Securing Thin Air

1
Securing Thin Air
Duncan S. Wong City University of Hong Kong
2
Outline

• Wi-Fi
• Security of WLAN
• War Driving
• WEP
• Breaking WEP
• Countermeasures
• New Development

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Wi-Fi (IEEE 802.11)

• Wireless Local Area Network (WLAN)
• Two popular standards
• 802.11b using 2.4GHz, 11Mbps
• 802.11g using 2.4GHz, 54Mbps

Wired Network
PDA
Access Point
Laptop
Smartphone
Tablet computer
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Some WLAN Products
WLAN Adapters
Access Points
Antenna
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Security of WLAN
Wired Network
PDA
Access Point
Laptop
Smartphone
Tablet computer

• IEEE 802.11 standard defines
• a Wired Equivalent Privacy (WEP) algorithm
• Encrypts all messages to and from the Access
  Point and WLAN users
• Supports 40 bits or 104 bits of secret key

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Software Setup with WEP Enabled
WEP Key ABCDEF1234
SSID 00074035FABC
WEP Key ABCDEF1234
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Using 40-bit/104-bit WEP is OPTIONAL

• By default, WEP is not enabled.
• My neighbors Access Points

• If an antenna is installed, more Access Points
  would be found.
• Illegal of using others Access Points without
  prior permission.

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When WEP is Not Enabled

• All messages are sent in clear over the wireless
  network.
• Eavesdroppers may be stealing your messages
  secretly

eavesdropper
gateway.cs
telnet gateway.cs
login
58888888
password
asdfg888

• Other insecure applications pop3, web-based
  email services, etc.

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How likely would one find an Access Point with
WEP turned off?
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WarDriving
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WarDriving Equipment
18dB Antenna
Poor guys
A laptop with WLAN adapter
Rich guys
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WarDriving Results

• Number of Access Points captured from Victoria
  Peak 257
• Those using WEP 28.8
• Radio signals of almost 20 access points captured
  were from Kowloon Peninsula (captured using a
  18dB antenna).
• Those access points are 3.6km to 7.2km away

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WEP (Wired Equivalent Privacy)

• An AP (Access Point) with WEP turned off is like
  letting a door open unattended
• WEP is the minimum level of security measure one
  should make when setting up a WLAN
• Is WEP secure enough?
• Depends
• on who our potential attackers are and how
  capable they are
• Also
• Need to learn the available attacking techniques
  that our enemies may launch.
• Review
• Two versions of WEP 40 bits / 104 bits of secret
  key

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WEP

• WEP encipherment block diagram

• Secret Key 40 bits or 104 bits
• Distributed to access points and wireless
  stations via external key management service
  (e.g. manually key in)

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WEP

• Initialization Vector (IV) 24 bits
• Pad subfield all zeros
• Key ID subfield specify one of four possible
  secret key values
• Integrity Algorithm
• CRC-32
• WEP PRNG
• RC4
• Initialized by Seed
• Outputs a long binary stream called Key Sequence

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WEP
Ciphertext generation WEP (K, IV, M) RC4(IV
K) ? (M c(M))
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Attack Exhaustive Key Search
40-bit key
WEP Encrypt
101101101
01011010
ciphertext
plaintext
Cracking machine
try next
Ki1 Ki Ki-1
NO
WEP Encrypt
101101101
01011010?
plaintext
YES

• Number of keys 240
• If the cracking machine tries 10 million keys per
  sec, it takes less than 31 hours to find the
  correct key.

succeed
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WEP using 104-bit keys
104-bit key
WEP Encrypt
101101101
01011010
ciphertext
plaintext

• Number of keys 2104
• If a cracking machine tries 10 million keys per
  sec, it takes more than 6x1016 years to find the
  correct key!
• So, exhaustive key search does not work against
  104-bit WEP.
• However, in 2001, a flaw of WEP was found
• The 104-bit secret key can be calculated by
  collecting 500,000 to 2,000,000 packets (just
  eavesdrop the radio channel)
• For an Access Point which sends/receives 100
  packets per second (e.g. transporting video
  streaming data), the attacker will take 1.4hrs
  to 5.6hrs for breaking the Access Point.
• Open-source Implementation http//airsnort.shmoo.
  com

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WEP is insecure, both 40 bits and 104 bits
WEP is still commonly used on current WLAN
products
? Most current WLAN products do not provide
security to our wireless traffic
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Countermeasures

• Rely on upper layer security solutions
• E.g. network layer, transport layer or
  application layer
• Upper layer solutions
• VPN with IPSec secure virtual private network

gateway.cs

• Using POP3 in the VPN tunnel
• http//www.cs.cityu.edu.hk/cslab/services/vpn/
  (CSLab VPN)
• http//www.cityu.edu.hk/csc/deptweb/facilities/ctn
  et/vpn/winxp/vpnsetup.htm (CityU VPN)
• SSH for remote access instead of telnet
• all messages are encrypted during transmission
• SFTP for file transfer instead of FTP

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Other Countermeasures and New Development

• Use stronger encryption algorithm to replace WEP
• Enforce access control

00-05-3E-66-4E-A1
Access Control . . 03-1E-25-34-7D-C2 . . . 00-05-3
E-66-4E-A1 . . .
03-1E-25-34-7D-C2
06-27-C5-21-63-EB

• Renew secret key frequently using some key
  management protocol

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Learn More about Information Security

• CS4286 Internet Security and E-commerce Protocols
• http//www.cs.cityu.edu.hk/cs4286
• CS4288 Cryptographic Algorithms and Protocols
• http//www.cs.cityu.edu.hk/cs4288