Mary is using asymmetric cryptography to send a message to Sam so that only Sam can read it. Which key should she use to encrypt the message?

Mary is using asymmetric cryptography to send a message to Sam so that only Sam can read it. Which key should she use to encrypt the message?

• A. Mary's private key
• B. Sam's public key
• C. Sam's private key
• D. A symmetric key shared between Mary and Sam

Answer: (B)

In asymmetric cryptography, you encrypt the message with the recipient’s public key so that only the recipient can decrypt it with their private key. Mary should use Sam’s public key to encrypt the message, ensuring that even if someone intercepts it, only Sam—who holds the matching private key—can read it. Encrypting with Mary’s private key would not keep the content secret; that approach is used for digital signatures to prove origin, not confidentiality, because anyone with Mary’s public key could decrypt. Sam’s private key isn’t something Mary has access to for encryption, and private keys are kept secret for decryption or signing, not for encryption from the sender. A symmetric key shared between them would bypass the asymmetric system entirely and isn’t appropriate when the scenario calls for public-key encryption to ensure only the intended recipient can read the message.

In asymmetric cryptography, you encrypt the message with the recipient’s public key so that only the recipient can decrypt it with their private key. Mary should use Sam’s public key to encrypt the message, ensuring that even if someone intercepts it, only Sam—who holds the matching private key—can read it.

Encrypting with Mary’s private key would not keep the content secret; that approach is used for digital signatures to prove origin, not confidentiality, because anyone with Mary’s public key could decrypt. Sam’s private key isn’t something Mary has access to for encryption, and private keys are kept secret for decryption or signing, not for encryption from the sender. A symmetric key shared between them would bypass the asymmetric system entirely and isn’t appropriate when the scenario calls for public-key encryption to ensure only the intended recipient can read the message.