Migration to PQC: Hybrid Approaches

Transitioning from classical to post-quantum cryptography is the largest cryptographic migration in history — more complex than the 2048-bit RSA upgrade or SHA-1 deprecation. It requires updating protocols, libraries, HSMs, certificates, and code simultaneously.

Hybrid KEM = Classical KEM + PQC KEM. Shared secret = KDF(classical_secret || pqc_secret). Security: requires breaking both. A quantum adversary breaks ECDH but not Kyber. A classical adversary breaks neither. Provides defense-in-depth during the transition period.