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 intentionally distributed to anyone who needs to send encrypted data and its exposure is expected and does not compromise security.

The private key is the only component that must remain secret in a public key cryptography system. If the private key is exposed or stolen, an attacker can decrypt all messages encrypted with the paired public key, completely undermining confidentiality. The entire security model of asymmetric encryption depends on the private key never being disclosed.

Modulus length determines the computational strength of the key but is not itself a secret - it is a parameter of the key, not the key material that must be protected.

The email server certificate authenticates the server identity but is not the element protecting the confidentiality of individual encrypted email messages.