The use of private and public encryption keys is fundamental in the implementation of which of the following?

The Diffie-Hellman algorithm is a key-exchange method that allows two parties to derive a shared secret over an insecure channel using mathematical exponentiation, but it does not itself use pre-existing private/public key pairs in the same PKI sense - it generates ephemeral values rather than relying on asymmetric encryption keys.

SSL (Secure Sockets Layer) is fundamentally built on asymmetric (public/private key) cryptography: the server presents a certificate containing its public key, the client uses it to encrypt a pre-master secret or verify a key exchange, and the corresponding private key is used to decrypt it. This PKI-based handshake establishes a secure channel, making private and public key pairs central to SSL's design and operation.

AES (Advanced Encryption Standard) is a symmetric encryption algorithm, meaning it uses a single shared secret key for both encryption and decryption, with no concept of separate public and private keys.

MD5 (Message Digest 5) is a cryptographic hash function that produces a fixed-length digest from input data; it does not use encryption keys of any kind - public or private - as it is a one-way hashing algorithm, not an encryption scheme.