What do digital certificates contain that is essential for PKI?

Digital certificates play a crucial role in Public Key Infrastructure (PKI) by providing a means to securely transmit a user's public key along with information that verifies the authenticity of that key. The inclusion of a user’s public key ensures that it can be used for establishing secure communications, such as encrypting data or verifying digital signatures.

In addition, a signed hashed value (often referred to as a digital signature) is included in the certificate to provide integrity and authenticity. This signature is generated by a trusted Certificate Authority (CA) using their own private key, which allows users to verify that the public key contained in the certificate indeed belongs to the entity identified by the certificate. Thus, the combination of a public key and a signed hashed value is essential for establishing trust in the digital communication environment that PKI aims to create.

The other options do not adequately represent the content or purpose of digital certificates. Only private keys, network passwords, or employee identification numbers do not provide the necessary components for secure communication or identity verification that a public key and a signed hash provide.