Outline for March 3, 2003

Reading: text, §12

Discussion Problem

"To fight and conquer in all your battles is not supreme excellence; supreme excellence consists in breaking the enemy's resistance without fighting. In the practical art of war, the best thing of all is to take the enemy's country whole and intact; to shatter and destroy it is not so good. So, too, it is better to capture an army entire than to destroy it, to capture a regiment, a detachment, or a company entire than to destroy it."1

What does this paragraph say to a system administrator or security officer seeking insight to defend her systems?

Outline for the Day

  1. Passwords
    1. How UNIX does selection
    2. Problem: common passwords
    3. May be pass phrases: goal is to make search space as large as possible, distribution as uniform as possible
    4. Other ways to force good password selection: random, pronounceable, computer-aided selection
    5. Go through problems, approaches to each, esp. proactive
  2. Attack Schemes Directed to the Passwords
    1. Exhaustive search: UNIX is 1-8 chars, say 96 possibles; it's about 7e16
    2. Inspired guessing: think of what people would like (see above)
    3. Random guessing: can't defend against it; bad login messages aid it
    4. Scavenging: passwords often typed where they might be recorded (as login name, in other contexts, etc.
    5. Ask the user: very common with some public access services
    6. Expected time to guess
  3. Password aging
    1. Pick age so when password is guessed, it's no longer valid
    2. Implementation: track previous passwords vs. upper, lower time bounds
  4. Ultimate in aging: One-Time Password
    1. Password is valid for only one use
    2. May work from list, or new password may be generated from old by a function
    3. Example: S/Key
  5. 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: monkey-in-the-middle
    5. Defense: mutual authentication
  6. Biometrics
    1. Depend on physical characteristics
    2. Examples: pattern of typing (remarkably effective), retinal scans, etc.
  7. Location
    1. Bind user to some location detection device (human, GPS)
    2. Authenticate by location of the device

1 Sun Tzu, The Art of War, James Clavell, ed., Dell Publishing, New York, NY ©1983, p. 15

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