Quantum threat lingers over legacy BTC as Ark flags structural tail risk

Ark Invest and Unchained say about 34.6% of Bitcoin—mainly early, reused and Taproot addresses—could be vulnerable if future quantum computers crack today’s cryptography.

Roughly one-third of all Bitcoin (BTC) in circulation could still be vulnerable if future quantum computers break today’s core cryptography, according to a new joint report from Ark Invest and Unchained.​

Ark warns on quantum risk to legacy BTC

The report estimates that about 34.6% of BTC supply remains at potential risk under a credible quantum-computing breakthrough scenario. That slice includes around 5 million BTC (about 25% of total supply) exposed through address reuse, roughly 1.7 million BTC (8.6%) held in early pay-to-public-key (P2PK) addresses, and about 200,000 BTC (around 1%) tied to Taproot’s P2TR address type. In each of these cases, public keys have been revealed on-chain, meaning a quantum-capable adversary who can break elliptic curve cryptography (ECC) could, in theory, derive private keys and sweep funds.​

Ark and Unchained stress that most existing Bitcoin is already safe from near-term quantum threats, as modern usage patterns minimize unnecessary key exposure. However, the legacy buckets—early coins, heavily reused addresses, and certain advanced script types—represent a structurally trapped cohort that may never fully move, especially where owners are lost, dead, or simply offline. That creates a long-lived attack surface that could distort supply expectations if quantum capability arrives earlier than anticipated.

Long-term problem, slow-moving fix

Crucially, the report frames quantum as a “long-term risk”: the industry still expects it will take years before any machine can realistically break Bitcoin’s ECC in real time. That lead time gives the Bitcoin community scope to research and deploy quantum-resistant schemes, including new address types, migration incentives, and protocol-level signals to discourage key reuse.​

For investors, the takeaway is not imminent doom but structural tail risk that needs to be priced and managed. If and when credible quantum attacks near viability, pressure will mount on long-dormant coins, and narratives around “lost” supply, Satoshi-era wallets, and institutional custody standards will likely reprice. Ark’s message is blunt: Bitcoin’s cryptography does not need replacing tomorrow, but serious work on quantum mitigation must happen well before the math breaks.