Which of the following relationships best describes public-key cryptography?

• A. Encryption and decryption use the same key
• B. It uses a pair of keys for encryption and decryption
• C. It relies on a single secret key
• D. It is used only for digital signing

Answer: (B)

Public-key cryptography is characterized by the use of a pair of keys: a public key and a private key. This dual-key approach allows one key to be used for encryption while the other key is utilized for decryption. Specifically, the public key is distributed and can be shared freely, allowing anyone to encrypt a message intended for the owner of the corresponding private key. The private key, kept secret by the owner, is then used to decrypt the message.

This system enhances security; even if the public key is known, it is computationally infeasible for anyone to derive the private key, thus maintaining the confidentiality of the communication. This relationship is fundamental to the operation of public-key cryptography, distinguishing it from symmetric cryptography, which relies on a single shared key for both encryption and decryption.

Other options presented do not accurately capture the essence of public-key cryptography: one suggests that the same key is used for both tasks, which is characteristic of symmetric encryption; another implies reliance on a single secret key, also indicative of symmetric systems; and the last option limits the function of public-key cryptography to digital signing, neglecting its role in secure message encryption and transmission.