cit 380: securing computer systemsslide #1 cit 380: securing computer systems passwords

CIT 380: Securing Computer Systems Slide #1

CIT 380: Securing Computer Systems

Passwords


Topics

1. Password Systems

2. Password Cracking

3. Hashing and Salting

4. Password Selection

5. Graphical Passwords

6. 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


Password System Example

User authenticates with 8-character alphanumeric password.

System compares against stored cleartext password.

A = [A-Za-z0-9]{8}C = AF = { I }L = { = }

Not a system that anyone should actually use.


Biometric System Example

User authenticates with fingerprint.

System compares w/ digital fingerprint template.

A = { user fingerprints }

C = { digital fingerprint templates }

F = { fingerprint reader + analog->digital }

L = { tunable similarity function }


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 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


Password CrackingGet Hashed Password pwhash

word = Next word from list

wordhash = Hash(word)

wordhash == pwhash

False

True

word is pw

List of potentialpasswords.


Cracking Methods

1. List of common passwords2. List of English/foreign words3. 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


Making Password Guessing Easier

Web sites will e-mail 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?


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 106 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 106 / 0.5 = 6.31 x 1013

– N = 96i, where i ranges from 1 to length of password

– 96i >= N = 6.31 x 1013 is true when largest i >= 8– The minimum required password length is 8.


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


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.


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.


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)


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.


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


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| = 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


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.


User Selection

Allow users to choose passwords.

Reject insecure passwords based on ruleset:1. Based on account, user, or host names2. Dictionary words3. Permuted dictionary words4. Patterns from keyboard5. Shorter than 6 characters6. Digits, lowercase, or uppercase only passwords7. License plates or acronyms8. Based on previously used passwords


Human Randomness?


Bad Passwords• 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

• 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


How to Select Good Passwords

1. Long passwords, consisting of multiple words..Use nth letter of each word if phrase too long.

2. Themes:1. Word combinations: 3 blind katz

2. E-mail or URL: [email protected]

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

mailto:[email protected]


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.


Password AgingRequirement that password be changed after a period of time or after an event has occurredIf 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.


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 attemptResponse: 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) = k1, h(k1) = k2, …, h(kn-1) = kn

Passwords, in order used, are

p1 = kn, p2 = kn-1, …, pn-1= k2, pn= k1


S/Key

Attacker cannot derive pi+1 from pi since

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.


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 pi

4. 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


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


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

• 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.


References1. Ross Anderson, Security Engineering, Wiley, 2001.2. Matt Bishop, Introduction to Computer Security, Addison-Wesley,

2005.3. Mark Burnett and Dave Kleiman, Perfect Passwords, Syngress,

2006.4. Lorie Faith Cranor and Simson Garfinkel, Security and Usability,

O’Reilly, 2005.5. 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.

6. Neils Provos and David Mazieres, “A Future-Adaptable Password Scheme,” http://www.openbsd.org/papers/bcrypt-paper.pdf, 2006.

7. Ed Skoudis, Counter Hack Reloaded, Prentice Hall, 2006.8. Simson Garfinkel, Gene Spafford, and Alan Schwartz, Practical

UNIX and Internet Security, 3/e O’Reilly, 2003.

http://cups.cs.cmu.edu/soups/2006/proceedings/p67_kuo.pdf

http://www.openbsd.org/papers/bcrypt-paper.pdf

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