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Hands-On Network Security: Practical Tools & Methods Security Training Course Dr. Charles J. Antonelli The University of Michigan 2012 Hands-On Network Security Module 4 Password Strength & Cracking Roadmap Password

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SLIDE 1

Hands-On Network Security: Practical Tools & Methods

Security Training Course

  • Dr. Charles J. Antonelli

The University of Michigan 2012

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SLIDE 2

Hands-On Network Security

Module 4 Password Strength & Cracking

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SLIDE 3

Roadmap

  • Password Authentication
  • How Passwords are Cracked
  • Countermeasures

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SLIDE 4

Password Authentication

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SLIDE 5

Password Representations

  • UNIX
  • DES “Hashes”

 Old technology, but still around

  • Linux
  • Hashes

 Salted SHA-512, SHA-256, MD5, Blowfish

  • Mac OS X
  • Hashes

 Salted SHA-1

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SLIDE 6

UNIX “Hash” Generation

  • Password length 8 characters or less
  • 7 bits of each character used to generate 56-bit key
  • Key used to encrypt a constant using a variation of the

DES algorithm

Key MGoBlue1

Constant

(0x00000000)

DES’

UNIX Hash

zvktPWeeFzCVA 04/12

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SLIDE 7

UNIX “Hash” Considerations

  • It’s not a hash
  • Keyboard character set
  • Common alphanumeric set only
  • Character variations ≈ 126
  • Maximum entropy ≈ 6.3*1016 passwords
  • Salted

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SLIDE 8

Linux Hash Generation

  • Hash the password
  • Store it

SHA-512 Hash

$6$dmk52gd$TWOWIDs1q6/uZ.t49s.YkFQr3zeTGzrYwN33Ep2pdTKw HekN/O2hK0QuSTtUYNmS5Homqtp9lA/jf0hWRE7Bb/

MGoBlue1

SHA-512

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SLIDE 9

Linux Hash Considerations

  • Keyboard character set
  • Common alphanumeric set only
  • Character variations ≈ 126
  • Maximum length = 256 characters
  • Entropy for 256-character password ≈ 4.9*10538
  • Entropy for 20-character password from 126 character set

≈ 1.0*1042

  • Entropy for 20-character password from 69 “keyboard”

character set ≈ 6.0*1036

  • Salted

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SLIDE 10

Linux Passwords

  • Passwords stored in
  • /etc/shadow readable only by root
  • Other per-user information stored in
  • /etc/passwd world readable
  • UNIX stored both in /etc/passwd !

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SLIDE 11

Linux Hashes

  • Several hashes available
  • Use SHA-512!

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ID Method $1$ MD5 $2a$ Blowfish (some distros) $5$ SHA-256 $6$ SHA-512 (default) 11

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SLIDE 12

SHA-3 Hash Contest Update

  • MD5 broken, SHA-1&2 suspect
  • NIST competition for a SHA-3
  • Timeframe 2008-2012
  • 51 candidates submitted for Round 1
  • 14 candidates in Round 2

 BLAKE, Blue Midnight Wish, CubeHash, ECHO, Fugue, Grøstl, Hamsi, JH, Keccak, Luffa, Shabal, SHAvite-3, SIMD, and Skein

  • Final candidates announced December 10, 2010

 BLAKE, Grøstl, JH, Keccak, and Skein

  • Final SHA-3 candidate conference held March 2012

 http://csrc.nist.gov/groups/ST/hash/sha-3/Round3/March2012/index.html

  • SHA-3 standard to be published 2012

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SLIDE 13

Choosing A Password

  • Good
  • Pass phrases (much

longer than 8 characters)

  • miX cAsE
  • digits/punctuation
  • control characters
  • easy to remember
  • no words in any

language

  • Bad
  • people’s names
  • dictionary/technical

words or phrases

  • birth dates
  • places
  • common acronyms
  • backwards spelling
  • simple permutations
  • 8 characters or less

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SLIDE 14

Choosing A Password 2012

  • Good
  • Pass phrases (much

longer than 8 characters)

  • Bad
  • Everything else

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SLIDE 15

How Passwords are Cracked

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Passive Online Attacks

Man-in-the-Middle and Replay Attacks

  • Somehow get access to communications

channel

  • Wait for authentication sequence
  • Proxy authentication-traffic
  • No need to brute-force
  • Considerations
  • Relatively hard to perpetrate
  • Must be trusted by one or both sides
  • Some tools widely available

 Anyone remember MarketScore?

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SLIDE 17

Active Online Attacks

Password guessing

  • Try different passwords until one works
  • Made easier by
  • Bad passwords
  • Excessive information from server
  • Lack of password guessing controls
  • Considerations
  • Assuming good passwords, is this even feasible?

 Common 8 character password space (69^8)  Password Expires in 90 days  Need to guess 3,964,493,629 pwds/sec  Need throughput of 253,727,592,310 bits/sec  Gigabit Ethernet = 1B bits/sec

  • Easily detected and stopped
  • Core problem: Bad passwords

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SLIDE 18

Offline Attacks

  • Attacker has password database
  • Not that hard: Need to be admin (or steal the box)
  • Can attack at leisure – Attack types:
  • Dictionary attack

 Very Fast  Core Problem: Bad Passwords

  • Brute Force attack

 AlphaNumerics then AlphaNumerics + Upper Row Symbol, etc  Slow, but will eventually find all passwords

  • Hybrid

 Start with Dictionary, Insert Entropy

  • Pre-computed Hashes

 Rainbow tables  Time-space tradeoff

  • Considerations
  • Moore’s law

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SLIDE 19

John the Ripper

  • http://www.openwall.com/john/
  • Fast, open-source password cracker
  • Created by Solar Designer
  • Active development group
  • Runs on Linux, Mac OS X, Solaris, Android, …
  • Handles DES, BSDI DES, FreeBSD MD5, OpenBSD

Blowfish, Kerberos AFS DES, and LM DES hashes

  • Runs well on HPC clusters using Open MP
  • No GPU support yet
  • But see http://www.elcomsoft.com/edpr.html?r1=Openwall

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SLIDE 20

Lab: Crack Passwords

1. Install John the Ripper

  • cd; tar zxf /usr/local/lab/john/john-1.7.9.tar.gz; cd ~/john-1.7.9/doc
  • Follow directions in INSTALL & README

2. Create test account with a weak password using MD5 hashing

  • sudo vi /etc/pam.d/system-auth

 Change string sha512 to md5 in third paragraph

  • sudo useradd sucker
  • sudo passwd sucker

3. Undo the change to system-auth you made in step 2. 4. Create test account with a weak password using SHA-512 hashing

  • sudo useradd trout
  • sudo passwd trout

5. Obtain password hashes

  • cd ~/john-1.7.9/run; sudo ./unshadow /etc/passwd /etc/shadow >passwd.1

6. Crack

  • ./john passwd.1

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SLIDE 21

Lab: Crack Passwords

  • You can interrupt at any time, and restart with
  • ./john –restore
  • If you want to start over
  • rm john.pot restore
  • To display all passwords found so far
  • ./john --show /tmp/passwd.1
  • To see how fast John is on your machine
  • ./john --test
  • When done, delete the test accounts and the local password and

crack files!

  • sudo userdel sucker; sudo userdel trout
  • /bin/rm ~/john-1.7.9/run/{john.pot,passwd.1}

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SLIDE 22

Rainbow Tables

  • What if you precomputed the password hashes?
  • All Windows LM Hashes: 166 Terabytes
  • All Windows NT Hashes < 15 chars: 140,959,235,198 Exabytes
  • This would result in faster cracking, at the cost of storing

all those hashes

  • This is the Time-Memory tradeoff
  • Implemented using hash chains

 Clever way to link the hashes into chains  Only store 1 in 10,000 hashes

  • Rainbow tables improve on hash chains
  • Reduce collisions (overlapping chains)
  • Ineffective against salted hashes
  • Unix, Linux, and Mac OS X hashes are salted
  • Windows NT hashes are not

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SLIDE 23

Rainbow Tables

  • http://ophcrack.sourceforge.net/
  • Windows password cracker that uses rainbow tables
  • Cracks LM and NT hashes
  • Live CD support
  • Free tables for Windows XP and Vista (dictionary based)
  • For-fee tables for Vista (NTLM)
  • Seems to be moribund
  • http://www.freerainbowtables.com/
  • “Folding@home” distributed cracking model
  • Terabytes of tables
  • Free tables
  • For-fee tables
  • Seems to be quite active

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SLIDE 24

Countermeasures

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Policy-Based Mitigation

  • Develop a password policy
  • Require pass phrases
  • Greater than 15 mixed characters
  • Password expiration for all accounts
  • No password reuse (temporal and spatial)
  • Account lockout (where appropriate)
  • Physical security policy
  • Cornerstone for any security
  • No physical security = no security
  • No policy = no enforcement

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Pass Phrases v. Passwords

  • Pass phrases are long strings
  • “I wish we’d use 2Factor authentication instead of passwords”
  • Very strong protection against attacks
  • Easy to remember, a bit longer to type
  • Passwords are short complex strings
  • “@Rag0Rnrul3z”
  • Hard to remember
  • Often difficult to type
  • Not resistant against current attacks

 Obvious substitutions are quickly broken

  • Take-away: Long easily-remembered phrases are better

than short complex passwords

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http://xkcd.com/936/

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SLIDE 27

Technology-Based Mitigation

Multi-factor authentication

  • Why use passwords at all?
  • Smart cards
  • Two-factor authentication
  • Very difficult to thwart
  • High cost of initial deployment

 Smart cards, tokens, readers, software, …  Long-term cost benefit

  • Idea: use your smartphone as your token

 http://www.duosecurity.com/

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SLIDE 28

Technology-Based Mitigation

Multi-factor authentication

  • Biometrics
  • Measure some physical characteristic

 Fingerprint, iris color distribution, retinal pattern, …

  • Usually defeated with non-technical attacks
  • Historically unreliable

 False positives - bad guy authenticated  False negatives - legitimate user refused

  • Can be stolen
  • Iris scanners popular

Courtesy WIkipedia

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SLIDE 29

Summary

  • Bad passwords get broken, even when

using good storage and authentication methods!

  • Solutions
  • 1. Use better passwords
  • 2. Don’t let bad guys get the hashes
  • Combination of policy and technology

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SLIDE 30

References

  • http://en.wikipedia.org/wiki/NIST_hash_function_competition
  • http://csrc.nist.gov/groups/ST/hash/sha-3/Round2/submissions_rnd2.html
  • http://csrc.nist.gov/groups/ST/hash/sha-3/Round2/Aug2010/documents/

presentations/BURR_SHA-3Conf-day_1_wrapup.pdf

  • http://en.wikipedia.org/wiki/Rainbow_tables
  • http://ophcrack.sourceforge.net/
  • man 3 shadow
  • man 3 crypt

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SLIDE 31

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http://threatpost.com/en_us/blogs/social-engineering-attacks-prove-failure-user-education-042110

cja 2012