Feasibility of a Quantum 51% Attack on Bitcoin by a State Actor

Introduction

The main concern here is whether or not a state actor -- motivated only by the desire to destroy an adversary -- would be able to muster enough computational power to effectively destroy the Bitcoin network. Obviously, normal participants in the Bitcoin game have NO economic incentive to destroy the network. But an adversary of the US, who did not have a huge investment in a strategic Bitcoin reserve, wouldn't mind destroying it if American economic stability were highly dependent on US Bitcoin reserves.

A 51% attack on Bitcoin means an entity gains majority control of the network’s mining power, enabling them to rewrite recent blockchain history, double-spend coins, or censor transactions. This analysis examines whether a state actor could use quantum computing to achieve such an attack.

Current State of Quantum Computing and Timeline for Breaking Bitcoin

Bitcoin’s security relies on two cryptographic pillars: Elliptic Curve Digital Signature Algorithm (ECDSA) and the SHA-256 hash function. A sufficiently advanced quantum computer threatens these through Shor’s algorithm (breaking ECDSA) and Grover’s algorithm (weakening SHA-256).

Currently, quantum computers lack the necessary qubit count and stability to break Bitcoin’s cryptography. Experts estimate that a quantum threat to Bitcoin is at least 10–20 years away, giving ample time for the network to prepare.

Could a Quantum Computer Achieve a 51% Hash Power Attack?

A 51% attack requires controlling the majority of Bitcoin’s mining hash rate. While Grover’s algorithm could theoretically provide a quadratic speed-up in mining, a quantum attacker would still require an enormous computational advantage.

Bitcoin’s difficulty adjustment mechanism would counteract sudden increases in mining efficiency. Additionally, sustaining such an attack would require continuous quantum dominance, making it extremely challenging.

Computational and Energy Requirements for a Quantum Attack

• Massive Qubit Count: Thousands to millions of stable, error-corrected qubits needed.
• High Speed Processing: Quantum operations would need GHz clock speeds.
• Enormous Energy & Cooling: Quantum machines require significant power and refrigeration.
• Technical Expertise and Secrecy: A nation-state would need extensive R&D resources.

The cost of building and operating such a system would be astronomical, making it unlikely that any state actor could maintain quantum dominance over Bitcoin mining.

Bitcoin’s Quantum-Resistant Countermeasures

Bitcoin is preparing for quantum threats through post-quantum cryptography (PQC). Proposals such as Pay-to-Quantum-Resistant-Hash (P2QRH) would transition Bitcoin addresses to quantum-resistant signatures. Additionally, consensus protocol changes could be introduced if quantum mining dominance emerges.

Global Network Resilience and Strategic Viability

Even if a state actor obtained a quantum computer capable of attacking Bitcoin, they would need to consider:

• Network Adaptation: Bitcoin can coordinate responses and fork if needed.
• Collateral Damage: Destroying Bitcoin would affect global financial markets.
• Alternative Uses for Quantum Power: Mining dominance for wealth accumulation could be a more rational strategy.
• Political and Economic Fallout: A state-sponsored attack could provoke international responses.

Given Bitcoin’s decentralized nature, a quantum 51% attack would not necessarily destroy the network but could instead lead to adaptation and countermeasures.

Conclusion

Currently, a quantum-based 51% attack on Bitcoin is not feasible due to the immense technical, financial, and logistical hurdles involved. Quantum computing is still far from achieving the power necessary to break Bitcoin’s cryptography or dominate mining.

Bitcoin’s planned security upgrades and community-driven adaptability further reduce the likelihood of a successful attack. While vigilance is necessary, the likelihood of a state actor disrupting or destroying Bitcoin via a quantum attack in the foreseeable future is extremely low.

For more insight into the game theory behind Bitcoin, see our last post.