1. Chapter 11: Authentication
Basics
Passwords
Challenge-Response
Biometrics
Location
Multiple Methods
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

2. Establishing Identity
One or more of the following
What entity knows (eg. password)
What entity has (eg. badge, smart card)
What entity is (eg. fingerprints, retinal characteristics)
Where entity is (eg. In front of a particular terminal)

3. What entity knows

4. What entity has

5. What entity is

6. Where entity is

7. Authentication System
(A, C, F, L, S)
A (authentication information): the set of specific information with which entities proves their identity;
C (complementary information): the set of information that the system stores and uses to validate the authentication information;
F (complementation function): generate the complementary information from the authentication information; f : A  C;
L: authentication functions that prove identity;
S: selection function enabling an entity to create or alter information in A or C;
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

8. Authentication System
(A, C, F, L, S)
A (authentication information): the set of specific information with which entities proves their identity;

9. Authentication System
(A, C, F, L, S)
C (complementary information): the set of information that the system stores and uses to validate the authentication information;

10. Authentication System
(A, C, F, L, S)
F (complementation function): generate the complementary information from the authentication information; f : A  C;

11. Authentication System
(A, C, F, L, S)
L: authentication functions that prove identity;

12. Authentication System
(A, C, F, L, S)
S: selection function enabling an entity to create or alter information in A or C;

13. Example Password system, with passwords stored on line in clear text
A set of strings making up passwords
C = A
F singleton set of identity function { I }
L single equality test function { eq }
S function to set/change password
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

14. Passwords
Example of an authentication mechanism based on what people know;
Sequence of characters
Examples: 10 digits, a string of letters, etc.
Generated randomly, by user, by computer with user input
Sequence of words
Examples: pass-phrases
Algorithms
Examples: challenge-response, one-time passwords
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

15. Storage of Passwords Store as cleartext Encipher file
If password file compromised, all passwords revealed
Encipher file
Need to have decipherment, encipherment keys in memory
Reduces to previous problem
Store one-way hash of password
If file read, attacker must still guess passwords or invert the hash
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

16. SHA-1 Hashing Example

17. Anatomy of Attacking Goal: find a  A such that:
For some f  F, f(a) = c  C
c is associated with entity
Two ways to determine whether a meets these requirements:
Direct approach: as above
Indirect approach: as l(a) succeeds iff f(a) = c  C for some c associated with an entity, compute l(a)
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

18. Preventing Attacks How to prevent this: Hide one of a, f, or c
Prevents obvious attack from above
Example: UNIX/Linux shadow password files
Hides c’s
Block access to all l  L or result of l(a)
Prevents attacker from knowing if guess succeeded
Example: preventing any logins to an account from a network
Prevents knowing results of l (or accessing l)
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

19. Guessing Through L Cannot prevent these Make them slow
Otherwise, legitimate users cannot log in
Make them slow
Backoff: the amount of time to wait before next attempt keep increasing;
Disconnection: after some number of failed authentication attempts, the connection is broken and the user must reestablish it;
Disabling: if n consecutive attempts to log in to an account fail, the account is disabled until a security manager can reenable it;
Be very careful with administrative accounts!
Jailing: Allow in, but restrict activities and record the attacker’s actions;
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

20. Dictionary Attack vs Brute Force
Definition 11–3. A dictionary attack is the guessing of a password by repeated trial and error.

21. Anderson’s Formula
The goal of defenders is to maximize the time needed to guess a password.
Anderson’s formula:
P: probability of guessing a password in specified period of time
G: number of guesses tested in 1 time unit
T: number of time units
N: number of possible passwords (|A|)
Then P ≥ TG/N

22. Example Goal Solution Passwords drawn from a 96-char alphabet
Can test 104 guesses per second
Probability of a success to be 0.5 over a 365 day period
What is minimum password length?
Solution
N ≥ TG/P = (365246060)104/0.5 = 6.311011
Choose s such that 96s ≥ N
So s ≥ 6, meaning passwords must be at least 6 chars long

23. Approaches: Password Selection
Random selection
Any password from A equally likely to be selected
Pronounceable passwords
User selection of passwords
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

24. Random Selection of Passwords
Theorem 11-1: let the expected time to guess a password be T. Then T is a maximum when the selection of any of a set of possible passwords is equiprobable;
Short passwords should be avoided;
Size of password space could be reduced due to the limited period of pseudo-random number generator;
Human factor is also an important role: a person who is assigned two random passwords must write them down;
Use a transformation algorithm to keep the password safe: t: X  A;
Transformation algorithm is “capitalize the third letter in the word, and append the digit 2.”
“Swqgle3”  ? (the transformation algorithm needs to be memorized).
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

25. Pronounceable Passwords
Generate phonemes randomly
Phoneme is unit of sound, eg. cv, vc, cvc, vcv (c: consonant, v: vowel)
Examples: helgoret, juttelon are; przbqxdfl, zxrptglfn are not
Problem: too few
Solution: key crunching
Run long key through hash function and convert to printable sequence
Use this sequence as password
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

26. User Selection Problem: people pick easy to guess passwords
Based on account names, user names, computer names, place names
Dictionary words (also reversed, odd capitalizations, control characters, “elite-speak”, conjugations or declensions, swear words, Torah/Bible/Koran/… words)
Too short, digits only, letters only
License plates, acronyms, social security numbers
Personal characteristics or foibles (pet names, nicknames, job characteristics, etc.
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

27. Picking Good Passwords
“LlMm*2^Ap”
Names of members of 2 families
“OoHeO/FSK”
Second letter of each word of length 4 or more in third line of third verse of Star-Spangled Banner, followed by “/”, followed by author’s initials
What’s good here may be bad there
“DMC/MHmh” bad at Dartmouth (“Dartmouth Medical Center/Mary Hitchcock memorial hospital”), ok here
Why are these now bad passwords? 
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

28. Proactive Password Checking
Analyze proposed password for “goodness”
Always invoked
Can detect, reject bad passwords for an appropriate definition of “bad”
Discriminate on per-user, per-site basis
Needs to do pattern matching on words
Needs to execute subprograms and use results
Spell checker, for example
Easy to set up and integrate into password selection system
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

29. Password Aging
Force users to change passwords after some time has expired
How do you force users not to re-use passwords?
Record previous passwords
Block changes for a period of time
Give users time to think of good passwords
Don’t force them to change before they can log in
Warn them of expiration days in advance

30. Challenge-Response
User, system share a secret function f (in practice, f is a known function with unknown parameters, such as a cryptographic key)

31. Pass Algorithms Challenge-response with the function f itself a secret
Example:
Challenge is a random string of characters such as “abcdefg”, “ageksido”
Response is some function of that string such as “bdf”, “gkip”
Can alter algorithm based on ancillary information
Network connection is as above, dial-up might require “aceg”, “aesd”
Usually used in conjunction with fixed, reusable password
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop

32. One-Time Passwords Password that can be used exactly once Problems
After use, it is immediately invalidated
Problems
Synchronization of user, system
Generation of good random passwords
Password distribution problem

33. Biometrics Fingerprints: optical or electrical techniques
Optical Scanner vs Capacitive Scanner;
Maps fingerprint into a graph, then compares with database

34. Biometrics – Voice ID Voices: speaker verification or recognition
Verification: uses statistical techniques to test hypothesis that speaker is who is claimed (speaker dependent)
Recognition: checks content of answers (speaker independent)

35. Other Characteristics
Eyes: patterns in irises unique
Measure patterns, determine if differences are random; or correlate images using statistical tests

36. Other Characteristics
Faces: image, or specific characteristics like distance from nose to chin
Lighting, view of face, other noise can hinder this

37. Other Characteristics
Keystroke dynamics: believed to be unique
Keystroke intervals, pressure, duration of stroke, where key is struck
Statistical tests used

38. Cautions These can be fooled!
Assumes biometric device accurate in the environment it is being used in!
Transmission of data to validator is tamperproof, correct

39. Key Points Authentication is not cryptography
You have to consider system components
Passwords are here to stay
They provide a basis for most forms of authentication
Protocols are important
They can make masquerading harder
Authentication methods can be combined
Example: PAM
November 1, 2004
Introduction to Computer Security
©2004 Matt Bishop