Chapter 15: Access Control Mechanisms

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Chapter 15 Access Control Mechanisms

• Dr. Wayne Summers
• Department of Computer Science
• Columbus State University
• Summers_wayne_at_colstate.edu
• http//csc.colstate.edu/summers

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Access Control Lists

• Let S be the set of subjects and R the set of
  rights of a system. An access control list (ACL)
  l is a set of pairs l (s,r) s e S r R.
  Let acl be a function that determines the access
  control list l associated with a particular
  object o. The interpretation of the access
  control list acl(o) si,ri) 1 i n
• Ex. UNIX uses three sets of users (owner, group,
  world) and three types of rights (read (r), write
  (w), and execute (x). ACL of a file can be
  represented by rw-r--r--

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Access Control Lists Issues

• Which subjects can modify an objects ACL?
• If there is a privileged user (root,
  administrator), do the ACLs apply to that user?
• Does the ACL support groups or wildcards?
• How are contradictory access control permissions
  handled?
• If a default setting is allowed, do the ACL
  permissions modify it, or is the default used
  only when the subject is not explicitly mentioned
  in the ACL?

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Capabilities

• Let O be the set of objects and R the set of
  rights of a system. A capability list c is a set
  of pairs c (o,r) o e O r R. Let cap be a
  function that determines the capability list c
  associated with a particular subject s. The
  interpretation of the capability list cap(s)
  oi,ri) 1 i n is that subject s may access
  oi using any right in ri

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Implementation of Capabilities

• Tagged architecture set of bits associated with
  each word that controls whether a word can be
  read/written
• Protection bits associated with paging /
  segmentation of memory
• Cryptographic checksums - used to check the
  integrity of the data

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Comparing Access Control Lists Capabilities

• Given a subject, what objects can it access, and
  how?
• Better to use capabilities
• Given an object, what subjects can access it, and
  how?
• Better to use ACLs

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Locks and Keys

• A piece of information (lock) is associated with
  the object and a second piece of information
  (key) is associated with the subject.
• Dynamic system
• Ex. Type checking label data different from
  instructions and only allow instructions to be
  executed