What is a necessary action if unauthorized persons gain access to PKI keys?

In the context of PKI, if unauthorized persons gain access to PKI keys, the immediate and necessary action is to report the incident for investigation. This step is crucial because it initiates a formal process to analyze the breach, determine the extent of the unauthorized access, and implement appropriate responses to mitigate any potential damage. Reporting ensures that security measures can be assessed, and further unauthorized access can be prevented. It also helps in maintaining accountability and compliance with security policies, which is especially important in military and sensitive environments.

While notifying a trusted agent and changing passwords could be elements of a response plan, these actions are often part of the broader procedure that follows the reporting of the incident. Not addressing the breach properly by investigating it can lead to ongoing vulnerabilities and further security risks. Ignoring the situation, as indicated in one of the options, is never appropriate in a security context, especially when dealing with sensitive information and infrastructures.