Which one of the following is not a Public Key cryptography algorithm?

Key concept: Cryptographic algorithms are classified by how they use keys. Public-key (asymmetric) cryptography uses a mathematically linked key pair -- a public key for encryption, signature verification, or encapsulating a secret, and a private key for decryption, signing, or recovering that secret -- so no raw secret key ever needs to be transmitted or pre-shared between the two parties. Symmetric-key cryptography, by contrast, requires both parties to already hold one identical shared secret key for both encryption and decryption.

Applying it here: Among the given options, RSA, ECC, and Kyber are each built around a mathematically linked public/private key pair, making them public-key (asymmetric) systems. AES, however, uses a single shared secret key for both encryption and decryption -- the hallmark of symmetric-key cryptography, not public-key cryptography.

• RSA -- asymmetric; the public key encrypts data or verifies a signature, the private key decrypts or signs, and its security rests on the difficulty of factoring large prime products.

• ECC (Elliptic Curve Cryptography) -- asymmetric; derives its public/private key pair from scalar multiplication of a point on an elliptic curve, giving RSA-level security with much shorter keys.

• AES (Advanced Encryption Standard) -- symmetric; standardized in NIST FIPS 197, it uses a single shared secret key for both encrypting and decrypting data.

• Kyber -- asymmetric; a lattice-based key-encapsulation mechanism selected by NIST for post-quantum standardization, where the public key encapsulates a shared secret that only the matching private key can recover.

Cross-check: This agrees with NIST's own classification: FIPS 197 defines AES as a symmetric-key standard, while NIST's public-key/post-quantum guidance (NIST IR 8105 and the Kyber standardization documents) classifies RSA, ECC, and Kyber as public-key algorithms.

Answer: Therefore, AES is the option that is not a Public Key cryptography algorithm.