Wireless Security
802.11, RFID, WTLS
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
Infrastructure mode
Access Point
Basic Service Set (BSS) –
Single cell
Station
Extended Service Set (ESS) –
Multiple cells
SMU
CSE 5349/7349
Ad-hoc mode
Independent Basic Service Set (IBSS)
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
MAC Address Locking
• Access points have Access Control Lists
(ACL).
• ACL is list of allowed MAC addresses.
– E.g. Allow access to:
• 00:01:42:0E:12:1F
• 00:01:42:F1:72:AE
• 00:01:42:4F:E2:01
• But MAC addresses are sniffable and
spoofable.
• Access Point ACLs are ineffective control.
SMU
CSE 5349/7349
Interception Range
Station outside
building perimeter.
Basic Service Set (BSS) –
Single cell
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349
802.11b Security Services
• Two security services provided:
– Authentication
• Shared Key Authentication
– Encryption
• Wired Equivalence Privacy
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
Initialization Vector
• IV must be different for every message
transmitted.
• 802.11 standard doesn’t 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.
SMU
CSE 5349/7349
WEP Encryption
IV
Initialisation
Vector (IV)
Secret key
||
Seed
PRNG
Key Stream

Cipher
text
Plaintext
||
32 bit CRC
ICV
Message
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
Security Attacks
•
Targeted network segment
•
Malicious association
•
Interference Jamming
•
Attack against MAC authentication
•
Vulnerability through ad hoc mode
– Free Internet
– Malicious use of identity
– Access to other network resources
– Host AP
– 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)
SMU
– Can spoof MAC with loadable firmware
– Defense?
CSE 5349/7349
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
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
128 bit WEP
• Vendors have extended WEP to 128 bit
keys.
– 104 bit secret key.
– 24 bit IV.
• Brute force takes 10^19 years for 104-bit
key.
• Effectively safeguards against brute force
attacks.
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
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.
SMU
CSE 5349/7349
Airsnort
• Automated tool
–
–
–
–
–
SMU
Does it all!
Sniffs
Searches for weaker IVs
Records encrypted data
Until key is derived.
CSE 5349/7349
Safeguards
•
•
•
•
•
•
•
Security Policy & Architecture Design
Treat as untrusted LAN
Discover unauthorised use
Access point audits
Station protection
Access point location
Antenna design
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349
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!
–
–
–
–
SMU
Sniff 802.11 packets
Identify IP addresses
Detect signal strength
May sniff your neighbours…
CSE 5349/7349
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.
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349
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
SMU
CSE 5349/7349