Topics. Authentication System. Passwords

Transcription

1 Passwords Topics 1. Password Systems 2. Password Attacks 3. Mitigating Attacks 4. Graphical passwords 5. One-time passwords 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 1

2 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. 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) Classic UNIX Passwords Format: Up to 8 ASCII characters A contains 6.9 x possible passwords C contains crypt hashes, strings of length 13 chosen from alphabet of 64 characters, 3.0 x strings Storage /etc/passwd (0644) was traditionally used /etc/shadow (0600) in modern systems 2

3 Attacks on Password Systems 1. Offline password cracking 2. Online dictionary attacks 3. Observe passwords 4. Attacking password infrastructure Offline Password Cracking Get Hashed Password pw hash word = Next word from list List of potential passwords. word hash = Hash(word) word hash == pw hash False True word is pw Cracking Methods 1. List of common passwords 2. List of English/foreign words 3. Permutation rules Substitute numbers/symbols for letters Change case, pluralize, reverse words, character shifts, digit/symbol prefix/postfix,joining words 4. Brute force All possible passwords 3

4 Online Password Guessing Use application s login interface Much slower than offline attack, but Always possible to do. Two types of attack Guess passwords for a specific known user account Know that it s a valid account Use information about user (name, bday) to guess Guess usernames and guess passwords for all accounts Don t know if account is valid Failures may be due to invalid username or password Someone probably has an easy to guess password Observe Passwords Sniff passwords off network Must be able to access a network node in path between source and destination of communication Impossible if connection is encrypted Find passwords in a file or database Identify files/databases that likely to contain passwords May or may not be encrypted Shoulder surf Watch for someone to type a password Desk search Passwords may be written down near workspace Attacking Password Infrastructure Web sites will you password if you answer a simple secret question: 1. What is your favorite color? 2. What is your pet s name? 3. What is your mother s maiden name? Violation of fail-safe defaults Failover to less secure protocol. How many favorite colors are there? 4

5 Mitigating Password Attacks 1. Enable system to use long passwords with a large alphabet of possible characters 2. Hash passwords to protect against file attacks 3. Salt passwords to protect against offline cracking 4. Select hard to guess passwords 5. Mitigating online attacks with backoff and lockout 6. Protect password management functions 7. Password aging 8. One-time passwords Countering Password Guessing Choose A, C, and F to select suitably low probability P(T) of guessing in time T. 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 Calculating Minimum Password Length Password System There are 96 allowable characters in password. System allows 10 6 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 10 6 / 0.5 = 6.31 x N = 96 i, where i ranges from 1 to length of password 96 i >= N = 6.31 x is true when largest i >= 8 The minimum required password length is 8. 5

6 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 2 53 possible passwords. How to resist dictionary attacks? Salting 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 2 66 Too small for today; modern UNIX doesn t use crypt. Salting (cont.) Prevents pre-calculated dictionary attack 2 66 passwords requires millions of terabytes crypt() 2 18 passwords/second Brute force would require 8000 machines for 48 days. 6

7 Modern UNIX Passwords Format: long ASCII string Hashing techniques: MD5 (unlimited length, bit salt) SHA1 (unlimited length, bit salt) Bcrypt (55 chars, 128-bit salt, adjustable cost) 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 Windows LM Hash Algorithm 1. Password fitted to 14 character length by truncating or padding with 0s. 2. Password converted to upper case. 3. Password divided into two 7-byte halves. 4. Each half used as DES key to encrypt same 8-byte constant. 5. Resultant strings merged to form a 16-byte hash value. 7

8 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. 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. Password Selection 1. Random Selection 2. Pronounceable Passwords 3. User Selection 8

9 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 Random Selection (Bad)Example PDP-11 password generator 16-bit machine 8 upper-case letters and digits P = 36 8 = 2.8 x At sec/encryption, 140 years to brute force PRNG had period of Only 65,535 possible passwords Requires 102 seconds to try all passwords 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. 9

10 User Selection Allow users to choose passwords. Reject insecure passwords based on ruleset: 1. Based on account, user, or host names 2. Dictionary words 3. Permuted dictionary words 4. Patterns from keyboard 5. Shorter than 6 characters 6. Digits, lowercase, or uppercase only passwords 7. License plates or acronyms 8. Based on previously used passwords Human Randomness? Most Common Passwords 10

11 How to Select Good Passwords 1. Long passwords, consisting of multiple words.. Use n th letter of each word if phrase too long. 2. Themes: 1. Word combinations: 3 blind katz 2. or URL: yoda@strong-this-password-is.net 3. Phone number: (888) 888-eight eight 4. Bracketing: Starfleet -> *!-Starfleet-!* 5. Add a word: shopping -> Goin shopping 6. Repetition: Pirate--PirateShip 7. Letter swapping: Sour Grape -> Gour Srape Mitigating Online Attacks If complements not accessible, attacker cannot use offline attacks and 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. Protecting Password Management Always send password to a predetermined location Do not let user change /phone when requesting a password reset or change. Always notify the user of a password change User can be alerted in case an attacker has changed the password. Let user create questions and answers Don t provide a list of pre-determined questions. 11

12 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 1. If change time too short, users have difficulty recalling passwords. 2. Cannot allow users to change password to current one. 3. Also prevent users from changing passwords too soon. 4. Give notice of impending password change requirement. 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. Password Reuse 12

13 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. 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 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) = k 1, h(k 1 ) = k 2,, h(k n-1 ) = k n Passwords, in order used, are p 1 = k n, p 2 = k n-1,, p n-1 = k 2, p n = k 1 13

14 S/Key Attacker cannot derive p i+1 from p i since p i = k n-i+1, p i+1 = k n-i, and h(k n-i ) = k n-i+1 which would require inverting h. Once user has used all passwords, S/Key must be reinitialized with a new seed. S/Key Login 1. User supplies account name to server 2. Server replies with number i stored in skeykeys file 3. User supplies corresponding password p i 4. Server computes h(p i ) = h(k n-i+1 ) = k n-i+2 = p i-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 p i 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 14

15 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 Key Points Passwords can be attacked by Offline cracking Online guessing Observation Attacking password management functions Stored passwords are secured via Hashing (crypt, MD5, SHA1, bcrypt) Salting Good passwords need to be Complex Unique Secret Changed on a regular basis One-time passwords offer greater security. References 1. Ross Anderson, Security Engineering, Wiley, Matt Bishop, Introduction to Computer Security, Addison- Wesley, Mark Burnett and Dave Kleiman, Perfect Passwords, Syngress, Lorie Faith Cranorand Simson Garfinkel, Security and Usability, O Reilly, Daswani, Foundations of Security, APress, Cynthia Kuo et. al., Human Selection of Mnemonic Phrasebased Passwords, SOUPS 2006, Neils Provosand David Mazieres, A Future-Adaptable Password Scheme, Ed Skoudis, Counter Hack Reloaded, Prentice Hall, Simson Garfinkel, Gene Spafford, and Alan Schwartz, Practical UNIX and Internet Security, 3/e O Reilly,