John wants to send Marie an email that includes sensitive information, and he does not trust the network that he is connected to. Marie gives him the idea of using PGP. What should John do to…

Using John's own private key would perform a digital signature operation (proving authenticity), not encryption for confidentiality - it would not protect the message content from eavesdroppers.

Using John's own public key to encrypt would mean only John could decrypt it with his private key, making it unreadable to Marie and defeating the purpose of sending her a secure message.

Marie's private key must remain secret and should never be shared with John or anyone else; sharing a private key completely compromises the security of the asymmetric encryption system.

In asymmetric/PGP encryption, John must use Marie's public key to encrypt the message. Only Marie's corresponding private key can decrypt it, ensuring that even if the message is intercepted on the untrusted network, no one else can read it. This is the fundamental principle of public-key cryptography used in PGP.