Which key exchange protocol is based on elliptic curve cryptography and is similar in operation to standard Diffie-Hellman?

ECDH, or Elliptic-Curve Diffie-Hellman, is the correct answer because it utilizes elliptic curve cryptography to enable secure key exchange between parties. This protocol is an adaptation of the traditional Diffie-Hellman method but leverages the mathematical properties of elliptic curves to provide a similar mechanism with enhanced security and efficiency.

In standard Diffie-Hellman, the security is based on the difficulty of solving the discrete logarithm problem in a finite field. ECDH, however, uses the elliptic curve group, which allows for smaller key sizes while still maintaining a high level of security. This means that ECDH can achieve the same security level as standard Diffie-Hellman with significantly shorter keys, resulting in faster computations and reduced bandwidth for key exchanges.

This makes ECDH particularly well-suited for environments where computational power and bandwidth are limited, while still ensuring robust cryptographic security. Therefore, it is widely used in modern secure communications, including in protocols for secure messaging and data encryption.

Considering the other options, S/MIME is primarily about securing email messages, API refers to a set of protocols for building software applications, and SSH is a protocol for secure network services, but none of these specifically