6.5 Million Bitcoin, Including Satoshi's 1.7 Million, Face Quantum Vulnerability

The long-anticipated threat of quantum computing to Bitcoin's security is accelerating, with recent estimates suggesting "Q-Day"—the moment quantum computers can break current public-key cryptography—could arrive within the next decade. This timeline is significantly shorter than previous projections of 20-30 years, prompting urgent discussions within the cryptocurrency community about potential vulnerabilities and mitigation strategies.

A substantial portion of Bitcoin, specifically an estimated 6.5 million coins, faces immediate quantum vulnerability due to their storage in addresses with exposed public keys. This includes approximately 1.7 million coins attributed to Bitcoin's pseudonymous creator, Satoshi Nakamoto. These legacy addresses, particularly those using Pay-to-Public-Key (P2PK) formats, Taproot, and instances of rampant address reuse, reveal public keys that could be exploited by sufficiently powerful quantum computers using algorithms like Shor's to derive private keys.

Experts warn that the threat extends beyond just the underlying cryptography; it challenges Bitcoin's fundamental principle of "mathematically enforced property rights." While the development of cryptographically relevant quantum computers is still years away, the concept of "harvest now, decrypt later" poses an immediate risk. Attackers could collect encrypted blockchain data today and decrypt it once quantum capabilities mature, potentially exposing past transactions and wallet ownership.

Among the proposed solutions, implementing a soft fork to declare P2PK coins unspendable is being considered. Other strategies include the adoption of post-quantum cryptography (PQC) standards, hybrid signature schemes, and new address types. The National Institute of Standards and Technology (NIST) has already finalized initial post-quantum cryptography standards, with governments and organizations beginning to plan their migration. However, the decentralized nature of Bitcoin makes consensus on significant protocol changes, such as a hard fork for mandatory migration, a complex challenge.