Lecture 24 Outline

Reading: §16*
Assignments: Homework 4, due Nov. 18; Lab 4, due Nov. 18

  1. Access Control Lists
    1. UNIX method
    2. Full ACLs: describe, revocation issue
  2. Capabilities
    1. Capability-based addressing
    2. Inheritance of C-Lists
    3. Revocation: use of a global descriptor table
  3. Lock and Key
    1. Associate with each object a lock; associate with each process that has access to object a key (it’s a cross between ACLs and C-Lists)
    2. Example: cryptographic (Gifford). X object enciphered with key K. Associate an opener R with X. Then:
      OR-Access: K can be recovered with any Di in a list of n deciphering transformations, so R = (E1(K), E2(K), …, En(K)) and any process with access to any of the Di’s can access the file
      AND-Access: need all n deciphering functions to get K: R = E1(E2(… En(K)) …))
    3. Types and locks
  4. Secret sharing
  5. MULTICS ring mechanism
    1. Rings, gates, ring-crossing faults
    2. Used for both data and procedures; rights are REWA
      (b1, b2) access bracket—can access freely; (b3, b4) call bracket—can call segment through gate; so if a’s access bracket is (32, 35) and its call bracket is (36, 39), then assuming permission mode (REWA) allows access, a procedure in:
      rings 0–31: can access a, but ring-crossing fault occurs
      rings 32–35: can access a, no ring-crossing fault
      rings 36–39: can access a, provided a valid gate is used as an entry point
      rings 40–63: cannot access a
    3. If the procedure is accessing a data segment d, no call bracket allowed; given the above, assuming permission mode (REWA) allows access, a procedure in:
      rings 0–32: can access d
      rings 33–35: can access d, but cannot write to it (W or A)
      rings 36–63: cannot access d

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