Which statement is true about AES?

• A. It is a symmetric-key algorithm
• B. It uses public-key cryptography
• C. It is a type of hash function
• D. It is used for digital signatures

Answer: (A)

The key idea is that AES is a symmetric-key algorithm. In symmetric-key cryptography, the same secret key is used to both encrypt and decrypt data, which is exactly how AES operates. AES is a block cipher that works on fixed-size blocks (128 bits) and can use 128-, 192-, or 256-bit keys, applying multiple rounds of substitutions, permutations, and mixing with the key to transform plaintext into ciphertext and then reverse the process with the same key. This is different from public-key cryptography, where encryption and decryption use different keys (a public key for encryption and a private key for decryption). AES is not a hash function, which would produce a fixed-size digest in a one-way manner, nor is it used by itself for digital signatures (digital signatures rely on public-key cryptography and usually involve signing a hash with a private key). So the statement that AES is a symmetric-key algorithm is the accurate description.