced, positing that it can partially mitigate asymmetric advantages an adversary might obtain through hashing power, stemming from factors such as algorithmic advances. These advantages also include potential threats from quantum attacks, like Grover’s algorithm, or vulnerabilities arising from hardware backdoor attacks.

Since its inception in 2008, the Bitcoin protocol has evolved into a global database, currently securing over one trillion USD in value. Recent work proposes that an adversary could potentially leverage financial instruments to disrupt the system. This research builds upon existing understanding of blockchain security and proposes methods to enhance resilience against evolving threats. The contributions focus on both theoretical security analysis and practical protocol improvements.

Multi-Resource Consensus for Enhanced Blockchain Security

The paper introduces Merged Bitcoin, a novel blockchain protocol designed to enhance security and trust beyond existing systems such as Bitcoin and Ethereum. The central idea of the protocol is to leverage multiple consensus resources—such as different hashing algorithms or combinations of proof-of-work and proof-of-stake—rather than relying on a single resource. By requiring an attacker to simultaneously compromise all underlying resources, Merged Bitcoin significantly raises the cost and difficulty of successful attacks, thereby improving overall system security.

A key contribution of the work is the formal and rigorous security analysis of the proposed protocol. The authors provide theoretical guarantees under assumptions such as bounded network delay and limited adversarial power, and they mathematically derive the growth rate of honest blocks to show how security depends on block arrival rates and network conditions. The concept of a “Big-And” security region is introduced, acknowledging that while no permissionless blockchain can fully eliminate the risk of a 51% attack, Merged Bitcoin can make such attacks exponentially unlikely. The protocol is also evaluated in comparison with Bitcoin, Ethereum’s proof-of-stake model, and Minotaur, highlighting its superior security and trust properties.

The paper further addresses practical concerns, including the risk of hardware backdoors, and discusses mitigation strategies enabled by the multi-resource design. By distributing trust across diverse resources, the protocol aims to foster broader confidence among users and reduce dependence on any single entity or technology. The analysis also touches on transaction finality, noting that the optimal k-confirmation rule for Merged Bitcoin remains an open question requiring further study.

In terms of limitations and future work, the authors identify opportunities to explore non-linear security regions, as seen in protocols like Minotaur, and to achieve strong “Big-And” security under weaker adversarial assumptions, such as scenarios where block withholding is not possible. Overall, the paper presents Merged Bitcoin as a theoretically grounded and more resilient blockchain architecture, offering a promising direction for improving the security and trustworthiness of permissionless consensus systems.

Merged Bitcoin Enhances Blockchain Security Significantly

Scientists have demonstrated a novel approach to bolstering the security of proof-of-work blockchain protocols, introducing a system called Merged Bitcoin. The research proves that the security region of a protocol utilising multiple hash types cannot be defined simply as the intersection of 51% attacks across all hash types, a significant finding for distributed ledger technology. Experiments reveal that Merged Bitcoin, an extension of the existing Bitcoin protocol allowing blocks to be linked using diverse hash types, maximises the cost of attack within a linear cost-per-hash economic model. The team measured the security region of Merged Bitcoin within a ∆-bounded delay network model, deriving closed-form bounds and comparing these to simulation results to validate their accuracy.

Results demonstrate that the protocol effectively increases the threshold for a successful attack, mitigating risks associated with centralisation of mining power and potential hardware backdoor vulnerabilities. A newly introduced difficulty adjustment method aims to address asymmetric advantages an adversary might gain through advancements in hashing power for specific hash types, including those arising from algorithmic improvements or quantum computing attacks like Grover’s algorithm. This work addresses the increasing threat to the Bitcoin protocol, currently securing over one trillion USD in value, by proposing an alternative to simply increasing SHA-256 mining circuit production. Scientists proved that the mining power required to compromise Merged Bitcoin correlates directly with the ability to mine faster than the honest, fully-delayed score growth rate, reducing to the sum of mining powers for each hash type as delay approaches zero.

Measurements confirm that utilising ‘n’ different hash types can exponentially decrease the probability of a hardware backdoor attack, assuming independence in the probability of attacking each block type. The study establishes formal bounds on the fully-delayed score growth rate, delivering closed-form upper and lower limits on the protocol’s security region. While an information-theoretic approach suggesting exponentially decreasing error probabilities through an ‘AND’ of 51% thresholds proved unfeasible in permissionless distributed protocols, the research demonstrates that Merged Bitcoin still significantly increases the cost of attack. The protocol’s design allows for viable participation from personal computers, diversifying mining resources beyond specialised SHA-256 circuits and reducing reliance on a limited number of ASIC producers.