What enables secure email communication using encryption?

The correct response highlights the role of asymmetric encryption in securing email communication. Asymmetric encryption employs a pair of keys: a public key and a private key. In this method, the sender uses the recipient's public key to encrypt the email content. This ensures that only the recipient, who possesses the corresponding private key, can decrypt and read the message. This system not only protects the confidentiality of the email but also enables authentication and non-repudiation because the signature of the sender can be verified using the sender's public key.

In contrast, symmetric encryption uses a single key for both encryption and decryption, which can present challenges in key distribution and management. While it can be secure, it does not inherently provide the same level of authentication or non-repudiation that asymmetric encryption does.

Certificate authorities help establish trust in encryption methods by issuing digital certificates but do not themselves handle the encryption of email; they verify identity instead. Firewall systems are crucial for network security and can help in securing overall systems but do not directly facilitate the encryption of email content. Therefore, asymmetric encryption is the key technology that ensures secure email communication through its robust method of managing keys and verifying identities.