Lecture 24: May 21, 2021

Reading: text, §16
Due: Homework 4, due May 24; Lab 3, due May 26

1. Access Control Lists
   1. Issues in the way lists work
   2. Revocation issue

2. Capabilities
   1. Capability-based addressing
   2. Capabilities as security mechanisms
   3. Inheritance of C-Lists

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 D_i in a list of n deciphering transformations, so
      R = (E₁(K), E₂(K), …, E_n(K)) and any process with access to any of the D_i’s can access the file
      AND-Access: need all n deciphering functions to get K: R = E_n(E_n(… E_n(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 (35, 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

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

ECS 135, Computer Security Version of May 21, 2021 at 7:48PM

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