Q-Day
Q-Day is the hypothetical point at which quantum computers become powerful enough to break the public-key cryptography that secures most of the internet. It refers to a capability threshold rather than a fixed calendar date.[1] After Q-Day, encryption schemes such as RSA and elliptic-curve cryptography, which protect web traffic, messaging and the digital signatures behind cryptocurrencies, could no longer be relied on.
Background
The threat rests on an algorithm published by the mathematician Peter Shor in 1994, which showed that a quantum computer could efficiently factor large numbers and solve discrete logarithms, the mathematical problems that RSA and elliptic-curve cryptography depend on.[2] Symmetric encryption is much less exposed, and can be protected by using longer keys.[1]
No existing quantum computer is close to the required scale, but estimates of what is needed have fallen. In May 2025, the Google researcher Craig Gidney estimated that 2048-bit RSA "could be factored in less than a week by a quantum computer with less than a million noisy qubits", far fewer than earlier estimates.[3] Expert estimates of when Q-Day might arrive vary widely, with most placing it in the 2030s or later.[1][4]
Harvest now, decrypt later
Because encrypted data can be recorded today and decrypted once quantum computers are capable, Q-Day is a threat before it arrives. Adversaries may already be collecting encrypted traffic to read later, a strategy known as "harvest now, decrypt later". Governments cite this risk as a reason to move to quantum-resistant encryption now.[5] In August 2024, the US National Institute of Standards and Technology published its first three post-quantum encryption standards, noting that "some experts predict that a device with the capability to break current encryption methods could appear within a decade".[6]
Cryptocurrencies
Blockchains such as Bitcoin and Ethereum secure funds with elliptic-curve signatures, so a capable quantum computer could steal funds from addresses whose public keys are visible on-chain. In 2024, Vitalik Buterin outlined how Ethereum could hard-fork to protect users in a sudden "quantum emergency", arguing that "we are already well-positioned to make a pretty simple recovery fork".[7] Making Ethereum quantum-safe is part of the long-term roadmap described in Buterin's essay "The Cryptographic World Computer".
See also
- Cryptographic Asymmetry
- The Cryptographic World Computer
- Zero-Knowledge Proof
- Black October
- Cypherpunk
References
- ↑ 1.0 1.1 1.2 "What Is Q-Day?". Palo Alto Networks. Retrieved November 13, 2026.
- ↑ Peter W. Shor (1995). "Polynomial-Time Algorithms for Prime Factorization and Discrete Logarithms on a Quantum Computer". arXiv. Retrieved November 13, 2026.
- ↑ Craig Gidney (May 21, 2025). "How to factor 2048 bit RSA integers with less than a million noisy qubits". arXiv. Retrieved November 13, 2026.
- ↑ Michele Mosca and Marco Piani (December 6, 2024). "Quantum Threat Timeline Report 2024". Global Risk Institute. Retrieved November 13, 2026.
- ↑ "Post-Quantum Cryptography Initiative". Cybersecurity and Infrastructure Security Agency. Retrieved November 13, 2026.
- ↑ August 13, 2024. "NIST Releases First 3 Finalized Post-Quantum Encryption Standards". National Institute of Standards and Technology. Retrieved November 13, 2026.
- ↑ Vitalik Buterin (March 9, 2024). "How to hard-fork to save most users' funds in a quantum emergency". Ethereum Research. Retrieved November 13, 2026.