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Chapter 11: Authentication. Basics Passwords. Establishing Identity. Authentication: binding of identity to subject One or more of the following What entity knows ( eg. Password, SSN) What entity has ( eg. badge, smart card) What entity is ( eg. fingerprints, biometrics)

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Chapter 11: Authentication


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establishing identity
Establishing Identity
  • Authentication: binding of identity to subject
  • One or more of the following
    • What entity knows (eg. Password, SSN)
    • What entity has (eg. badge, smart card)
    • What entity is (eg. fingerprints, biometrics)
    • Where entity is (eg. In front of a particular terminal)
slide3
An example:
    • You know some password
    • The computer also knows it (could be the password itself or some post computation result)
    • A function maps your password to the stored information
    • Examine whether they match
    • You can also change the password
authentication system
Authentication System
  • Authentication system contains: (A, C, F, L, S)
    • A: information that proves identity (what you know)
    • C: information stored on computer and used to validate authentication information (what computer knows)
    • F: complementation function; f : AC (function maps your knowledge to stored information)
    • L: functions that prove identity: A x C  {T, F}
    • S: functions enabling entity to create, alter information in A or C
example
Example
  • Password system, with passwords stored in clear text
    • A: set of strings making up passwords
      • All 8 character strings
    • C = A
      • All 8 character strings
    • F: Mapping an input password to itself
    • L: single equality test function { eq }
      • Input ?= stored password
    • S: function to set/change password
passwords
Passwords
  • 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
  • Algorithms
    • Examples: challenge-response, one-time passwords
storage
Storage
  • Password stored in cleartext
    • If password file compromised, all passwords revealed
  • Encipher file
    • Need to have decipherment, encipherment keys in memory
    • If attackers get a hold of the keys, reduces to previous problem
  • Store one-way hash of password
    • If file is read, attacker must still guess passwords or invert the hash
anatomy of attacking
Anatomy of Attacking
  • Goal: locate 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:
    • Approach 1: if attacker knows function f, she/he can try as above
      • E.g., attacker gets the file containing hashed passwords, she/he can start to hash all possible passwords
    • Approach 2: try to login by guessing a password
preventing attacks
Preventing Attacks
  • How to prevent this:
    • Hide one of a, f, or c
      • Prevents attack from above
      • Example:
        • You do not know the function f
        • You cannot get c
    • Block access to all lL or result of l(a)
      • Restrict the number of trying you can conduct in every unit time
      • Prevent attacker from knowing if guess succeeded
      • Prevent any logins to an account from a network
dictionary attacks
Dictionary Attacks
  • Trial-and-error from a list of potential passwords
    • Off-line: know f and c, and repeatedly try different guesses gA until the list is done or passwords guessed
      • Examples: pre-compute the hash of possible password
    • On-line: have access to functions in L and try guesses g until some l(g) succeeds
      • Examples: trying to log in by guessing a password
guessing probability
Guessing probability
  • P: probability of guessing a password,
    • ≥ tried password / total number of password
  • Put it in a formula:
    • G: number of guesses tried in 1 time unit
    • T: number of time units
    • N: number of possible passwords (|A|)
    • Then P ≥ TG/N
example1
Example
  • Goal
    • 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
    • P >= GT/N
      • GT/N = 0.5, G= 104 , T=60 * 60 * 24 * 365, N= 96^length
    • N ≥ TG/P = (365246060)104/0.5 = 6.311011
    • Choose length such that 96^length ≥ N
    • So length ≥ 6, meaning passwords must be at least 6 chars long
pronounceable passwords
Pronounceable Passwords
  • Generate phonemes randomly
    • Phoneme is unit of sound, something easy to pronounce
    • Examples: helgoret, juttelon are; przbqxdfl, zxrptglfn are not
    • Remember the names of brands: Mazda, Toshiba, etc
  • Problem: too few
    • For example, there are about 440 phonemes
    • If every phoneme contains 3 characters, then a 9-char password contains only 3 phonemes
    • Attacker needs to search only 440^3 = 85 million passwd
  • Solution: key crunching
    • Run long key through hash function and convert to printable sequence
    • Use this sequence as password
salting
Salting
  • Goal: mitigate dictionary attacks
  • Problem: Say all passwords are 8-char long, the attacker can pre-compute hash values of all 8-char strings. Later when she/he gets the hash file, she/he can easily determine the string
  • Method:
    • Introduce additional information so that it is very difficult for attackers to conduct pre-computation
    • Still very efficient for the system to authenticate a user
    • Example
      • Use salt as first part of input to hash function
guessing through l
Guessing Through L
  • Cannot prevent these
    • Otherwise, legitimate users cannot log in
  • Make them slow
    • Backoff
    • Disconnection
    • Disabling
      • Be very careful with administrative accounts!
      • Can be used to conduct DoS attacks
    • Jailing
      • Allow in, but restrict activities
password aging
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
slide18
Key management in some UNIX systems
    • don't use the shadow passwordfiles
    • the passwords are stored encrypted in the file /etc/passwd
    • Format of the stored record
      • Account; coded password data; homedir;
      • Gigawalt; fURfuu4.4hY0U; /home/gigawalt
slide20
It becomes more difficult for attackers to try all possible password offline
  • If an attacker gets the /etc/passwd file and knows all salt, then it is still ok to him/her
  • Make the salt private: stored in a file with restricted access permissions
  • Challenges
    • Keep the salt private
    • Maintain salt for users
key points
Key Points
  • Authentication is not cryptography
    • You have to consider system components
  • Passwords are important
    • They provide a basis for most forms of authentication
  • Protocols are important
    • They can make attacks harder
  • Authentication methods can be combined