Re: I-0451: When To Use IFF/IFC And ACF/ACC

["NIAP Interpretations Board" <faigin@aero.org>]

  The following is a proposal for a NIAP Interpretation of, or formal
  guidance about, a Common Criteria document that has been approved by the
  NIB and is being submitted to the CCIMB for concurrence. It is being
  posted for informational purposes.




                      CCITSE/CEM  GUIDANCE (PROPOSED)

     _________________________________________________________________

                    I-0451: When To Use IFF/IFC And ACF/ACC
     _________________________________________________________________

TYPE:                 Guidance
NUMBER:               I-0451
STATUS:               Ready to Send to Management/CCIMB

TITLE:                When To Use IFF/IFC And ACF/ACC
PREVIOUS POSTING:      [cc-cmt 00357]


SOURCE REFERENCE:     CC v2.1 Part 2 Subclause 6.1 FDP_ACC
                      CC v2.1 Part 2 Subclause 6.2 FDP_ACF
                      CC v2.1 Part 2 Subclause 6.5 FDP_IFC
                      CC v2.1 Part 2 Subclause 6.6 FDP_IFF
RELATED TO:           <None>

ISSUE:

   When is it appropriate to use IFF/IFC or ACF/ACC?

STATEMENT

   ACF/ACC should be used when the intent is to control access to an
   object; IFF/IFC should be used when the intent is to control the flow
   of information.

   Access to an object means that there is a set of rules that define
   whether some entity (a "subject") may have a particular form of access
   to a data container (an "object") for some particular type of
   operation. There are no controls based on the information itself; that
   is: if the subject is permitted to write the object, it may write any
   data into the object; similarly, if a subject is permitted to read an
   object, it can do whatever it wishes with the data it has read.

   Information flow control is based on some fundamental characteristic
   associated with the information (not the container), and may not
   involve an active subject. Information flow policies dictate whether
   information with a particular characteristic can move from one
   controlled entity to another.

SUPPORT:

   When the Common Criteria was first written, the goal on the functional
   side was to capture the policies that were present in the previous
   criteria that served as inspiration. In the case of the Trusted
   Computer System Evaluation Criteria, this was Discretionary Access
   Control (DAC) and Mandatory Access Control (MAC). However, at this
   time, there was an effort not to use terms that brought with them
   specific connotations--hence, many TCSEC terms were not used in the
   CC.

   While the TCSEC separated controls based upon who was allowed to make
   changes (discretionary verses mandatory), the CC has separated
   controls based upon the type of control. Since IFF/IFC type controls
   were often non-discretionary and ACF/ACC were often discretionary the
   confusion arose that this was always the case.

   Access controls (ACF/ACC) may be used to model what in the
   TCSEC-paradigm was called Discretionary Access Control, but this is
   not their sole use. They could also be used to implement Role-Based
   Access Controls, as well as a variety of other controls. The key
   characteristic is that the controls are based on the object containing
   the information: they address the fundamental question of whether a
   particular subject can access a particular object. Note that this is
   independent of the actual implementation: it could be an access
   control list on an object, but it could also be a permitted access
   list on a subject, or some fixed rule set stored completely
   independently of either subject or object. Access control policies
   typically have an element of active decision making.

   Information flow controls (IFF/IFC) may be used to model what in the
   TCSEC-paradigm was called Mandatory Access Control (more properly,
   non-Discetionary Access Control, although even that is a misnomer),
   but that is not their sole use. The key characteristic is that the
   controls are based on a characteristic associated with the
   information, such as a sensitivity label, the quality of the data, or
   the source of the data. More importantly, the characteristic stays
   associated with the data as it moves through the TSF, and serves to
   provide the basis for the controls (although this characteristic may
   be discarded, or the association destroyed, after the decision has
   been made). Note that the characteristic need not be embedded in the
   information; for example, consider unlabelled information entering a
   multi-level system through a labelled port.

   In Information Flow, subjects need not be involved, as might happen in
   a network device that connects two ports. In fact, there might not be
   a decision made during run-time at all; the decision may have been
   made ahead of time by the TOE design and captured in the TOE design
   (for example, an information diode).

   In determining which policy to use in writing a security target or
   profile, it is extremely important not to let the actual or planned
   implementation affect the choice of policy. The type of policy should
   be chosen based on the fundamental type of control: is it subject
   access to an object, or is it based on characteristics of the
   information.