In public-key cryptography, which pair of operations is typically used?

• A. Encrypt with the recipient's public key; Decrypt with the recipient's private key
• B. Encrypt with the recipient's private key; Decrypt with the recipient's public key
• C. Encrypt with a symmetric key; Decrypt with a symmetric key
• D. No encryption is used

Answer: (A)

In public-key cryptography, you rely on a publicly available key to protect confidentiality: encrypt the message with the recipient’s public key so that only the recipient can decrypt it with their private key. This works because the private key is kept secret by the owner, ensuring that only they can read what was encrypted, even if others intercept it.

Encrypting with the private key and decrypting with the public key would let anyone using the public key decrypt the message, which defeats confidentiality; that approach is used for digital signatures to prove origin, not to keep content secret. Symmetric encryption uses the same secret key for both directions, which requires a secure way to share that key beforehand and isn’t the public-key encryption model described here. And not encrypting at all does not provide confidentiality in a public-key framework.