量子ギャップ：BTC と ETH がセキュリティにおいて異なる道を歩む理由

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The quantum divide between Bitcoin and Ethereum Quantum computing has long been viewed as a distant, largely theoretical threat to blockchain systems. However, that perspective is now starting to change. With major technology companies such as Google establishing timelines for post-quantum cryptography, and crypto researchers re-examining long-held assumptions, the discussion is shifting from abstract theory to concrete planning. However, Bitcoin and Ethereum, two major blockchain networks, are addressing the quantum computing threat in different ways. Both networks depend on cryptographic systems that could, in principle, be compromised by sufficiently powerful quantum computers. However, their approaches to addressing this shared vulnerability are evolving in markedly different directions. This divergence, often referred to as the “quantum gap,” has less to do with mathematics and more to do with how each network handles change, coordination and long-term security. Did you know? Quantum computers do not need to break every wallet at once. They only need access to exposed public keys, which means older Bitcoin addresses that have already transacted could theoretically be more vulnerable than unused ones. Why quantum computing matters for blockchains Blockchains rely heavily on public-key cryptography, particularly elliptic curve cryptography (ECC). This framework allows users to derive a public address from a private key, enabling secure transactions while keeping sensitive information protected. If quantum computers achieve sufficient scale and capability, they could fundamentally weaken this foundation. Algorithms such as Shor’s algorithm could, in theory, allow quantum systems to compute private keys directly from public keys, thereby jeopardizing wallet ownership and overall transaction security. The consensus among most researchers is that cryptographically relevant quantum computers are still years or even decades away. Nevertheless, blockchain platforms present a distinct challenge. They cannot be updated instantaneously. Any substantial migration requires extensive coordination, rigorous testing and broad adoption over multiple years. This situation highlights a key paradox: Although the threat is not pressing in the near term, preparation needs to begin well in advance. External pressure is accelerating the debate The discussion has moved well beyond crypto-native communities. In March 2026, Google announced a target timeline to transition its systems to post-quantum cryptography by 2029. It cautioned that quantum computers pose a significant threat to existing encryption and digital signatures. This development is particularly relevant for blockchain systems because digital signatures play a fundamental role in verifying ownership. While encryption is vulnerable to “store-now, decrypt-later” attacks, digital signatures face a distinct risk. If compromised, they could increase the risk of unauthorized asset transfers. As major institutions begin preparing for quantum resilience, blockchain networks face growing pressure to outline their own mitigation strategies. This is where the differences between Bitcoin and Ethereum become more apparent. Did you know? The term “post-quantum cryptography” does not refer to quantum technology itself. It refers to classical algorithms designed to resist quantum attacks, allowing existing computers to defend against future quantum capabilities without requiring quantum hardware. Bitcoin’s approach: Conservative and incremental Bitcoin’s approach to quantum risk is guided by its core philosophy: minimize changes, maintain stability and avoid introducing unnecessary complexity at the base layer. One of the most widely discussed proposals in this context is Bitcoin Improvement Proposal 360 (BIP-360), which introduces the concept of Pay-to-Merkle-Root (P2MR). Instead of fundamentally altering Bitcoin’s cryptographic foundations, the proposal seeks to limit exposure by changing the structure of certain transaction outputs. The objective is not to achieve full quantum resistance for Bitcoin in a single move. Rather, it aims to create a pathway for adopting more secure transaction types while preserving backward compatibility with the existing system. This approach mirrors the broader mindset within the Bitcoin community. Discussions often reflect extended time horizons, ranging from five years to several decades. The community is focused on ensuring that any changes do not undermine Bitcoin’s core principles: decentralization and predictability. Nevertheless, this strategy has attracted criticism. Some argue that delaying more comprehensive measures could leave the network vulnerable if quantum advances arrive faster than expected. Others contend that making hasty changes could introduce avoidable risks into a system designed for long-term resilience. Ethereum’s approach: Roadmap-driven and adaptive Ethereum, by contrast, is pursuing a more proactive and structured strategy. The Ethereum ecosystem has begun formalizing a post-quantum roadmap that treats the challenge as a multi-layered system upgrade rather than a single technical adjustment. A key element in Ethereum’s approach is “cryptographic agility,” which refers to the ability to replace core cryptographic primitives without undermining the stability of the network. This aligns with Ethereum’s broader design philosophy, which emphasizes flexibility and continuous iterative improvement. The roadmap covers multiple layers: Execution layer: Investigating account abstraction and alternative signature schemes that can support post-quantum cryptography. Consensus layer: Assessing replacements for validator signature mechanisms, including hash-based options. Data layer: Modifying data availability structures to ensure security in a post-quantum setting. Ethereum developers have positioned post-quantum security as a long-term strategic priority, with timelines extending toward the end of the decade. In contrast to Bitcoin’s incremental proposals, Ethereum’s approach resembles a staged migration plan. The goal is not immediate rollout but gradual preparation, allowing the network to transition when the threat becomes more concrete. Why Bitcoin and Ethereum are taking different approaches to the quantum threat The divergent approaches of Bitcoin and Ethereum are not a coincidence. They arise from fundamental differences in architecture, governance and philosophy. Bitcoin’s base layer design emphasizes robustness and predictability, fostering a cautious attitude toward significant upgrades. Any change must meet a high bar for consensus and, even then, is usually limited in scope. Ethereum, by contrast, has a track record of coordinated upgrades and protocol evolution. From the shift to proof-of-stake to ongoing scaling improvements, the network has demonstrated a willingness to execute complex changes when needed. This distinction shapes how each network views the quantum threat. Bitcoin generally sees it as a remote risk that warrants careful, minimal intervention. Ethereum treats it as a systems-level issue that requires early planning and architectural adaptability. In this context, the “quantum gap” is less about disagreement over the nature of the threat and more about how each ecosystem defines responsible preparation. Did you know? Some early Bitcoin transactions reused addresses multiple times, unintentionally increasing their exposure. Modern wallet practices discourage address reuse partly because of long-term risks such as quantum attacks, even though the threat is not immediate. An unresolved challenge for both Bitcoin and Ethereum Despite their differing strategies, neither Bitcoin nor Ethereum has fully resolved the quantum threat. Bitcoin continues to examine various proposals and weigh trade-offs, yet no clear migration path has been formally adopted. Ethereum, although more advanced in its planning, still faces substantial technical and coordination hurdles before its roadmap can be fully implemented. Several open questions remain relevant to both ecosystems: How to migrate existing assets protected by vulnerable cryptography How to coordinate upgrades within decentralized communities How to balance backward compatibility and forward security These difficulties underscore the complexity of the issue. Post-quantum security represents more than a technical upgrade. It is also a test of long-term adaptability, governance and coordination. Could security posture influence market narratives? As institutional interest in quantum risk continues to grow, differences in preparedness could eventually shape how markets assess blockchain networks. The reasoning is simple: A network that demonstrates greater adaptability to threats may be viewed as more resilient over the long term. However, this idea remains largely speculative. Because quantum threats are still seen as a long-term concern, any near-term market effects are more likely to stem from narrative than from concrete technical developments. Nevertheless, the fact that the discussion is now entering institutional research and broader public discourse suggests that it could become a more prominent consideration in the future.

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