ECDHE (Elliptic Curve Diffie-Hellman Ephemeral) (ECDHE)

ECDHE is the key exchange algorithm used in virtually every modern TLS connection. It lets the client and server agree on a shared secret over an insecure channel without ever transmitting the secret itself. The "ephemeral" part is what matters most: new keys are generated for every connection and discarded after use.

Here's why ephemeral keys matter. With older key exchange methods (static RSA), the server's long-lived private key was used directly to decrypt the session. If that key was ever compromised (stolen, leaked, or obtained via court order), an attacker could decrypt all previously recorded traffic. With ECDHE, each session uses a fresh key pair. Even if someone steals the server's private key, they can't decrypt past sessions. This property is called forward secrecy (or perfect forward secrecy).

The math uses elliptic curves, which provide the same security strength as traditional Diffie-Hellman but with much smaller key sizes. A 256-bit elliptic curve key provides roughly the same security as a 3072-bit RSA key. Smaller keys mean less computation and less data transmitted during the handshake, which directly impacts TLS connection time on CDN edges handling millions of handshakes per second.

In TLS 1.3, ECDHE is the only key exchange method. TLS 1.3 removed static RSA key exchange entirely, making forward secrecy mandatory. TLS 1.2 supports both, but all modern configurations prefer ECDHE cipher suites. If you see a cipher suite name like TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, the ECDHE part is the key exchange, RSA is for authentication (verifying the server's identity), and AES-GCM is the symmetric cipher used after key exchange.

CDN edges perform an enormous number of ECDHE operations. Cloudflare reports billions of TLS handshakes per day. The two most common curves are X25519 (fast, modern, designed by djb) and P-256 (NIST standard, widely supported). X25519 is faster and preferred by most CDNs. Some CDN providers use hardware acceleration or custom implementations to handle the load.

For CDN operators, the ECDHE overhead is real but manageable. On modern hardware with AES-NI and optimized elliptic curve implementations, an ECDHE key exchange adds about 1ms of server-side computation. TLS session resumption (via session tickets or PSK) avoids repeating the full key exchange on subsequent connections, reducing the overhead to near zero for returning visitors.