The security service that is served by the process of encrypting plaintext with the sender’s private key and decrypting ciphertext with the sender’s public key is identification. Identification is the process of verifying the identity of a person or entity that claims to be who or what it is. Identification can be achieved by using public key cryptography and digital signatures, which are based on the process of encrypting plaintext with the sender’s private key and decrypting ciphertext with the sender’s public key. This process works as follows:

The sender has a pair of public and private keys, and the public key is shared with the receiver in advance.

The sender encrypts the plaintext message with its private key, which produces a ciphertext that is also a digital signature of the message.

The sender sends the ciphertext to the receiver, along with the plaintext message or a hash of the message.

The receiver decrypts the ciphertext with the sender’s public key, which produces the same plaintext message or hash of the message.

The receiver compares the decrypted message or hash with the original message or hash, and verifies the identity of the sender if they match.

The process of encrypting plaintext with the sender’s private key and decrypting ciphertext with the sender’s public key serves identification because it ensures that only the sender can produce a valid ciphertext that can be decrypted by the receiver, and that the receiver can verify the sender’s identity by using the sender’s public key. This process also provides non-repudiation, which means that the sender cannot deny sending the message or the receiver cannot deny receiving the message, as the ciphertext serves as a proof of origin and delivery.

The other options are not the security services that are served by the process of encrypting plaintext with the sender’s private key and decrypting ciphertext with the sender’s public key. Confidentiality is the process of ensuring that the message is only readable by the intended parties, and it is achieved by encrypting plaintext with the receiver’s public key and decrypting ciphertext with the receiver’s private key. Integrity is the process of ensuring that the message is not modified or corrupted during transmission, and it is achieved by using hash functions and message authentication codes. Availability is the process of ensuring that the message is accessible and usable by the authorized parties, and it is achieved by using redundancy, backup, and recovery mechanisms.