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Safety integrity

Immunity from dangerous breakdowns, Inerrancy. Safety feature. In computerized systems in security it is the characteristic for which the system is designed and built to be immune from failures that cause the unsafe state of the system itself. Indicates the suitability of a system to comply with the specified safety requirements, in any given operating condition and for a given period of time. In order to define this characteristic, it is necessary in principle to consider all the causes of Failure, random and systematic, which can lead to a state of danger (Hazard). Many of these can be defined quantitatively (for example in probabilistic terms), but others may depend on several factors, which cannot be accurately quantified, and can only be considered qualitatively. In particular, during the design and construction phases of a safe system, preliminary and final hazard analyzes (PHA and HA) are carried out, referring to the use of the system itself in its life cycle. The suitability of security, for the systems that require its specification, is defined through levels (Safety Integrity Level, SIL), through the Safety Integrity Level Definition (SIL-D), conventionally from 0 to 4, which specify the “degree of immunity from dangerous failures”: level 4, very high, vital, critical for safety (safety critical), Fail Safe; level 3, high, vital, critical for safety (safety critical), high immunity (high integrity); level 2, medium, semi-vital, connected to safety (safety involved), average immunity (medium integrity); level 1, low, semi-vital, connected to safety (safety involved), low integrity (low integrity); level 0, not defined, not vital, not connected to safety (non safety related), not in safety (non safety). In particular, the following can be defined: the preliminary system SIL (pSysSIL or pSIL), the global and definitive system SIL (SysSIL or SIL), the SIL of the hardware and software parts (HwSIL and SwSIL). A secure system must globally have SIL 4, while the component parts, depending on their competition to safe operation, can have both SIL 4 and SIL 3, 2, 1 and 0

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