Citi said in a report released on Friday that quantum computing is starting to challenge the cryptographic foundations used across blockchains, financial systems, and digital communications. The bank argued that Bitcoin stands out as a key area of concern as advances in quantum technology begin to test assumptions long treated as mathematically secure.
Cryptography specialists cited in the report said a sufficiently powerful quantum computer could break the public-key encryption algorithms used on networks such as Bitcoin and Ethereum. If that happens, digital wallets, crypto exchanges, and core blockchain infrastructure would face a higher level of exposure. The report also warned that data encrypted and stored today could be collected now and decrypted later.
Exposed Bitcoin keys account for roughly one-third of supply
Citi said Bitcoin is especially vulnerable because of its governance structure and the slow pace of protocol changes. The report highlighted previously exposed public keys, dormant wallets, and legacy address types as the main pressure points on the network. It added that wallets believed to belong to Bitcoin creator Satoshi Nakamoto fall within that group.
Based on the latest estimates, between 6.5 million and 6.9 million BTC are vulnerable to quantum attacks because their keys have been exposed. That equals about one-third of the circulating supply. At current market prices, the report put the potential losses at roughly $450 billion.
Private keys could be extracted before transactions are confirmed
Experts cautioned that future quantum machines may be able to extract private keys before blockchain transactions complete validation. That would create a serious problem during the period between transaction broadcast and final confirmation. For networks and users relying on public-key cryptography to secure assets, that shift would alter the current threat model in a direct way.
The report framed the issue as broader than crypto alone. If conventional encryption methods are broken by quantum capability, the impact would also reach financial networks and digital communications systems that depend on the same class of cryptographic tools.
Ethereum may adapt faster as post-quantum debate grows
Citi noted that proof-of-stake chains such as Ethereum are relatively more agile in rolling out protocol upgrades. Even so, the report said their security could still be compromised if quantum advances make validator keys breakable. Faster governance does not remove the underlying cryptographic risk.
While the report said quantum attacks are not expected to become mainstream in the near term, it urged blockchain networks to move toward post-quantum encryption in advance. The crypto community is already discussing BIP-360 and BIP-361 as possible long-term responses for Bitcoin. Citi said cryptocurrencies can adapt over time, but that depends on broad implementation of quantum-resistant tools and protocol updates across the industry.