A Certificate Authority (CA) generates a key pair that will be used for encryption and decryption of email. The integrity of the encrypted email is dependent on the security of which of the following?

The public key is intended to be distributed freely and its exposure does not compromise the security of the system; it is expected to be known to all parties.

In a public-key infrastructure, the private key is the single secret component that must never be disclosed. If an attacker obtains the private key generated by the CA, they can decrypt any message encrypted with the corresponding public key and can impersonate the key owner by signing arbitrary data. All integrity and confidentiality guarantees of the email system collapse the moment the private key is exposed.

Modulus length determines the computational strength of the key pair, but it is a parameter chosen at key generation time and does not represent something whose ongoing security must be protected.

The email server certificate authenticates the mail server to clients but is separate from the key pair used for message encryption and decryption, so its security does not directly protect the encrypted email content.