Insider Brief
- A Citi Institute report estimates that a quantum-enabled cyberattack disrupting a major U.S. bank’s access to Fedwire could put $2.0–$3.3 trillion of U.S. GDP at risk, reframing quantum computing as a systemic financial threat rather than a distant technology issue.
- The report argues that the primary risk is already active through “harvest now, decrypt later” attacks, making long-lived financial, government, and personal data vulnerable even before quantum computers reach full cryptographic capability.
- While post-quantum cryptography standards exist, Citi finds that regulatory pressure, legacy systems, and the scale of required upgrades make execution and coordination the central challenge for banks, governments, and digital infrastructure providers.
A single quantum-enabled cyberattack on a major U.S. bank could trigger trillions of dollars in economic damage, according to a new study by the Citi Institute, reframing quantum computing from a distant research concern into an immediate financial stability risk.
In a report, analysts at Citi Group estimate that a one-day disruption to a top-five U.S. bank’s access to the Fedwire Funds Service — the real-time payment system operated by the Federal Reserve — could generate $2 trillion to $3.3 trillion in indirect economic losses, equivalent to 10% to 17% of U.S. gross domestic product. The impact, measured as GDP-at-risk, could spill into a six-month recession driven by cascading failures across the financial system, according to the analysis.
While much of the debate about quantum computing is when the technology will appear, the report attempts to move from theoretical timelines and into the reality of economic exposure. Rather than asking when quantum computers might arrive, the report focuses on what would happen if today’s encryption systems were compromised at scale and how costly even a short disruption could be.
According to Citi, modern economies depend heavily on public-key cryptography, the mathematical foundation used to secure payments, digital identities, communications and data storage. A sufficiently powerful quantum computer could break those systems far faster than classical machines, exposing financial institutions, governments and critical infrastructure to widespread disruption.
The analysts describe the quantum threat as a low-probability but high-severity risk, comparable to rare financial crises that carry outsized consequences. Even a modest chance of success is enough to justify urgent preparation, they add.
The effect, in other words, would dwarf the fears around the Y2K threat in the late 20th century.
“Quantum computing will trigger the largest upgrade of cryptography in human history, far bigger than the Y2K transition,” said Steve Suarez, CEO of HorizonX, as quoted by the analysts.
How Close Is “Q-Day”?
The report refers to “Q-day” as the point when quantum computers become powerful enough to break widely used public-key encryption. Based on estimates cited from regulators and risk institutes, the probability of such a breakthrough is placed at 19% to 34% by 2034, rising to 60% to 82% by 2044.
Yet the report emphasizes that Q-day should not be treated as a single future event. According to the analysts, the most serious risk is already underway through so-called “harvest now, decrypt later” attacks, in which adversaries collect encrypted data today with the expectation of decrypting it later using quantum systems.
“Q-Day is often treated as a future event,” the report quotes Rebecca Krauthamer, Co-Founder and CEO, QuSecure. “From a risk perspective, it is already here. Data stolen today can be decrypted later. Organizations don’t need quantum systems to respond; PQC is deployable today.”
This creates an immediate vulnerability for information with long-term value, such as financial records, medical data, biometric identifiers, government archives and intellectual property. While upgrading encryption can protect future data flows, the report notes that historical data already captured cannot be retroactively secured.
For financial institutions and governments, this means the timeline for action is not driven by the arrival of quantum computers, but by the shelf life of the data they are trying to protect.
System-wide Effects?
Citi’s analysis also goes beyond individual data breaches to examine system-wide effects. According to the report, a successful quantum-enabled attack would likely spread rapidly across sectors, undermining authentication systems, payment networks and digital trust mechanisms that underpin daily economic activity
In finance, the exposure is particularly acute mainly because banks rely on cryptographic systems for interbank messaging, transaction settlement, identity verification and secure communications. A disruption to any of these layers could impair liquidity, freeze payments and erode confidence, amplifying the initial shock.
The Fedwire scenario highlighted in the report illustrates how quickly local failures could become national crises. Even without direct theft, the inability to process high-value payments could cascade through markets, disrupt supply chains and force emergency interventions.
Beyond finance, the report identifies healthcare, telecommunications, energy and defense systems as high-risk sectors due to their reliance on long-lived data and infrastructure that is difficult to upgrade quickly.
A Regulatory Shift Is Already Underway
The report points to growing regulatory momentum as a key driver of change. While few jurisdictions have imposed hard deadlines for quantum-safe security across the private sector, governments are increasingly treating post-quantum preparedness as a compliance issue rather than a voluntary upgrade.
According to Citi, U.S. federal agencies are expected to begin migrating high-risk systems to post-quantum cryptography by 2030, with full adoption targeted by 2035. Similar timelines are emerging in Europe, where coordinated national strategies are required by the end of 2026 and high-risk systems must transition by 2030.
Other regulators, including central banks, have begun issuing directives requiring institutions to assess and manage quantum-related cyber risks. The report notes that these policies are shifting the conversation from awareness to execution, particularly in highly regulated industries such as banking and payments.
The analysts report that quantum readiness is moving from a technical discussion into the boardroom, where it is increasingly framed as an operational and governance issue.
Crypto Exposure Is Uneven
The report also assesses the impact of quantum computing on public blockchains, challenging the notion that all crypto assets face the same level of risk.
According to Citi, about 25% of Bitcoin supply is potentially quantum-exposed because the associated public keys have already been revealed on-chain. That translates to roughly 4.5 million to 6.7 million bitcoins, worth an estimated $500 billion to $600 billion at current prices.
Other blockchains face higher exposure. The report estimates that more than 65% of Ethereum’s current supply could be vulnerable, while Solana’s exposure is described as effectively total due to differences in transaction design.
Still, the analysts caution against oversimplified conclusions. Unlike banks or governments, most blockchains have limited exposure to harvest-now, decrypt-later attacks because transaction data is public by design. Instead, their vulnerability centers on digital signatures and governance speed.
Chains that can coordinate protocol upgrades quickly may be able to migrate to quantum-resistant signature schemes faster than traditional institutions can overhaul legacy systems.
Solutions Exist — Execution Is the Bottleneck
The report concludes — importantly — that the quantum threat is no longer a problem of missing technology. According to Citi, post-quantum cryptography standards have already been finalized by international bodies, including the U.S. National Institute of Standards and Technology.
The challenge lies in deploying those standards at scale across complex, aging systems. Large institutions may need to inventory thousands of applications, coordinate with vendors, retrain staff and redesign authentication layers over multi-year timelines.
The report compares the coming transition to the Y2K remediation effort of the late 1990s, but argues that the quantum upgrade will be broader and more uneven. Unlike Y2K, which had a fixed deadline and limited scope, quantum-safe migration requires replacing cryptographic foundations embedded across nearly every digital system.
According to Citi, this could make the quantum transition the largest cryptographic overhaul in history, with costs extending well beyond software upgrades to include organizational change and long-term risk management.
