Which statement is true about RSA?

• A. It is a symmetric-key algorithm
• B. It is a public-key algorithm
• C. It is a hash function
• D. It is a block cipher

Answer: (B)

RSA is an asymmetric, public-key algorithm. It uses a pair of keys—a public key that can be shared openly and a private key that remains secret. Data encrypted with the public key can be decrypted only with the corresponding private key, and data signed with the private key can be verified with the public key. This enables secure key exchange and digital signatures without needing a shared secret in advance. It contrasts with symmetric-key algorithms, which use one secret key for both encryption and decryption, and with hash functions, which produce fixed-size digests and are not reversible. RSA is also not a block cipher, since its primary design is for public-key encryption and signatures rather than the reversible transformation of fixed-size blocks with a shared key. In practice, RSA is often used to securely exchange a symmetric key for faster data encryption, while the bulk data is encrypted with a symmetric algorithm.