Authentication Systems: Passwords, Challenge-Response, Biometrics, and Security - Prof. Da, Study notes of Electrical and Electronics Engineering

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Authentication

CS461/ECE

Reading

• Chapter 4.5 from Security in Computing

• Chapter 10 from Handbook of Applied

Cryptography

http://www.cacr.math.uwaterloo.ca/hac/abo

ut/chap10.pdf

Ivanhoe, Sir Walter Scott

• Paraphrased: (Wamba gains entry to the castle dressed as a friar) Wamba: Take my disguise and escape, I will stay and die in your place. Cedric: I can’t possibly impersonate a friar, I only speak English. Wamba: If anyone says anything to you, just say “ Pax vobiscum .” Cedric: What does that mean? Wamba: I don’t know, but it works like a charm!

Basics

• Authentication: binding of identity to

subject

• Identity is that of external entity (my identity,

the Illini Union Bookstore, etc .)

• Subject is computer entity (process, network

connection, etc .)

Authentication System

• ( A , C , F , L , S )
  • A : information that proves identity
  • C : information stored on computer and used to validate authentication information
  • F : set of complementation functions that generates C ; f : A → C
  • L : set of authentication functions that verify identity; l : A × C → { true , false }
  • S : functions enabling entity to create, alter information in A or C

Authentication System

A: identity proving info Computer F:Complementation C: identity validating info L: Authentication True or False S: Update

Storage

• Store as cleartext
  • 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

Example

• Original UNIX system standard hash function
  • Hashes password into 11 char string using one of 4096 hash functions
• As authentication system:
  • A = { strings of 8 chars or less }
  • C = { 2 char hash id || 11 char hash }
  • F = { 4096 versions of modified DES }
  • L = { login , su , … }
  • S = { passwd , nispasswd , passwd+ , … }

Preventing Attacks

• How to prevent this:
  • Hide information so that either A , F , or C cannot be found - Prevents obvious attack from above - Example: UNIX/Linux shadow password files - Hides C
  • Block access to all l ∈ L or result of l ( a,c )
    • 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 )

Using Time

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 If passwords are chosen randomly, how many (required) characters r make a brute force attach fail with probability at least 1-P? With an n character alphabet, so

Approaches: Password Selection

• Random selection
  • Any password from A equally likely to be selected
  • See previous example
  • Make sure it’s random! (e.g. period of 2 32 is not enough for (26+10) 8 passwords)
• Key crunching ( e.g. hashing) a long key to a

sequence of shorter keys

• Pronounceable passwords
• User selection of passwords

Pronounceable Passwords

• Generate phonemes randomly

• Phoneme is unit of sound, e.g. cv , vc , cvc , vcv
• Examples: helgoret, juttelon are; przbqxdfl,

zxrptglfn are not

• ~ 440 possible phonemes

6

possible keys with 6 phonemes (12-

characters long), about the same as 96

8

• Used by GNU Mailman mailing list

software (?)

Picking Good Passwords

• Examples from textbook
  • “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? 

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