Which model is considered upside down compared to the Bell-LaPadula model?

The Biba Integrity Model is considered the "upside down" version of the Bell-LaPadula model because it focuses on maintaining data integrity as opposed to confidentiality. While the Bell-LaPadula model is designed to protect the confidentiality of information by preventing subjects at lower security levels from accessing higher-level information (also known as the "no read up" principle), the Biba model enforces integrity through a different set of rules.

In the Biba model, the primary emphasis is on ensuring that users cannot corrupt data, which is achieved through "no write up" and "no read down" principles. This means that a subject at a lower integrity level cannot write to an object at a higher integrity level, thus preventing the introduction of potentially corrupt information. Conversely, it also restricts subjects from reading from lower integrity levels, which helps maintain the integrity of data being accessed.

This inversion reflects the different goals of each model: Bell-LaPadula's focus on preventing unauthorized access to sensitive information contrasts with Biba's focus on preventing unauthorized modification of that information. The other models, while also dealing with aspects of security and integrity, do not represent this particular take on reversing the priorities laid out in the Bell-LaPadula framework.