OASIS eXtensible Access Control Markup Language (XACML) TC

  • 1.  Ordered Combinators

    Posted 07-10-2003 18:42
    
    Greetings, 
    
    I would still like to issue a caution to these ordered Permit-Overrides
    and Deny-Overrides. 
    
    The current combinator algorithms imply that it may return
    Permit-Indeterminate, Deny-Indeterminate, or NotApplciable-Indeterminate
    based on the availability of the arguments or errors in evaluating their
    consitiuents, being careful to make sure one does not return the wrong
    thing, i.e. Deny in the face of and Indeterminate in Permit-Overrides, or
    Permit in the face of an Indterminate in Deny-Overrides
    
    So, why the ordered combinators?
    
    Is it only there to make sure that if you are evaluating in the face of
    errors or availability of arguments, so that you will get the same answer
    from different implementations based on the same inputs/availability or
    error conditions?
    
    This requires a lot more than just specifying an ordering to the combining 
    algorithm. Otherwise, you are giving a "false sense of integrity." I think 
    you are biting off more than you can chew.
    
    In PolicySets, this requires that all Policies be evaluated by the same
    evaluation engine. If a "sub" policy's combinator evaluates a rule that
    generates an Indeterminate, then the PolicySet may return Indeterminate.  
    However, under another implementation although the same "sub" policy will
    get evaluated at the same point, but indeterminate may not be the case due
    to a different evaluation of the rules of the "sub" policy, but you have
    no real control of this.
    
    So, I think it that really doesn't take the problem you are seeking to
    fix.
    
    Also, I'm a little concerned about the definitions of and, or, and n-of,
    which also affects this.  I'll write that under a separate thread.
    
    
    Cheers,
    -Polar