Lecture 17: May 8, 2026

Reading: text, §13.3.1–13.9, 16.1
Assignments: Homework 3, due May 11, 2026

1. Greetings and felicitations!

2. Passwords
   1. Ways to force good password selection: random, pronounceable, computer-aided selection
   2. Best: use passphrases: goal is to make search space as large as possible, distribution as uniform as possible

3. Attacks
   1. Exhaustive search
   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

4. Defenses
   1. For trial and error at login: dropping or back-off
   2. For thwarting dictionary attacks: salting

5. 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 Pick age so when password is guessed, it’s no longer valid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

6. 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

7. 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 over network

8. Biometrics
   1. Depend on physical characteristics
   2. Examples: pattern of typing (remarkably effective), retinal scans, etc.

9. Location
   1. Bind user to some location detection device (human, GPS)
   2. Authenticate by location of the device

10. Multi-factor authentication

11. Access Control Lists
    1. Full access control lists
    2. Abbreviations (UNIX method)

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

ECS 153A, Computer & Information Security & Privacy I Version of May 7, 2026 at 12:41PM

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