Bishop: Chapter 12 Authentication
Outline <ul><li>Introduction </li></ul><ul><li>Passwords-based authentication </li></ul><ul><li>Challenge-response </li></...
Authentication <ul><li>Authentication:  Binding of  identity  to the  subject </li></ul><ul><ul><li>Entity  knows  somethi...
Authentication <ul><li>Five components in an  authentication system (AS) </li></ul><ul><ul><li>A :  set of  authentication...
Authentication <ul><li>(Cont.) </li></ul><ul><ul><li>L :  set of  authentication functions </li></ul></ul><ul><ul><ul><li>...
Passwords-based Authentication <ul><li>A  password  is information associated with an entity that confirms its identity. <...
Passwords-based Authentication <ul><li>Impersonating  by guessing passwords </li></ul><ul><ul><li>The goal: To find an a  ...
Passwords-based Authentication <ul><ul><li>A  dictionary attack  is the guessing of a password by repeated trial and error...
Passwords-based Authentication <ul><li>Countering  dictionary attack </li></ul><ul><ul><li>The  goal : To maximize the tim...
Passwords-based Authentication <ul><li>An example: </li></ul><ul><ul><li>Let S be the length of the password. </li></ul></...
Passwords-based Authentication <ul><li>Analysis of the Anderson Formula:  </li></ul><ul><ul><li>The goal is to maximize th...
Passwords-based Authentication <ul><li>Password Selection </li></ul><ul><ul><li>Random </li></ul></ul><ul><ul><li>Pronounc...
Outline <ul><li>Introduction </li></ul><ul><li>Passwords-based authentication </li></ul><ul><li>Challenge-response </li></...
Challenge-Response <ul><li>The fundamental problem with  passwords  is that they are  reusable . That is, a valid password...
Challenge-Response <ul><li>A function  f  is called a  pass algorithm , if  f  is the secret in a challenge-response AS. <...
Challenge-Response <ul><li>One-Time Passwords </li></ul><ul><ul><li>A  one-time password  is a password that is invalidate...
Challenge-Response <ul><li>Hardware-supported C/R authentication </li></ul><ul><ul><li>One or more hardware devices are us...
Challenge-Response <ul><li>Advantages of hardware-supported authentication? </li></ul><ul><ul><li>Two-factor authenticatio...
Challenge-Response <ul><li>Attacks on Challenge-Response Authentication </li></ul><ul><ul><li>If the attacker have suffici...
Outline <ul><li>Introduction </li></ul><ul><li>Passwords-based authentication </li></ul><ul><li>Challenge-response </li></...
Biometrics-Based Authentication <ul><li>Automatic measurement of biological or behavioral features that identify a person ...
Location-Based Authentication <ul><li>Location signature  =  unique location  +  unique time </li></ul><ul><ul><li>Describ...
Location-Based Authentication <ul><li>Strengths of LBA: </li></ul><ul><ul><li>Hardware-based authentication </li></ul></ul...
Hybrid Authentication <ul><ul><li>The principle of  multiple factors </li></ul></ul><ul><ul><li>Examples: </li></ul></ul><...
Summary <ul><li>Passwords-based authentication </li></ul><ul><li>Challenge/Response </li></ul><ul><li>Other methods: </li>...
Next <ul><li>Chapter 14: Identity  </li></ul>
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Authentication

  1. 1. Bishop: Chapter 12 Authentication
  2. 2. Outline <ul><li>Introduction </li></ul><ul><li>Passwords-based authentication </li></ul><ul><li>Challenge-response </li></ul><ul><li>Other methods: </li></ul><ul><ul><li>Biometrics-based authentication </li></ul></ul><ul><ul><li>Location-based authentication </li></ul></ul><ul><ul><li>Hybrid approach </li></ul></ul>
  3. 3. Authentication <ul><li>Authentication: Binding of identity to the subject </li></ul><ul><ul><li>Entity knows something </li></ul></ul><ul><ul><ul><li>Passwords, id numbers </li></ul></ul></ul><ul><ul><li>Entity has something </li></ul></ul><ul><ul><ul><li>Badge, smart card </li></ul></ul></ul><ul><ul><li>Entity is something </li></ul></ul><ul><ul><ul><li>Biometrics </li></ul></ul></ul><ul><ul><li>Entity is someplace </li></ul></ul><ul><ul><ul><li>Source IP, restricted area terminal </li></ul></ul></ul>
  4. 4. Authentication <ul><li>Five components in an authentication system (AS) </li></ul><ul><ul><li>A : set of authentication information </li></ul></ul><ul><ul><ul><li>used by the entity to prove its identify </li></ul></ul></ul><ul><ul><ul><li>e.g., id/password </li></ul></ul></ul><ul><ul><li>C : set of complementary information </li></ul></ul><ul><ul><ul><li>used by the AS to validate A </li></ul></ul></ul><ul><ul><ul><li>e.g., passwords </li></ul></ul></ul><ul><ul><li>F : set of complementation functions </li></ul></ul><ul><ul><ul><li>f : A -> C </li></ul></ul></ul><ul><ul><ul><li>Given a  A, generate appropriate c  C </li></ul></ul></ul><ul><ul><ul><li>e.g., Given id & password, the AS retrieves the stored password. </li></ul></ul></ul>
  5. 5. Authentication <ul><li>(Cont.) </li></ul><ul><ul><li>L : set of authentication functions </li></ul></ul><ul><ul><ul><li>l : A  C -> { true , false } </li></ul></ul></ul><ul><ul><ul><li>verify the entity’s identity </li></ul></ul></ul><ul><ul><ul><li>e.g., Given the id/password provided by the entity, and the retrieved password, the AS compares the two passwords to determine if the entity should be authenticated. l in this case is the equivalent( ) function. </li></ul></ul></ul><ul><ul><li>S : set of selection functions </li></ul></ul><ul><ul><ul><li>Enables the entity to generate/alter A and/or C </li></ul></ul></ul><ul><ul><ul><li>e.g., mechanisms allowing a user to change its password (such as passwd in UNIX) </li></ul></ul></ul>
  6. 6. Passwords-based Authentication <ul><li>A password is information associated with an entity that confirms its identity. </li></ul><ul><li>How can passwords be protected? </li></ul><ul><li>A solution: one-way hashing </li></ul><ul><ul><li>A user’s password is encrypted and then stored. The stored password is never decrypted. </li></ul></ul><ul><ul><li>It should be difficult for an attacker to revert the stored password to the plaintext password. </li></ul></ul><ul><ul><li>A user A may try to guess the password of another user, B, and thus impersonate B. (next slide) </li></ul></ul>
  7. 7. Passwords-based Authentication <ul><li>Impersonating by guessing passwords </li></ul><ul><ul><li>The goal: To find an a  A such that, for f  F, f(a) = c  C, and c is associated with an entity. </li></ul></ul><ul><li>General approaches </li></ul><ul><ul><li>Hide enough information so that one of a, c, or f cannot be found. </li></ul></ul><ul><ul><li>Prevent access to the authentication functions L. </li></ul></ul><ul><ul><li>Other approaches? The 3-tries “principle” </li></ul></ul>
  8. 8. Passwords-based Authentication <ul><ul><li>A dictionary attack is the guessing of a password by repeated trial and error. </li></ul></ul><ul><ul><li>The dictionary may be a set of strings in random order, or a set of strings in decreasing order of probability of selection. </li></ul></ul><ul><ul><li>Type one dictionary attack : Given c  C and f  F , the attack takes each guess g and computes f(g) for each f  F. If f(g) corresponds to the c for the entity, then g authenticates the entity under f . </li></ul></ul><ul><ul><li>Type 2 dictionary attack : Given l  L , the attack takes a guess g . If l returns true, given g , then g is the correct password. </li></ul></ul>
  9. 9. Passwords-based Authentication <ul><li>Countering dictionary attack </li></ul><ul><ul><li>The goal : To maximize the time needed to guess the password </li></ul></ul><ul><ul><li>Anderson’s Formula: </li></ul></ul><ul><ul><ul><li>P : The probability that an attacker guesses a password in a specified period of time </li></ul></ul></ul><ul><ul><ul><li>G : The number of guesses that can be tested in one time unit </li></ul></ul></ul><ul><ul><ul><li>T : The number of time units during which guessing occurs </li></ul></ul></ul><ul><ul><ul><li>N : The number of possible passwords </li></ul></ul></ul><ul><ul><li>A generalized form of the formula: </li></ul></ul><ul><ul><li>Examples: next slide </li></ul></ul>
  10. 10. Passwords-based Authentication <ul><li>An example: </li></ul><ul><ul><li>Let S be the length of the password. </li></ul></ul><ul><ul><li>Let A be the number of characters in the alphabet from which the characters of the password are drawn. </li></ul></ul><ul><ul><ul><li>Then N = A S . </li></ul></ul></ul><ul><ul><li>Let E be the number of characters exchanged when logging in. </li></ul></ul><ul><ul><li>Let R be the number of bytes per minute that can be sent over a communication link. </li></ul></ul><ul><ul><li>Let G be the number of guesses per minute. Then G = R / E. </li></ul></ul><ul><ul><li>If the attack extends over M months, T = 30 x 24 x 60 x M. </li></ul></ul><ul><ul><li>Let P be the probability that the attack would succeed. </li></ul></ul><ul><ul><ul><li>Then </li></ul></ul></ul>
  11. 11. Passwords-based Authentication <ul><li>Analysis of the Anderson Formula: </li></ul><ul><ul><li>The goal is to maximize the time needed for the attacker to guess the password (T). </li></ul></ul><ul><ul><li>That is, to decrease the chance that the attack may succeed (P). </li></ul></ul><ul><li>Approaches: </li></ul><ul><ul><li>To increase N, the set of possible passwords </li></ul></ul><ul><ul><li>To decrease the time allowed to guess the passwords, that is, to reduce T </li></ul></ul><ul><ul><li>To decrease G </li></ul></ul><ul><li>Question: How can each of the approaches be implemented? See the 2 nd example on page 313. </li></ul>
  12. 12. Passwords-based Authentication <ul><li>Password Selection </li></ul><ul><ul><li>Random </li></ul></ul><ul><ul><li>Pronounceable nonsense </li></ul></ul><ul><ul><li>Use selection </li></ul></ul><ul><ul><ul><li>Controls on allowable </li></ul></ul></ul><ul><ul><li>Password checking, aging </li></ul></ul>
  13. 13. Outline <ul><li>Introduction </li></ul><ul><li>Passwords-based authentication </li></ul><ul><li>Challenge-response </li></ul><ul><li>Other methods: </li></ul><ul><ul><li>Biometrics-based authentication </li></ul></ul><ul><ul><li>Location-based authentication </li></ul></ul><ul><ul><li>Hybrid approach </li></ul></ul>
  14. 14. Challenge-Response <ul><li>The fundamental problem with passwords is that they are reusable . That is, a valid password may be replayed by an attacker. </li></ul><ul><li>Solution? To associate a password with only one session </li></ul><ul><li>A challenge-response AS is one in which the system S sends a random message m (the challenge) to a user U , and U replies with r = f(m) . f( ) is a secret function agreed upon by both S and U . </li></ul><ul><li>U  request to connect  S </li></ul><ul><li>S  challenge (c)  U </li></ul><ul><li>U  response (r)  S </li></ul><ul><li>// S validates r by determining whether r = f ( c ). </li></ul>
  15. 15. Challenge-Response <ul><li>A function f is called a pass algorithm , if f is the secret in a challenge-response AS. </li></ul><ul><li>A sample pass algorithm: </li></ul><ul><ul><li>Suppose f(c) = c c mod <the-current-year>. </li></ul></ul><ul><ul><li>On November 12, 2004, when the AS sends c = 5 to U, U should return 1121 in order to be authenticated. Otherwise the authentication fails. </li></ul></ul>
  16. 16. Challenge-Response <ul><li>One-Time Passwords </li></ul><ul><ul><li>A one-time password is a password that is invalidated as soon as it is used. </li></ul></ul><ul><ul><li>A form of challenge-response authentication. </li></ul></ul><ul><ul><li>The user and the AS agree on the mechanism of generating a sequence of passwords. </li></ul></ul><ul><ul><li>Every time a password in the sequence is used, it is invalidated. The next time the user logs in, the next password in the sequence is used. </li></ul></ul>
  17. 17. Challenge-Response <ul><li>Hardware-supported C/R authentication </li></ul><ul><ul><li>One or more hardware devices are used in the C/R validation procedure. </li></ul></ul><ul><li>Two forms </li></ul><ul><li>Tokens: A token is a device that provides mechanisms for hashing or enciphering information. </li></ul><ul><ul><li>a) AS  c  U b) U  c  token c) token  r  U d) U  r  AS </li></ul></ul><ul><li>Time-based: A device, which is attached to a computer, displays a different number every 60 seconds. To log in, the user enters the number displayed on the device, followed by his/her password. </li></ul><ul><ul><li>e.g., RSA SecurID Card </li></ul></ul>
  18. 18. Challenge-Response <ul><li>Advantages of hardware-supported authentication? </li></ul><ul><ul><li>Two-factor authentication </li></ul></ul><ul><li>Links: </li></ul><ul><ul><li>RSA SecurID Card (SD200): http://www.mocomsystems.com/rsa_securID_card.html </li></ul></ul><ul><ul><li>RSA SecurID® Card Studio: http://www.rsasecurity.com/company/news/releases/pr.asp?doc_id=880 </li></ul></ul><ul><ul><li>Authentication using RSA SecurID Card: http://www.safestone.com/downloads/datasheets/rsa_securid.pdf </li></ul></ul><ul><li>More Information: </li></ul><ul><ul><li>The Power Behind RSA SecurID® Two-factor User Authentication: RSA ACE/Server® (an RSA white paper) http://www.rsasecurity.com/products/securid/whitepapers/AS51_SB_0203.pdf#xml=http://www.rsasecurity.com/programs/texis.exe/webinator/search/xml.txt?query=RSA+SecurID+Card+&pr=default&order=r&cq=&id=3fb204b72 </li></ul></ul>
  19. 19. Challenge-Response <ul><li>Attacks on Challenge-Response Authentication </li></ul><ul><ul><li>If the attacker have sufficient number of challenges (c) and the respective responses (r), he can try to deduce the encryption algorithm. </li></ul></ul><ul><ul><li>Example: the compromised Kerberos AS </li></ul></ul><ul><ul><li>A solution: encrypted key challenge [Bellovin/Merritt, 1992] </li></ul></ul><ul><ul><ul><li>Use shared key to share session key </li></ul></ul></ul><ul><ul><ul><li>Session key encrypts challenge </li></ul></ul></ul><ul><ul><ul><li>Challenge thus indistinguishable from random data </li></ul></ul></ul>
  20. 20. Outline <ul><li>Introduction </li></ul><ul><li>Passwords-based authentication </li></ul><ul><li>Challenge-response </li></ul><ul><li>Other methods: </li></ul><ul><ul><li>Biometrics-based authentication </li></ul></ul><ul><ul><li>Location-based authentication </li></ul></ul><ul><ul><li>Hybrid approach </li></ul></ul>
  21. 21. Biometrics-Based Authentication <ul><li>Automatic measurement of biological or behavioral features that identify a person </li></ul><ul><li>Used for human subject identification </li></ul><ul><li>Based on physical characteristics that are tough to copy </li></ul><ul><ul><li>fingerprint </li></ul></ul><ul><ul><li>voice patterns </li></ul></ul><ul><ul><li>iris patterns / retina scans </li></ul></ul><ul><ul><li>face recognition </li></ul></ul><ul><ul><li>keystroke dynamics (interval, timing, pressure, duration, location, etc.) </li></ul></ul><ul><ul><li>Combined approach </li></ul></ul>
  22. 22. Location-Based Authentication <ul><li>Location signature = unique location + unique time </li></ul><ul><ul><li>Describes the physical location of an entity </li></ul></ul><ul><ul><li>Relies on GPS </li></ul></ul><ul><li>Location signature sensor (LSS) </li></ul><ul><ul><li>A hardware device used by the user’s host to obtain a LS for the user </li></ul></ul><ul><ul><li>The LS is transmitted from the user site to the AS site for authentication. </li></ul></ul><ul><li>Attacks at location-based authentication </li></ul><ul><ul><ul><li>Stolen LSS: impersonation attacks ? </li></ul></ul></ul><ul><ul><ul><li>Intercepted LS: replay attacks ? </li></ul></ul></ul>
  23. 23. Location-Based Authentication <ul><li>Strengths of LBA: </li></ul><ul><ul><li>Hardware-based authentication </li></ul></ul><ul><ul><li>A stolen LSS is useless. (True?) </li></ul></ul><ul><ul><li>The authentication can be done continuously. </li></ul></ul><ul><li>Any weakness ? </li></ul>
  24. 24. Hybrid Authentication <ul><ul><li>The principle of multiple factors </li></ul></ul><ul><ul><li>Examples: </li></ul></ul><ul><ul><ul><li>LSS + id/password </li></ul></ul></ul><ul><ul><ul><li>Challenge/response + smart card </li></ul></ul></ul><ul><ul><ul><li>What you are + what you have (examples?) </li></ul></ul></ul>
  25. 25. Summary <ul><li>Passwords-based authentication </li></ul><ul><li>Challenge/Response </li></ul><ul><li>Other methods: </li></ul><ul><ul><li>Biometrics-based authentication </li></ul></ul><ul><ul><li>Location-based authentication </li></ul></ul><ul><ul><li>Hybrid approach </li></ul></ul>
  26. 26. Next <ul><li>Chapter 14: Identity </li></ul>
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