Citi is ringing the alarm bell: quantum computers could pose a much bigger near-term risk to Bitcoin than many expect.
In a May 18 digital-asset research note, Citi analyst Alex Saunders warned that faster-than-expected advances in quantum computing are compressing the timeline for attacks against Bitcoin and wider internet infrastructure. The bank says Bitcoin is particularly exposed because its conservative governance and slow upgrade process make rapid defensive changes difficult to implement.
What’s vulnerable
- Citi flagged a concrete attack vector: public keys already exposed on-chain. Early Bitcoin address formats (pay-to-public-key outputs) revealed public keys permanently once funds were spent, leaving those keys collectible by attackers. That pool includes addresses believed to belong to Satoshi Nakamoto.
- Citi estimates 6.5–6.9 million BTC have already-exposed keys — a stash Citi values at roughly $450 billion under current market conditions.
- The bank also highlighted the “harvest now, decrypt later” threat: bad actors can hoover up encrypted data and signatures today, then use future quantum capabilities to crack them.
Broader system risk
Citi’s Institute has been tracking quantum risk across finance through 2026; as reported by The Quantum Insider in February, the bank estimated a quantum-enabled attack on a major U.S. bank could threaten $2–$3.3 trillion of GDP. Saunders cautioned: “While large-scale quantum attacks remain a medium-term concern, the pace of progress has shortened the horizon and warrants closer attention from investors.”
Not all blockchains are equally positioned
Proof-of-stake networks like Ethereum may be better able to respond because they can push protocol upgrades more frequently, Citi said — but faster upgrading also brings a larger attack surface in other ways. Bitcoin’s slow, consensus-heavy upgrade path makes it harder to roll out defenses quickly.
Defenses and the road ahead
Citi remains constructive that crypto can adapt through post-quantum cryptography. Proposed Bitcoin upgrades such as BIP-360 and BIP-361 aim to strengthen resilience, but they require broad agreement among miners and node operators — a process that historically takes years, not months.
Why this matters now
Bitcoin is already navigating multiple pressures: rising energy costs, an industry pivot toward AI among miners, and growing institutional scrutiny of its long-term cryptographic resilience (as covered in crypto.news Q1 2026 mining coverage). JPMorgan has separately warned that miners shifting to AI confront heavy capital demands and possible shareholder dilution — underscoring that Bitcoin’s infrastructure is under structural stress from several angles at once.
Bottom line
Quantum attacks aren’t imminent today, but Citi says the window for meaningful risk reduction is closing. With millions of BTC linked to exposed keys and upgrade proposals still years from wide adoption, investors, miners and node operators should treat quantum resilience as a strategic priority rather than a distant curiosity. Bitcoin currently trades around $76,900.
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