CRYSTALS-Kyber is primarily used as which cryptographic primitive?

• A. Digital signatures
• B. Key encapsulation mechanism
• C. Hash function
• D. Block cipher

Answer: (B)

CRYSTALS-Kyber is a lattice-based key encapsulation mechanism used to securely establish a shared secret between parties. In practice, the sender uses the recipient’s public key to encapsulate a fresh random key into a ciphertext. The recipient, with their private key, decapsulates that ciphertext to recover the same random key. That key is then used to encrypt the actual message with a separate symmetric cipher. The essential idea is to enable secure key exchange over an insecure channel, even against quantum adversaries.

It isn’t a digital signature, which would provide authentication and non-repudiation for data; it isn’t a hash function, which maps data to a fixed-size digest without enabling key exchange; and it isn’t a block cipher, which directly transforms plaintext to ciphertext using a symmetric key. Kyber’s role is specifically to enable secure key exchange through encapsulation, hence its classification as a key encapsulation mechanism.