Hybrid cryptographic approaches combine classical and post-quantum cryptographic algorithms in a single protocol or system to provide security against both conventional and quantum attacks during the transition period. A hybrid scheme ensures that the overall security holds as long as at least one of the component algorithms remains unbroken.
In hybrid key exchange, a TLS handshake performs both a classical ECDH key agreement and a PQC key encapsulation (such as ML-KEM) simultaneously. The two shared secrets are combined through a key derivation function to produce the session key. If the quantum threat does not materialize, the classical component provides proven security. If the PQC algorithm has an undiscovered weakness, the classical component provides fallback protection. Hybrid signatures concatenate classical and PQC signatures, with verification requiring both to pass. Certificate chains can include hybrid certificates containing dual key pairs. Protocol-level hybrid approaches use composite keys where the public key contains both algorithm types, while negotiation-based approaches allow endpoints to select algorithm combinations dynamically.
Hybrid approaches address the confidence gap in the PQC transition. Post-quantum algorithms are mathematically newer and have undergone less real-world cryptanalysis than classical algorithms with decades of scrutiny. Deploying PQC alone requires trusting that these newer assumptions will hold. Hybrid schemes eliminate this risk by ensuring security degrades gracefully -- an attacker must break both classical and post-quantum components. The tradeoff is increased computational overhead, larger key sizes, and more complex protocol implementations that require careful engineering.
CDA recommends hybrid approaches as the default migration strategy within Data Protection and Sovereignty missions. Our guidance helps organizations implement hybrid TLS configurations, evaluate the performance impact on their specific workloads, and plan the eventual transition from hybrid to pure PQC once confidence in post-quantum algorithms matures.