Which of the following best describes the relationship between a public and private key in public-key encryption?

• A. They are identical
• B. They are different yet complementary
• C. They are used for decryption only
• D. They serve the same purpose

Answer: (B)

In public-key encryption, the relationship between the public and private keys is fundamental to its security and functionality. The best description of this relationship is that they are different yet complementary.

The public key is used to encrypt data, and it can be shared openly with anyone who wants to send encrypted messages to the key owner. In contrast, the private key is kept secret and is used to decrypt the messages that have been encrypted with the corresponding public key. This complementary nature is what allows secure communications to take place; the public key's accessibility permits anyone to encrypt messages, while the private key's confidentiality ensures that only the owner can decrypt and read those messages.

Thus, the two keys work together in a way that enhances security: one key enables encryption, and the other enables decryption, and they cannot function properly without each other. This unique relationship is what differentiates public-key encryption from other encryption methods, such as symmetric encryption, where the same key is used for both encrypting and decrypting.