Which cipher is not a public-key cipher among the listed options?

• A. RSA
• B. ECC
• C. AES
• D. DES

Answer: (C)

Public-key cryptography relies on a pair of keys: a public key for encryption and a private key for decryption, which lets you share the public key openly and keep the private key secret. RSA and ECC are classic examples of this approach.

Symmetric encryption uses a single shared secret key for both encryption and decryption. AES fits in this category, so it is not a public-key cipher. DES is also a symmetric cipher, though it is older and less commonly used today.

The defining difference is whether a key pair or a single shared key is used. Since AES uses one key for both directions, it is not a public-key cipher.