Confidentiality in electronic communication is achieved by ensuring that only the intended recipient can read the message. In an asymmetric (public-key) cryptographic system, this is accomplished when the sender encrypts the message using the recipient's publicly available key. The resulting ciphertext can only be decrypted by the corresponding private key, which is held exclusively by the recipient. This process ensures that even if the message is intercepted, its contents remain secret from any unauthorized party. This is a fundamental principle of public-key infrastructure (PKI) used in secure email standards like S/MIME and PGP.

A. The sender encrypting it with its private key. This action creates a digital signature, which provides authentication, integrity, and non-repudiation, not confidentiality. Anyone with the sender's public key can decrypt it.
B. The sender encrypting it with its public key. Encrypting with one's own public key is not useful for communication, as only the sender (who holds the private key) could decrypt it.
D. The sender encrypting it with the receiver's private key. The sender should never have access to the receiver's private key. A private key must remain secret to its owner to maintain the security of the system. ---

1. National Institute of Standards and Technology (NIST) Special Publication 800-32

Introduction to Public Key Technology and the Federal PKI Infrastructure.

Section 2.2

"Public Key Cryptography

" Paragraph 3: "To provide confidentiality for a message

the sender encrypts the message with the public key of the intended recipient. The recipient then uses his/her private key to decrypt the message. Only the recipient has the private key that corresponds to the public key and is therefore the only person who can decrypt the message."

2. Internet Engineering Task Force (IETF) RFC 4880

OpenPGP Message Format.

Section 2.1

"Public-Key-Encrypted Messages": This section details the process where a one-time session key is generated

used to encrypt the message data

and then this session key itself is encrypted with the recipient's public key. This ensures that only the holder of the corresponding private key can decrypt the session key and

subsequently

the message.

3. Pfleeger

C. P.

Pfleeger

S. L.

& Margulies

J. (2015). Security in Computing (5th ed.). Pearson Education.

Chapter 2

"Cryptography

" Section 2.3

"Public Key Encryption": The text explains

"To send a secure message to [a recipient]

you fetch a copy of [their] public key... You then encrypt your message using that public key... When [the recipient] receives the ciphertext

[they] decrypt it with [their] private key." This academic text confirms the standard procedure for ensuring confidentiality.