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CSC 382/582: Computer Security

CSC 382/582: Computer Security. Passwords. Topics. Password Systems Password Cracking Hashing and Salting Password Selection Graphical Passwords One-time Passwords. Authentication System. A : set of authentication information information used by entities to prove identity

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CSC 382/582: Computer Security

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  1. CSC 382/582: Computer Security Passwords CSC 382/582: Computer Security

  2. Topics • Password Systems • Password Cracking • Hashing and Salting • Password Selection • Graphical Passwords • One-time Passwords CSC 382/582: Computer Security

  3. 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 382/582: Computer Security

  4. 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 382/582: Computer Security

  5. 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 382/582: Computer Security

  6. Classic UNIX Passwords Format: Up to 8 ASCII characters • A contains 6.9 x 1016 possible passwords • C contains crypt hashes, 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 382/582: Computer Security

  7. Threats to Password Systems • Interception of Passwords • Shoulder surfing • Keylogging • Network sniffing • Login Spoofing • Phishing • Password Guessing • Repeated trial and error logins • Repeated trial and error hashing of passwords CSC 382/582: Computer Security

  8. Password Cracking Attempt to discover passwords by for each word in list if have hashed password: hash word if hashed password == hashed word you know a valid password else attempt to login with word if login successful you know a valid password end CSC 382/582: Computer Security

  9. Cracking Methods • List of common passwords • List of 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 passwords CSC 382/582: Computer Security

  10. 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 382/582: Computer Security

  11. 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 382/582: Computer Security

  12. 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 382/582: Computer Security

  13. 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 382/582: Computer Security

  14. 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 doesn’t use crypt. CSC 382/582: Computer Security

  15. 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 382/582: Computer Security

  16. Modern UNIX Passwords • Format: long ASCII string • Hashing techniques: • MD5 (unlimited length, 12-48 bit salt) • SHA1 (unlimited length, 12-48 bit salt) • Bcrypt (55 chars, 128-bit salt, adjustable cost) CSC 382/582: Computer Security

  17. 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 382/582: Computer Security

  18. 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 382/582: Computer Security

  19. 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 382/582: Computer Security

  20. 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 382/582: Computer Security

  21. Password Selection • Random Selection • Pronounceable Passwords • User Selection CSC 382/582: Computer Security

  22. 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 382/582: Computer Security

  23. 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 382/582: Computer Security

  24. 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 382/582: Computer Security

  25. 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 382/582: Computer Security

  26. Human Randomness? CSC 382/582: Computer Security

  27. 123456 letmein password 12345678 dragon qwerty michael 654321 harley ranger iwantu xxxxxxx turtle united porsche guitar black diamond nascar jun0389 06031989 amanda phoenix mickey tigers purple xmen94 aaaaaa Bad Passwords • prince • beach • amateur • ncc1701 • tennis • startrek • swimming • kitty • rainbox • 112233 • 232323 • giants • enter • 0 • cupcake • 8675309 • marlboro • newyork • diablo • sexsex • access14 • abgrtyu • 123123 • dragon123 • applepie • 31415926 • 99skip • just4fun • xcvb • typewriter CSC 382/582: Computer Security

  28. How to Select Good Passwords • Long passwords, consisting of multiple words.. Use nth letter of each word if phrase too long. • Themes: • Word combinations: 3 blind katz • E-mail or URL: yoda@strong-this-password-is.net • Phone number: (888) 888-eight eight • Bracketing: Starfleet -> *!-Starfleet-!* • Add a word: shopping -> Goin’ shopping • Repetition: Pirate--PirateShip • Letter swapping: Sour Grape -> Gour Srape CSC 382/582: 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 382/582: 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 382/582: Computer Security

  31. Graphical Passwords • Face Scheme: Password is sequence of faces, each chosen from a grid of 9 faces. • Story Scheme: Password is sequence of images, each chosen from a grid of 9, to form a story. CSC 382/582: Computer Security

  32. 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 382/582: Computer Security

  33. 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 382/582: Computer Security

  34. 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 382/582: Computer Security

  35. 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 382/582: Computer Security

  36. 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 382/582: Computer Security

  37. 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 382/582: Computer Security

  38. 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 382/582: Computer Security

  39. Key Points • Good passwords need to be • Complex • Unique • Secret • Changed on a regular basis • Stored passwords are secured via • Hashing (crypt, MD5, SHA1, bcrypt) • Salting • One-time passwords offer greater security. CSC 382/582: Computer Security

  40. References • Ross Anderson, Security Engineering, Wiley, 2001. • Matt Bishop, Introduction to Computer Security, Addison-Wesley, 2005. • Mark Burnett and Dave Kleiman, Perfect Passwords, Syngress, 2006. • Lorie Faith Cranor and Simson Garfinkel, Security and Usability, O’Reilly, 2005. • Cynthia Kuo et. al., “Human Selection of Mnemonic Phrase-based Passwords,” SOUPS 2006, http://cups.cs.cmu.edu/soups/2006/proceedings/p67_kuo.pdf, 2006. • Neils Provos and David Mazieres, “A Future-Adaptable Password Scheme,” http://www.openbsd.org/papers/bcrypt-paper.pdf, 2006. • Ed Skoudis, Counter Hack Reloaded, Prentice Hall, 2006. • Simson Garfinkel, Gene Spafford, and Alan Schwartz, Practical UNIX and Internet Security, 3/e O’Reilly, 2003. CSC 382/582: Computer Security

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