Key Exchange
Key exchange is a cryptographic process by which two parties securely establish a shared secret key over an insecure communication channel, enabling subsequent symmetric encryption of their communications.
Key exchange protocols solve the fundamental challenge of establishing a shared secret between two parties who have no prior secure communication channel. The most well-known key exchange protocol is Diffie-Hellman (DH), proposed in 1976, which allows two parties to jointly derive a shared secret by exchanging public values that are computationally infeasible for an eavesdropper to use to reconstruct the secret.
Modern implementations primarily use Elliptic Curve Diffie-Hellman Ephemeral (ECDHE), which provides equivalent security to classical DH with significantly smaller key sizes and faster computation. The "ephemeral" designation means that fresh key pairs are generated for each session, providing perfect forward secrecy -- even if a long-term private key is later compromised, past session keys cannot be recovered.
In TLS 1.3, ECDHE is the only supported key exchange mechanism, reflecting the security community's consensus that ephemeral key exchange with forward secrecy should be mandatory. Common elliptic curves used include X25519 (Curve25519) and P-256. As quantum computing advances, post-quantum key exchange mechanisms like CRYSTALS-Kyber (ML-KEM) are being standardized to protect against future quantum attacks. Organizations should ensure their TLS configurations use ECDHE-based cipher suites and plan for migration to post-quantum key exchange algorithms as standards mature.