Post-Quantum Cryptography: The Race Before Q-Day The threat that keeps cryptographers awake isn’t a breach happening today. It’s the data that was stolen last year — sitting in an adversary’s archive, waiting for a quantum computer that doesn’t exist yet. By the time that computer arrives, the window to do anything about it will have already closed. Why This Problem Is Different Most security threats are reactive — an attacker exploits a vulnerability, a defender patches it.