Outline for March 7, 2003

Discussion Problem

The PGP secure mailing system uses both RSA and a classical cipher called IDEA. When one installs PGP, the software generates two large (512 bits or so) numbers, to produce a modulus of 1024 bits. Such a number is too large to be factored easily. The private and public keys are generated from these quantities. The private key is enciphered with a classical cipher using a user-supplied pass phrase as the key. To send a message, a 128-bit key is randomly generated, and the message enciphered using IDEA with that key; the key is enciphered using the recipient's public key, and the message and enciphered key are sent.

1. If you needed to compromise a user's PGP private key, what approaches would you take?
2. It's often said that PGP gets you the security of a key with length 1024. Do you agree?

Outline for the Day

1. Access Control Lists
   1. UNIX method
   2. ACLs: describe, revocation issue
2. 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
3. Privilege in Languages
   1. Nesting program units
   2. Temporary upgrading of privileges
4. 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 cryptography. 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)...))
   3. Types and locks
5. 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