A large-enough quantum computer would break the asymmetric crypto the internet runs on. Post-quantum cryptography (PQC) is the new generation of algorithms built to resist it — and the migration is already starting.
What breaks, what survives
| Crypto | Quantum impact |
|---|---|
| RSA, ECC (asymmetric) | Broken by Shor's algorithm |
| AES (symmetric) | Weakened — just use bigger keys (AES-256) |
| Hashes | Weakened — use longer outputs |
So the danger is concentrated in asymmetric key exchange and signatures — i.e. TLS and PKI.
"Harvest now, decrypt later"
Adversaries can record encrypted traffic today and decrypt it once quantum computers arrive. So data that must stay secret for years is already at risk — which is why migration is urgent even before the hardware exists.
The response
NIST has standardized PQC algorithms (e.g. ML-KEM / Kyber for key exchange, ML-DSA / Dilithium for signatures). Browsers and VPNs are rolling out hybrid modes (classical + PQC) now.
Related: Symmetric vs Asymmetric Encryption · The TLS Handshake · Public Key Infrastructure