Cracking WiFi Networks

1
Cracking Wi-Fi Networks

• Peter MacLellan
• Northeastern University
• Candidate BSCS 2008

2
Overview

• Quick Legal Issues
• Available Security Features
• Cracking WEP
• Algorithm Weakness
• Software/Techniques
• Cracking WPA
• Conclusions Best Practices
• Questions

3
Legal Issues

• Laws on WiFi access go generally on a state by
  state basis, are vague and generally untested
• Title 18 of the U.S.Code
• Illegal to connect to federal government
  agencies, financial and interstate commerce
  institutions
• Be smart-only connect to a network
• That is yours you own it
• You have legitimate permission to do so

4
Available Security Features

• MAC (Media Access Control) Filtering
• MAC can be spoofed
• WEP (Wired Equivalent Privacy)
• Known to be practically insecure since 2001
• WPA (Wi-Fi Protected Access)
• Practically secure interim fix for WEP
• WPA2
• Practically secure Implements full IEEE
  802.11i-2004 standard

5
Access Point Admin Application

• Security features are set in the Access Points
  administration application (available via a web
  browser)
• Its important to change the admin users
  password from the get go
• A compromised network is much more appealing to a
  hacker if the hacker has full control over its
  settings

6
Overview

• Quick Legal Issues
• Available Security Features
• Cracking WEP
• Algorithm Weakness
• Software/Techniques
• Cracking WPA
• Conclusions Best Practices
• Questions

7
RC4

• WEP security uses the RC4 stream cipher invented
  by Ron Rivest of RSA Security in 1987
• RC4 is a stream cipher
• A stream cipher takes a key and a message
• The key is used to generate a pseudo-random bit
  string which is then XORed with the message to
  produce ciphertext
• Decryption happens in the same manner

8
WEPs Use of RC4

• For WEP, the key used is a concatenation of an
  Initialization Vector (IV, pseudo-random bits)
  and the key you set on the AP
• The use of the IV adds security to WEP by
  eliminating a 1-to-1 mapping between message and
  ciphertext

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Attacks on WEP (1 of 3)

• In 2001, Fluhrer, Mantin and Shamir discovered
  that the first few bytes of RC4 data are
  non-random
• In order for this attack to work, the IVs need to
  fulfill a so-called "resolved condition, i.e.
  weak IVs
• In response, manufacturers made Access Points
  which avoided using weak IVs
• 4 million frames needed for this attack

10
Attacks on WEP (2 of 3)

• In 2004, a person using the pseudonym KoreK
  posted a family of statistical attacks against
  WEP that do not need weak IVs
• Frames needed for key-recovery was reduced to
  about 500,000 packets (down from 4 million)
• aircrack-ng is based on this attack

11
Attacks on WEP (3 of 3)

• In 2007, Erik Tews, Ralf-Philipp Weinmann, and
  Andrei Pyshkin optimized the previous attacks
• Working at 802.11g data rates, they showed they
  could crack 128-bit WEP with just 85,000 packets,
  a success rate of 95...in less than 60 seconds
• This is the basis for aircrack-ptw

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WEP Packets

• Frames contain a plaintext header followed by
  encrypted data
• Packet lengths arent hidden
• BSSID are easily spoofed for retransmission of
  packets
• For key recovery, both plaintext and ciphertext
  must be known

13
ARP Requests

• Address Resolution Protocol
• Standard method for finding a host's hardware
  address when only its network layer address is
  known.
• Example Your computer asking for its IP address
• We dont know the secret key so we cannot
  generate our own ARP request.
• The solution is to capture an ARP request and
  replay it

14
Aircrack-ng

• Found at www.aircrack-ng.org/
• Steps for installing software (for active
  attacks)
• Must be running Linux and have a compatible card
• Patch drivers for card to enable injection
• Install aircrack-ng
• Install aircrack-ptw

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Why Crack WEP?

• Though WEP was cracked in 2001, empirical
  evidence shows that 70 of networks still use it
• http//eprint.iacr.org/2007/120

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Example Site Survey
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Overview

• Quick Legal Issues
• Available Security Features
• Cracking WEP
• Algorithm Weakness
• Software/Techniques
• Cracking WPA
• Conclusions Best Practices
• Questions

18
WPA Encryption

• WPA still uses RC4 for encryption with the added
  security of TKIP
• TKIP Temporal Key Integrity Protocol
• Rekeying protocol ensures that every packet is
  encoded with a different key
• Message integrity check disallows forged packets
• Known WEP attacks not possible

19
WPA Attack

• Two types of WPA authentication
• WPA Business
• WPA Personal
• WPA Business uses RADIUS server for
  authentication
• WPA Personal uses four-way handshake for
  authentication
• Only WPA Personal can be cracked using
  information from the four-way handshake

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WPA Dictionary Attack

• Four-way handshake uses the network name as a
  step in masking the contents of the key
• Impossible to keep a full dictionary of
  password-ciphertext pairs for all networks (this
  can be done for common names)
• Instead, generate a list of probable passwords
  that can be used to compute handshake and compare
  it to known handshake offline, on the fly

21
Password Generation

• Research has shown that most passwords include
  English words
• Hackers/University Researchers have compiled
  lists that contain the most probable words and
  numbers
• I wrote a program that combines all the possible
  words with all the possible numbers which is then
  piped to aircrack-ng

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Overview

• Quick Legal Issues
• Available Security Features
• Cracking WEP
• Algorithm Weakness
• Software/Techniques
• Cracking WPA
• Conclusions Best Practices
• Questions

23
Best Practices

• Always change the password of the APs
  administrator application
• Select WPA as the encryption protocol and use a
  password not based on an English word
• Use MAC Address filtering
• Dont piggyback off of another persons network
• Check your ISPs license agreement to see if you
  can share your connection

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Overview

• Quick Legal Issues
• Available Security Features
• Cracking WEP
• Algorithm Weakness
• Software/Techniques
• Cracking WPA
• Conclusions Best Practices
• Questions

25
Cracking Wi-Fi Networks

• Peter MacLellan
• Northeastern University
• Candidate BSCS 2008

For more information http//www.ccs.neu.edu/home/
peter/wifi