$4.$ In public key$-$based encryption, which of the following is true?

A$)$ One uses the other$$s private key to encrypt messages for authenticity.

B$)$ One uses its own public key to encrypt messages for authenticity.

C$)$ One uses the other$$s public key to encrypt messages for confidentiality.

D$)$ One uses its own private key to encrypt messages for confidentiality.

Answer: C$)$ One uses the other$$s public key to encrypt messages for confidentiality.