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Security Assurance Levels: A SIL Approach to Security

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This paper takes a critical look at the SIL concept, its overall strengths and weaknesses as applied to security, and proposes general models for use within the security arena.

The continued deployment of Ethernet-enabled devices in industrial automation and control systems has exposed a number of new risks previously unrealized in IT or process control safety and security. For example, the dissimilar validation and verification requirements for integrated consumer-off-the-shelf (COTs) technologies can threaten availability and continuity of plant operations. The ease at which individual components can be networked facilitates both intentional and unintentional safety and security violations. The effects of tunneling critical time-sensitive plant communications over protocols such as Ethernet, which do not natively support quality-of-service (QoS), must also be considered.

The Safety Integrated Levels (SIL), as defined in ISA-84 and internationalized in ISO 61508 and IEC 61511, define a fourlayer model to deal with process safety requirements for hardware safety integrity and systematic safety integrity. This probabilistic model utilizes Failure Model and Effects Analysis to project risk and damages during a system failure. It can be used to assess and implement systems to achieve a desired level of safety risk reduction. The model, appropriately adjusted, can also be effectively applied to system security.

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