Outline for May 5, 2005

Reading: §12.3-12.6, §22.2, §15

Discussion

It has often been said tha the only way to decipher a message that has been enciphered using RSA is to factor the modulus n used by the cipher. If you were told that an enciphered message was on a computer that you controlled, and that the message was enciphered using RSA with an n of 1024 bits (about 309 decimal digits), how would you find the encrypter's private key?

Outline

1. Challenge-response systems
   1. Computer issues challenge, user presents response to verify secret information known/item possessed
   2. Example operations: f(x) = x+1, random, string (for users without computers), time of day, computer sends E(x), you answer E(D(E(x))+1)
   3. Note: password never sent on wire or network
   4. Attack: man-in-the-middle
   5. Defense: mutual authentication
2. Biometrics
   1. Depend on physical characteristics
   2. Examples: pattern of typing (remarkably effective), retinal scans, etc.
3. Location
   1. Bind user to some location detection device (human, GPS)
   2. Authenticate by location of the device
4. Combinations: PAM
5. Access Control Lists
   1. UNIX method
   2. ACLs: describe, revocation issue
6. 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
7. Privilege in Languages
   1. Nesting program units
   2. Temporary upgrading of privileges
8. 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)...))
   3. Types and locks
9. 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