Outline for March 11, 2002

Reading:  ß15.1-15.4

1.	Greetings and Felicitations

2.	Puzzle of the day

3.	Privilege in Languages

a.	Nesting program units

b.	Temporary upgrading of privileges

4.	Access Control Lists

a.	UNIX method

b.	ACLs:  describe, revocation issue

5.	MULTICS ring mechanism

a.	MULTICS rings:  used for both data and procedures; rights are REWA

b.	 (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

c.	If the procedure is accessing a data segment d, no call bracket allowed; given the above, assuming permis-
sion 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

6.	Capabilities

a.	Capability-based addressing: show picture of accessing object

b.	Show  process limiting access by not inheriting all parent's capabilities

c.	Revocation: use of a global descriptor table

7.	Lock and Key

a.	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)

b.	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 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)...))

c.	Types and locks