What technology can be defined as a passive device that listens for intrusions?

An Intrusion Detection System (IDS) is designed to monitor network or system activities for malicious activities or policy violations. It can be classified as a passive device because its primary function is to listen, analyze, and alert administrators to potential security breaches or intrusions without actively preventing them. When unusual activity is detected—such as unauthorized access attempts or abnormal data traffic patterns—the system records this information and may trigger alerts for further investigation.

In contrast, a firewall actively filters incoming and outgoing traffic based on predetermined security rules to prevent unauthorized access. A proxy server acts as an intermediary for requests from clients seeking resources from other servers, often used for anonymity or content filtering rather than intrusion detection. Lastly, encryption protocols are designed to secure data in transit or at rest by making it unreadable without decryption keys, thus they do not inherently monitor for intrusions. Therefore, the correct choice that accurately defines a passive device focused on intrusion detection is the Intrusion Detection System.