Which algorithm is a symmetric cipher?

• A. Diffie-Hellman
• B. Public-key Cryptography Standards (RSA)
• C. Advanced Encryption Standard (AES)
• D. Elliptic Curve Digital Signature Algorithm (ECDSA)

Answer: (C)

The main idea is recognizing a symmetric cipher, which uses the same secret key to both encrypt and decrypt data. AES is a prime example of this: with one shared key, it turns plaintext into ciphertext and back again, enabling fast and efficient confidentiality for large amounts of data. The other options aren’t symmetric ciphers: Diffie-Hellman is a method for establishing a shared secret between parties, not for encrypting data itself; RSA is an asymmetric algorithm that uses a public/private key pair for encryption and decryption; ECDSA is a digital signature algorithm that verifies authenticity and integrity rather than encrypting data. AES supports standard key sizes like 128, 192, or 256 bits and is widely used because it performs well in both hardware and software environments.