Which of the following is not a symmetric key cryptography algorithm?

Concept

Cryptographic algorithms are classified by how they manage keys. A symmetric-key algorithm uses one single shared secret key for BOTH encryption and decryption; the sender and receiver must already possess that same key. A key-exchange (key-agreement) protocol is a different category entirely: its purpose is not to encrypt a message, but to let two parties establish a shared secret over an insecure channel, using asymmetric (public/private) mathematics.

Applying it here

The question asks which option is NOT a symmetric-key algorithm. Test each name against the definition above: does it encrypt/decrypt data with a single shared key, or does it merely set up a key?

• RC4 — a stream cipher that encrypts data using one shared key. This is symmetric.

• Blowfish — a 64-bit block cipher (Bruce Schneier) using one variable-length key. Symmetric.

• DES — the classic block cipher encrypting 64-bit blocks with one 56-bit key. Symmetric.

• Diffie–Hellman — does NOT encrypt any data. It is a key-agreement protocol built on the discrete-logarithm problem (public/private values), used to derive a shared secret that a separate symmetric cipher then uses.

RC4, Blowfish and DES all fit the symmetric definition (encrypt with one shared key), so the odd one out — the one that is not a symmetric-key cipher — is Diffie–Hellman.

Cross-check

A quick test: can the algorithm, by itself, turn plaintext into ciphertext? RC4/Blowfish/DES can. Diffie–Hellman cannot — it only produces a shared key, which is why it is grouped with asymmetric/public-key methods, not with symmetric ciphers.