U4. Harrison–Ruzzo–Ullman

BLP - no policies for changing access rights or creation and deletion of subjects and objects
1. HRU addresses these issues
  1. Not an alternative to BLP, it captures a different set of Policies
HRU defines six primitive operations for manipulating subjects, objects and the access matrix
1. enter access right into Matrix
2. delete access right from Matrix
3. create subject s
4. delete subject s
5. create object o
6. delete object o
7. Using these primitive operations, you can build up more complex commands to allow a variety of functions to be carried out
Access rights
1. Read, Write, Own
  1. Owners can manipulate their objects
2. access matrix describes the state of the system
3. commands effect changes in the access
4. HRU can model policies for allocating access rights
Leakage and Safety
1. Leakage
  1. An access matrix M is said to leak the right a if there exists a command c that adds a into a position of the access matrix that previously did not contain a.
2. Safety
  1. An access matrix M is said to be safe with respect to the right a if no sequence of commands can transform M into a state that leaks a.
  2. Difficult to decide if the model is safe
    1. This introduces the concept of Decidability
      1. A problem is called undecidable if there is no algorithm that can guarantee to provide a solution to every instance of the problem
        No algorithm can possibly exist to solve every instance of the problem
        Decidability relates to every instance of the problem.
        The more expressive the security model, the more difficult it is to verify security
        Linked to the 3rd Fundamental design decision - Complexity or Assurance
3. the Concepts of 'leakage' and 'safety' are specifically concerned with whether existing rights can be used to add unsafe access rights to the matrix
4. To verify compliance with policy
  1. check that no undesirable access rights can be granted

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U4. Harrison–Ruzzo–Ullman

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	Created by Craig Parker over 10 years ago