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3 months ago in Quantum Computing By Pooja
As a cryptography student, I’m trying to grasp the real-world threat timeline: how long would it actually take a quantum computer to break SHA-256 encryption?
With all the headlines about “quantum supremacy,” it’s hard to separate hype from reality. I understand that, in theory, Grover’s algorithm reduces the attack time, but building such a machine seems enormously difficult. Could you walk me through the current estimates—how many reliable qubits, how much error correction, and how many years or decades experts realistically believe it will take before SHA-256 is truly vulnerable?
All Answers (2 Answers In All)
By Natasha Answered 1 month ago
Algorithmic Speedup: Grover’s algorithm provides a quadratic speedup, reducing SHA-256’s security from 128 bits (classical) to 128 bits (quantum) in terms of attack complexity. This means a quantum computer would still need to perform ~3.4×10³? operations to find a collision.
Required Qubits: To run Grover’s algorithm on a 256-bit hash, recent studies estimate a need for roughly 20–30 million physical qubits with low error rates—far beyond today’s ~1,000–2,000 noisy qubits.
Time to Break: If we assume a future quantum machine performing 10¹? Grover iterations per second (an optimistic projection), breaking SHA-256 would still take over 100 million years with today’s technology. Even with hypothetical advances, a realistic attack timeline is 15–25 years away.
Migration Timeline: Organizations like NIST are standardizing post-quantum algorithms now because replacing cryptographic systems industry-wide can take 10–20 years.
Bottom line: The cryptographic risk is low today, but the strategic risk of delayed preparation is high. The timeline isn’t about the algorithm’s weakness—it’s about the colossal engineering effort required to build a quantum computer powerful enough to threaten it.
Replied 1 month ago
By Pooja
Thank you so much for your response.Its really helpfull for me.
Reply to Natasha
By Heena Answered 1 month ago
From a hardware and engineering perspective, breaking SHA-256 with a quantum computer is currently—and for the foreseeable future—infeasible. Here’s why in numbers:
- Qubit Scale: Running Grover’s algorithm against SHA-256 would require approximately √(2²??) = 2¹²? iterations. To execute this, you'd need a fault-tolerant quantum computer with millions of logical qubits.
- Physical vs. Logical Qubits: Given current error rates, each logical qubit might require 1,000 to 10,000 physical qubits for error correction. That means you'd need billions of physical qubits in total.
- Current State: As of 2024, the largest public quantum processors have under 2,000 noisy physical qubits—orders of magnitude too small and too error-prone.
- Time Estimate: Even with rapid progress, most experts in quantum engineering estimate it will take at least 20 to 30 years before a machine capable of breaking SHA-256 in a practical timeframe (say, weeks or months) could exist.
So, while the theoretical threat is real, the engineering hurdles are enormous. Today’s focus is rightly on post-quantum cryptography migration, not because SHA-256 is broken, but because upgrading global cryptographic infrastructure takes decades.
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