Wireless Security

1
Wireless Security

• 802.11, RFID, WTLS

2
802.11

• 802.11 a, b,
• Components
• Wireless station
• A desktop or laptop PC or PDA with a wireless
  NIC.
• Access point
• A bridge between wireless and wired networks
• Radio
• Wired network interface (usually 802.3)
• Bridging software
• Aggregates access for multiple wireless stations
  to wired network.

3
802.11 modes

• Infrastructure mode
• Basic Service Set
• One access point
• Extended Service Set
• Two or more BSSs forming a single subnet.
• Most corporate LANs in this mode.
• Ad-hoc mode (peer-to-peer)
• Independent Basic Service Set
• Set of 802.11 wireless stations that communicate
  directly without an access point.
• Useful for quick easy wireless networks.

4
Infrastructure mode
Access Point
Basic Service Set (BSS) Single cell
Station
Extended Service Set (ESS) Multiple cells
5
Ad-hoc mode
Independent Basic Service Set (IBSS)
6
Open System Authentication

• Service Set Identifier (SSID)
• Station must specify SSID to Access Point when
  requesting association.
• Multiple APs with same SSID form Extended Service
  Set.
• APs broadcast their SSID.

7
MAC Address Locking

• Access points have Access Control Lists (ACL).
• ACL is list of allowed MAC addresses.
• E.g. Allow access to
• 0001420E121F
• 000142F172AE
• 0001424FE201
• But MAC addresses are sniffable and spoofable.
• Access Point ACLs are ineffective control.

8
Interception Range
Station outside building perimeter.
100 metres
Basic Service Set (BSS) Single cell
9
Interception

• Wireless LAN uses radio signal.
• Not limited to physical building.
• Signal is weakened by
• Walls
• Floors
• Interference
• Directional antenna allows interception over
  longer distances.

10
Directional Antenna

• Directional antenna provides focused reception.
• D-I-Y plans available.
• Aluminium cake tin.
• 11 Mbps at 750 meters.
• http//www.saunalahti.fi/elepal/antennie.html

11
802.11b Security Services

• Two security services provided
• Authentication
• Shared Key Authentication
• Encryption
• Wired Equivalence Privacy

12
Wired Equivalence Privacy

• Shared key between
• Stations.
• An Access Point.
• Extended Service Set
• All Access Points will have same shared key.
• No key management
• Shared key entered manually into
• Stations
• Access points
• Key management a problem in large wireless LANs

13
RC4

• Refresher
• RC4 uses key sizes from 1 bit to 2048 bits.
• RC4 generates a stream of pseudo random bits
• XORed with plaintext to create ciphertext.

14
WEP Sending

• Compute Integrity Check Vector (ICV).
• Provides integrity
• 32 bit Cyclic Redundancy Check.
• Appended to message to create plaintext.
• Plaintext encrypted via RC4
• Provides confidentiality.
• Plaintext XORed with long key stream of pseudo
  random bits.
• Key stream is function of
• 40-bit secret key
• 24 bit initialisation vector (more later)
• Ciphertext is transmitted.

15
Initialization Vector

• IV must be different for every message
  transmitted.
• 802.11 standard doesnt specify how IV is
  calculated.
• Wireless cards use several methods
• Some use a simple ascending counter for each
  message.
• Some switch between alternate ascending and
  descending counters.
• Some use a pseudo random IV generator.

16
WEP Encryption
IV Cipher text
Initialisation Vector (IV)
PRNG

Key Stream
?
Seed
Secret key
Plaintext

32 bit CRC
ICV
Message
17
WEP Receiving

• Ciphertext is received.
• Ciphertext decrypted via RC4
• Ciphertext XORed with long key stream of pseudo
  random bits.
• Check ICV
• Separate ICV from message.
• Compute ICV for message
• Compare with received ICV

18
Shared Key Authentication

• When station requests association with Access
  Point
• AP sends random number to station
• Station encrypts random number
• Uses RC4, 40 bit shared secret key 24 bit IV
• Encrypted random number sent to AP
• AP decrypts received message
• Uses RC4, 40 bit shared secret key 24 bit IV
• AP compares decrypted random number to
  transmitted random number

19
Security - Summary

• Shared secret key required for
• Associating with an access point.
• Sending data.
• Receiving data.
• Messages are encrypted.
• Confidentiality.
• Messages have checksum.
• Integrity.
• But SSID still broadcast in clear.

20
Security Attacks

• Targeted network segment
• Free Internet
• Malicious use of identity
• Access to other network resources
• Malicious association
• Host AP
• Interference Jamming
• Easy to jam the signals
• DOS through repeated, albeit unsuccessful access
  requests (management messages are not
  authenticated. Egs. Wlan-jack)
• DoS through disassociation commands
• Interference with other appliances (2.4 G
  spectrum)
• Attack against MAC authentication
• Can spoof MAC with loadable firmware
• Defense?
• Vulnerability through ad hoc mode

21
802.11 Insecurities

• Authentication two options
• Open
• Shared-key
• Shared-key more insecure?
• Static key management
• If one device is compromised/stolen, everyone
  should change the key
• Hard to detect
• WEP keys
• 40 or 128 can be cracked in less than 15 minutes

22
IV Collision attack

• If 24 bit IV is an ascending counter,
• If Access Point transmits at 11 Mbps, IVs
  exhausted in roughly 5 hours.
• Passive attack
• Attacker collects all traffic
• Attacker could collect two encrypted messages
• If two messages EM1, EM2, both encrypted with
  same key stream ( same key and same IV)
• EM1 ? EM2 M1 ? M2
• Effectively removes the key stream
• Can now try to derive plaintext messages

23
Limited WEP keys

• Some vendors allow limited WEP keys
• User types in a password
• WEP key is generated from passphrase
• Passphrases creates only 21 bits of 40 bit key.
• Reduces key strength to 21 bits 2,097,152
• Remaining 19 bits are predictable.
• 21 bit key can be brute forced in minutes.

24
Brute Force Key Attack

• Capture ciphertext.
• IV is included in message.
• Search all 240 possible secret keys.
• 1,099,511,627,776 keys
• 200 days on a modern laptop
• Find which key decrypts ciphertext to plaintext.

25
128 bit WEP

• Vendors have extended WEP to 128 bit keys.
• 104 bit secret key.
• 24 bit IV.
• Brute force takes 1019 years for 104-bit key.
• Effectively safeguards against brute force
  attacks.

26
IV weakness

• WEP exposes part of PRNG input.
• IV is transmitted with message.
• Initial keystream can be derived
• TCP/IP has fixed structure at start of packets
• Attack is practical.
• Passive attack.
• Non-intrusive.
• No warning.

27
Wepcrack

• First tool to demonstrate attack using IV
  weakness.
• Open source
• Three components
• Weaker IV generator.
• Search sniffer output for weaker IVs record 1st
  byte.
• Cracker to combine weaker IVs and selected 1st
  bytes.

28
Airsnort

• Automated tool
• Does it all!
• Sniffs
• Searches for weaker IVs
• Records encrypted data
• Until key is derived.

29
Safeguards

• Security Policy Architecture Design
• Treat as untrusted LAN
• Discover unauthorised use
• Access point audits
• Station protection
• Access point location
• Antenna design

30
Wireless as Untrusted LAN

• Treat wireless as untrusted.
• Similar to Internet.
• Firewall between WLAN and Backbone.
• Extra authentication required.
• Intrusion Detection
• WLAN / Backbone junction.
• Vulnerability assessments

31
Discover Unauthorised Use

• Search for unauthorised access points or ad-hoc
  networks
• Port scanning
• For unknown SNMP agents.
• For unknown web or telnet interfaces.
• Warwalking!
• Sniff 802.11 packets
• Identify IP addresses
• Detect signal strength
• May sniff your neighbours

32
Location of AP

• Ideally locate access points
• In centre of buildings.
• Try to avoid access points
• By windows
• On external walls
• Line of sight to outside
• Use directional antenna to point radio signal.

33
IPSec VPN

• IPSec client placed on every PC connected to the
  WLAN
• Filters to prevent traffic from reaching anywhere
  other than VPN gateway and DHCP/DNS server
• Can combine user authentication also

34
IEEE 802.11i

• A new framework for wireless security
• Centralized authentication
• Dynamic key distribution
• Will apply to 802.11 a,b g
• Uses 802.1X as authentication framework
• Extensible Authentication Protocol (EAP), RFC
  2284 (EAP-TLS LEAP)
• Mutual authentication between client and
  authentication server (RADIUS)
• Encryption keys dynamically derived after
  authentication
• Session timeout triggers reauthentication

35
802.11i Encryption Enhancements

• Temporal Key Integrity Protocol (TKIP)
• RC4 still used
• Per-packet keys
• Hash functions for MIC instead of CRC 32
• Only firmware upgrade required
• AES
• AES cipher replaces RC4
• Will require new hardware