ECDHERSA

ECDHERSA is a term sometimes used to describe a cryptographic key-exchange approach that combines Elliptic Curve Diffie-Hellman (ECDH) with RSA-based authentication. In practice, many protocols implement the same pattern under the designation ECDHE_RSA, where an ephemeral elliptic-curve Diffie-Hellman exchange is authenticated with an RSA-based signature. There is no widely adopted standard specifically named “ECDHERSA” as a separate specification, but the concept is commonly realized in TLS cipher suites such as those that use ECDHE with RSA for server authentication.

In a typical ECDHE_RSA-like setup, the client and server agree on an elliptic curve and generate ephemeral key pairs. The client and server exchange public keys and compute a shared secret from their private keys and the other party’s public key. The server signs its ephemeral public key with its RSA private key, and the client verifies the signature using the server’s RSA certificate, thereby authenticating the server. Once the signature is verified, both sides derive session keys from the shared secret using a key derivation function, enabling secure encrypted communication for the session. The use of ephemeral keys provides forward secrecy, so compromise of long-term keys does not reveal past sessions.

Security considerations include reliance on proper certificate validation and PKI for RSA authentication, the performance cost of RSA signatures, and the need to select appropriate elliptic curves and hash algorithms. ECDHE_RSA remains widely deployed in TLS alongside alternatives such as ECDHE_ECDSA or newer signature schemes (for example, Ed25519-based approaches). See also ECDH, ECDHE, RSA, TLS.