What does the simple security (ss) property mean in the Bell-LaPadula model?
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The ss (simple security) property of the Bell-LaPadula access control model states that reading of information by a subject at a lower sensitivity level from an object at a higher sensitivity level is not permitted (no read up)
Source: KRUTZ, Ronald L.
& VINES, Russel.
D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 5: Security Architectures and Models (page 202).
The Bell-LaPadula (BLP) model is a security model used for access control in computer systems. It is based on the concept of multilevel security, where information is classified into different security levels and users are granted access only to information that is at or below their security clearance level.
The simple security (ss) property in the BLP model refers to the rule that a subject (user or process) at a certain security level can read only information at the same or lower security level. In other words, the property states that there should be "no read up" from lower levels to higher levels.
The ss property is important because it prevents information from being leaked or accessed by unauthorized users. For example, if a user with a secret-level clearance is not allowed to read information at the top-secret level, then that user cannot accidentally or intentionally leak sensitive information to unauthorized parties.
It is worth noting that the ss property only applies to read access, not write access. Users with lower security clearance levels can still write to objects at higher security levels, but they cannot read the contents of those objects. To prevent unauthorized modification or destruction of higher-level objects, the BLP model also includes the *-property, which restricts write access based on security levels (specifically, it prohibits "no write down" from higher levels to lower levels).
In summary, the simple security (ss) property in the Bell-LaPadula model means that a user or process can read only information at the same or lower security level, and there should be "no read up" from lower levels to higher levels.