Notes for November 16, 1998

1. Greetings and Felicitations!
2. Puzzle of the Day
3. Privilege in Languages
1. Nesting program units
2. Temporary upgrading of privileges
4. MULTICS ring mechanism
1. MULTICS rings: used for both data and procedures; rights are REWA
2. (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
5. Capabilities
   1. Capability-based addressing: show picture of accessing object
   2. Show process limiting access by not inheriting all parent's capabilities
   3. Revocation: use of a global descriptor table
6. 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: use crypto (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 = E₁(E₂(...E_n(K)...))