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CSC 382: Computer Security

CSC 382: Computer Security. Authentication. Authentication. What is Authentication? Passwords Dictionary Attacks UNIX and Windows Passwords Password Selection Challenge-Response Biometrics Location. What is Authentication?. Binding of an identity to a subject Based on:

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CSC 382: Computer Security

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  1. CSC 382: Computer Security Authentication CSC 382: Computer Security

  2. Authentication • What is Authentication? • Passwords • Dictionary Attacks • UNIX and Windows Passwords • Password Selection • Challenge-Response • Biometrics • Location CSC 382: Computer Security

  3. What is Authentication? Binding of an identity to a subject Based on: • What the entity knows (e.g., passwords) • What the entity has (e.g., access card) • What the entity is (e.g., fingerprints) • Where the entity is (e.g., local terminal) CSC 382: Computer Security

  4. Authentication System A: set of authentication information • information used by entities to prove identity C: set of complementary information • information stored by system to validate A F: set of complementation functions f : A→ C • generate C from A L: set of authentication functions l: A  C→{T,F} • verify identity S: set of selection functions • enable entity to create or alter A or C CSC 382: Computer Security

  5. Password System Example User authenticates with 8-character alphanumeric password. System compares against stored cleartext password. A = [A-Za-z0-9]{8} C = A F = { I } L = { = } Not a system that anyone should actually use. CSC 382: Computer Security

  6. Passwords What you know Sequence of characters Complementation Function • Identity: requires access control to protect C • One-way Hash • easy to compute c = f(a) • difficult to compute a = f-1(c) CSC 382: Computer Security

  7. UNIX Passwords Format: Up to 8 ASCII characters • A contains 6.9 x 1016 possible passwords • C contains strings of length 13 chosen from alphabet of 64 characters, 3.0 x 1023 strings Storage • /etc/passwd (0644) was traditionally used • /etc/shadow (0600) in modern systems CSC 382: Computer Security

  8. Password Cracking Find a A, such that, for f  F, f(a) = c  C and c is associated with a particular entity. Methods of determining whether a is associated with an entity: • Compute f(a) • Defend by hiding info so a, c, or f unknown. • Authenticate via l(a) • Defend by preventing access to auth functions L. CSC 382: Computer Security

  9. Dictionary Attack Guess password by repeated trial and error. Types of Dictionary Attacks: • Compute f(g), requires password hashes. • Compute f(g) for each guess g and each f  F. • Success if f(g) matches complementary info for any entity E. • Compute l(g) • Attempt l(g) for each guess g. • Success if l(g) true. CSC 382: Computer Security

  10. Types of Dictionary Attacks • Common passwords • English/foreign words • Permutation rules • Substitute numbers/symbols for letters • Change case, pluralize, reverse words, character shifts, digit/symbol prefix/postfix,joining words • Brute force • All possible strings CSC 382: Computer Security

  11. Making Password Guessing Easier Web sites will e-mail you password if you answer a simple “secret” question: • What is your favorite color? • What is your pet’s name? • What is your mother’s maiden name? Violation of fail-safe defaults Failover to less secure protocol. How many favorite colors are there? CSC 382: Computer Security

  12. Countering Password Guessing Choose a, c, and f to select suitably low probability of successful guessing P(T) >= TG / N • G is number of guess per time unit T • T is number of time units in attack • N is number of possible passwords CSC 382: Computer Security

  13. Anderson’s Formula AS >= 4.32 x 104 M R / P E • N = AS • A is number of characters in password alphabet • S is length of password • G = R/E • R number of bytes/minute that can be transmitted • E number of characters exchanged when logging in • T = 4.32 x 104M is time (min) for M months CSC 382: Computer Security

  14. Example Password System • A = 96 characters • System allows 104 guesses/second • Requirement: probablility of success guess should be 0.5 over 365-day period What should the minimum password length be? • N >= TG/P • N >= (365 x 24 x 60 x 60) x 104 / 0.5 = 6.31 x 1011 • S96i >= N = 6.31 x 1011 is true when S >= 6 • The minimum required password length is 6. CSC 382: Computer Security

  15. Assumptions • Time to test password is constant • Reasonable for f F • Choose minimum time if variable. • All passwords are equally likely to be selected. • Dependent on password selection mechanism CSC 382: Computer Security

  16. UNIX Password Hashing crypt() function used for hashing • DES encrypts 64-bit block of 0s (25 rounds) using your password for the key. • Modified DES incompatible with DES hardware cracking tools. • Limited to 8 characters or less. • If limited to 95 printable characters, only 253 possible passwords. • How to resist dictionary attacks? Salting CSC 382: Computer Security

  17. Salting Adds a 2-character (12-bit) random, public data to password to create key. Any word may be encrypted in 4096 possible ways (i.e., there are 4096 f  F). • Your password always uses same salt. • Someone else with same password (a) probably has different salt, and thus different c = f(a). Number of possible keys increased to 266 • Too small for today; modern UNIX uses MD5/SHA-1. CSC 382: Computer Security

  18. Salting (cont.) Prevents pre-calculated dictionary attack • 266 passwords requires millions of terabytes crypt() 218 passwords/second • Brute force would require 8000 machines for 48 days. CSC 382: Computer Security

  19. Windows 2000/XP Passwords Storage • %systemroot%\system32\config\sam • locked while NT running • %systemroot%\repair\sam_ backup file • may be accessible via remote registry calls Format • LAN Manager (LM) Hash • NT (MD4) Hash CSC 382: Computer Security

  20. Windows LM Hash Algorithm • Password fitted to 14 character length by truncating or padding with 0s. • Password converted to upper case. • Password divided into two 7-byte halves. • Each half used as DES key to encrypt same 8-byte constant. • Resultant strings merged to form a 16-byte hash value. CSC 382: Computer Security

  21. Windows LM Hash Problems Last 8 bytes of c known if password < 7 chars. Dividing password into halves reducing problem of breaking 14-character password to breaking two 7-character passwords. Conversion to upper case reduces character set. Dictionary of password hashes can be prebuilt • Number of possible passwords much smaller than DES space. • No salt is used. CSC 382: Computer Security

  22. Windows NT Hash Converts to Unicode, MD4 hashes result Caveat: Often used in conjunction with LM hash, which is required for backwards compatibility. No salt: identical passwords generate identical hashes. CSC 382: Computer Security

  23. Password Selection • Random Selection • Pronounceable Passwords • User Selection CSC 382: Computer Security

  24. Random Selection Yields equal distribution of passwords for maximum difficulty in cracking • What about short passwords? Random passwords aren’t easy to remember • Short term memory holds 7 +/- 2 items • People have multiple passwords • Principle of Psychological Acceptability Requires a good PRNG CSC 382: Computer Security

  25. Random Selection (Bad)Example PDP-11 password generator • 16-bit machine • 8 upper-case letters and digits • |P| = 368 = 2.8 x 1012 • At 0.00156 sec/encryption, 140 years to brute force PRNG had period of 216 – 1 • Only 65,535 possible passwords • Requires 102 seconds to try all passwords CSC 382: Computer Security

  26. Pronounceable Passwords Generate passwords from random phonemes instead of random characters. • People can remember password as sequence of audible phonemes instead of characters, allowing easy recall of longer passwords. • Fewer pronounceable passwords exist than random passwords. CSC 382: Computer Security

  27. User Selection Allow users to choose passwords. Reject insecure passwords based on ruleset: • Based on account, user, or host names • Dictionary words • Permuted dictionary words • Patterns from keyboard • Shorter than 6 characters • Digits, lowercase, or uppercase only passwords • License plates or acronyms • Based on previously used passwords CSC 382: Computer Security

  28. How to Select Good Passwords • Use a long password (at least 8 chars). • Avoid dictionary words and permutations. • Avoid using passwords based on old ones. • Incorporate numbers and/or punctuation. • Use a pass phrase • Use nth letter of each word if phrase too long. • Manual random generation • Roll dice to select word from list. CSC 382: Computer Security

  29. Guessing via Authentication Fns If complements not accessible, attacker must use authentication functions. Cannot be prevented. Increase difficulty of auth function attack: Backoff: increasing wait before reprompting. Disconnection: disconnect after n failures. Disabling: disable account after n failures. Jailing: permit access to limited system, so admins can observe attacker. CSC 382: Computer Security

  30. Password Aging Requirement that password be changed after a period of time or after an event has occurred If expected time to guess is 180 days, should change password more frequently than 180 days • If change time too short, users have difficulty recalling passwords. • Cannot allow users to change password to current one. • Also prevent users from changing passwords too soon. • Give notice of impending password change requirement. CSC 382: Computer Security

  31. Challenge-Response Problem: passwords are reusable, and thus subject to replay attacks. Solution: authenticate in such a way that the transmitted password changes each time. CSC 382: Computer Security

  32. One-Time Passwords A password that’s invalidated once used. Challenge: number of auth attempt Response: one-time password Problems • Generation of one-time passwords • Use hash or crytographic function • Synchronization of the user and the system • Number or timestamp passwords CSC 382: Computer Security

  33. S/Key One-time password system based on a hash function h (MD4 or MD5). User initializes with random seed k. Key generator calculates: h(k) = k1, h(k1) = k2, …, h(kn-1) = kn Passwords, in order used, are p1 = kn, p2 = kn-1, …, pn-1= k2, pn= k1 CSC 382: Computer Security

  34. S/Key Attacker cannot derive pi+1 from pisince pi = kn-i+1, pi+1 = kn-i, and h(kn-i) = kn-i+1 which would require inverting h. Once user has used all passwords, S/Key must be re-initialized with a new seed. CSC 382: Computer Security

  35. S/Key Login • User supplies account name to server • Server replies with number i stored in skeykeys file • User supplies corresponding password pi • Server computes h(pi) = h(kn-i+1) = kn-i+2 = pi-1 and compares result with stored password. If match, user is authenticated and S/Key updates number in skeykeys file to i-1 and stores pi CSC 382: Computer Security

  36. S/Key Login FreeBSD/i386 (example.com) (ttypa) login: <username> s/key 97 fw13894 Password: Use S/Key calculator on local system to calculate response: % key 97 fw13894 Enter secret password: WELD LIP ACTS ENDS ME HAAG CSC 382: Computer Security

  37. Other One Time Password Systems Software: OPIE • Backwards compatible with S/Key (if same hash used). Hardware: RSA SecurID card • Displayed password changes every 60sec. • Password = constant password + SecurID CSC 382: Computer Security

  38. Biometrics Identification by human anatomic or behavioral characteristics. A biometric characteristic should be: • universal: everyone should have it • unique: not two people should share it • permanent: it should not change with time • quantifiable: it must be practically measurable CSC 382: Computer Security

  39. Biometric Measurement Possible Outcomes: • Correct person accepted • Imposter rejected • Correct person rejected (False Rejection) • Imposter accepted (False Acceptance) Tradeoff exists between False Accept Rate and False Reject Rate CSC 382: Computer Security

  40. Fingerprints Capacitive measurement, using differences in electrical charges of whorls on finger to detect those parts touching chip and those raised. Attacks: • Forcing authorized user to grant access. • Recover latent fingerprint impression. • Artificial gummy fingers made from molding plastic/gelatin 68-100% successful when tested against 11 types of fingerprint auth systems. CSC 382: Computer Security

  41. Types of Biometrics Voice Characteristic Recognition • statistical analysis of voice characteristics Eyes • iris patterns • retina patterns • pattern of blood vessels observed by laser Face Recognition • Can it distinguish a face from a picture of that face? CSC 382: Computer Security

  42. Types of Biometrics (cont.) Keystroke Dynamics • Signature based on keystroke intervals, pressure, duration, and where key is struck. • Belived to be unique as written signatures. • Dynamic detection constantly observes user. Combinations • Combine multiple biometric techniques. • Not necessarily an improvement: typically can reduce either False Accept Rate or False Reject Rate, not both. CSC 382: Computer Security

  43. Biometrics are not infallible What are False Accept and Reject Rates? Do the characteristics change over time? • Retina changes during pregnancy. • Fingerprint damage due to work/pipe smoking. • Young and old people have fainter fingerprints. Is it accurate in the installed environment? • Is someone observing fingerprint or voiceprint checks? • i.e., did you collect biometric from the person? CSC 382: Computer Security

  44. Biometrics can be compromised. Unique identifiers, not secrets. • You can change a password. • You can’t change your iris scan. Examples: • You leave your fingerprints every place. • It’s easy to take a picture of your face. Other compromises. • Use faux ATM-style devices to collect biometrics. • Obtain all biometric templates from server. CSC 382: Computer Security

  45. Use and Misuse of Biometrics Employee identification. • Employee enters login name. • System uses fingerprint to verify employee is who he claims to be. • Problem: Does biometric match the employee? Criminal search (Superbowl 2001) • System uses face recognition to search for criminals in public places. • Problem: Does any biometric in database match anyone in a crowd of people? • Assume system is 99.99% accurate and 1 in 10million people is a terrorist. Result: 1000 false positives for each terrorist. CSC 382: Computer Security

  46. Location Classic: only allow access from a particular terminal or a particular set of remote hosts. Modern: GPS-based • Location Signature Sensor (LSS) for host and user. • Access rules permit user only to access host with specific LSS values. • Cell-phones track location, and some states use them to track drivers’ speed and locations. CSC 382: Computer Security

  47. Key Points • Authentication consists of an entity, the user, attempting to convince another entity, the verifier, of the user’s identity • something you know, something you have, something you are, somewhere you are located • Passwords are the most common and basic type of authentication • Dictionary Attacks • Challenge Response: One-Time Passwords • Biometrics verify identity based on measured physical characteristics of the user CSC 382: Computer Security

  48. References • Phil Agre. “Your Face is not a Bar Code,” http://polaris.gseis.ucla.edu/pagre/bar-code.html, 2003. • Ross Anderson, Security Engineering, Wiley, 2001. • Matt Bishop, Introduction to Computer Security, Addison-Wesley, 2005. • Ben Mook, “Md. pilot program tracks drivers’ speed, location via cell phones,” The Daily Record, October 21, 2005, http://www.mddailyrecord.com/pub/5_398_friday/businessnews/172883-1.html • Bruce Schneier, “Biometrics: Truths and Fictions,” Cryptogram, http://www.schneier.com/crypto-gram-9808.html#biometrics, 1998. • Bruce Schneier, “The Curse of the Secret Question,” http://www.schneier.com/essay-081.html, 2005. • John Viega and Gary McGraw, Building Secure Software, Addison-Wesley, 2002. • David Wheeler, Secure Programming for UNIX and Linux HOWTO, http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/index.html, 2003. CSC 382: Computer Security

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