There are three core goals of cybersecurity: confidentiality, integrity, and availability. They are commonly known as the security triad, or CIA. Confidentiality is the idea that information can be kept secret and known only to those people or systems who need that information to perform their duties. Integrity is the idea the information is valid and verifiably correct. Availability is the idea that a system or data is running or available when it is needed. To obtain true security, all three of these core concepts are required.
Grouping an industrial control system (ICS) into zones of security is a logical way to begin planning for availability, integrity, and confidentiality in the system. First, we break the system into three regions: 1) the analog data and devices region, 2) the region encompassing devices with machine-to-machine communications, and 3) a region where humans interact with the system.
The second of these, the machines-only region, we further separate into three zones based on the application and function of the devices therein. The first or lowest zone is analog and includes devices that directly interact with the analog region and perform real-time monitoring and issue corrective instructions. The second machine-only zone performs nonreal-time automated functions such as event collection and upstream reporting. The third machine-only zone is the barrier that deters humans from directly interacting with the first two zones. Each of these regions and zones has specific considerations for the operations and security of that zone and, therefore, the entire system.
By logically breaking up the network in this fashion, we are able to apply security tools, techniques, and procedures to ensure the confidentiality, integrity, and availability required at each of the individual devices or levels. We are also able to take advantage of the network itself to support our security. Finally, this system allows us to consider methods for implementing security, not only in newly commissioned ICSs but also in existing ICSs, by identifying and prioritizing critical regions.
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