What does Elliptic-Curve Diffie-Hellman (ECDH) represent in a cipher suite?

Elliptic-Curve Diffie-Hellman (ECDH) represents a method for secure key exchange or session key agreement using elliptic curve cryptography. In the context of cipher suites, ECDH is specifically used to establish a shared secret between two parties over an insecure channel, allowing them to create a symmetric session key securely. This key can then be used for symmetric encryption of data transmitted between the parties, ensuring confidentiality.

The use of elliptic curves provides a high level of security with smaller key sizes compared to traditional methods, which enhances performance and reduces computational overhead. ECDH is crucial in securing communication protocols like TLS (Transport Layer Security), where establishing a secure session key is essential for protecting data in transit.

The other options include symmetric bulk encryption, which involves using a single key for both encryption and decryption of data. A signature cipher refers to cryptographic algorithms that provide authentication and integrity through digital signatures. Lastly, a hash function for HMAC (Hash-based Message Authentication Code) is key for ensuring data integrity and authenticity but does not represent key exchange. Each of these plays a different role in cryptographic operations, but ECDH distinctly facilitates the establishment of session keys, making it vital for secure communications.