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WLAN
• What is WLAN?
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Wireless Local Area Network
Extension of a wired LAN
Uses high frequency radio waves (RF)
Speed: 2 MB/s to 54 Mb/s
Distance:100 feet to 15miles
• Physical vs. Wireless LAN
– Physical LAN has defined borders
– Wireless is difficult to determine where access stops
– Both are vulnerable to attacks
802.11 Standard
• 802.11
– IEEE family of specifications for WLANs
– 2.4GHz 2Mb/s
– Includes WEP
• 802.11a
– 5GHz, 54Mb/s
• 802.11b
– Often called Wi-Fi, 2.4GHz, 11Mb/s
• 802.11e
– QoS & Multimedia support to 802.11b & 802.11a
• 802.11g
– 2.4GHz, 54Mb/s
• 802.11i
– An alternative of WEP, known as WPA2
Modes of WLANs
• Ad-Hoc
– Peer to Peer
• Communication only to computers within
transmission range
• If communication to the internet is required then one
of the members must act as a router
– Called the Independent Basic Service Set
(IBSS)
Modes of WLANs
• Infrastructure
– Each client sends its data to an access point
– The access points acts as a bridge and forwards
the packets to other clients or to the wired
network.
– Called the Basic Service Set (BSS)
War Driving & Chalking
• People drive around looking for access
points.
• When an access point is found, a chalk
mark is placed on the sidewalk or building
WEP
• WEP – Wired Equivalent Privacy
• Supposed to give the same amount of
privacy as a wired LAN
• Used to prevent eavesdropping
• Used to prevent unauthorized access to the
LAN (not explicitly a goal of WEP)
Secure Protocols For Encryption
Application
Application
SSL
SSL
Transport
(TCP, UDP)
Network (IP)
Transport
(TCP, UDP)
Router
Network (IP)
Network (IP)
(VPN)
(VPN)
802.11b Link
WEP
802.1b
Physical
Network (IP)
802.11b Link
WEP
802.1b
Physical
Ethernet
Link
Ethernet
Link
Ethernet
Physical
Ethernet
Physical
Beacon Frame
• Each access point broadcasts a beacon frame
several times a second
• It contains:
– The beacon interval – How often does the frame get
broadcast
– A time stamp
– Service Set Identifier (SSID)
– Supported transmission rates
– Parameter sets – frequency hops, delay, etc
– Compatibility info – such as: all clients must use WEP
– Traffic Map – What AP are in power saver mode
Probe Frame
• A client may broadcast a probe frame to
find the AP associated with an SSID.
• The SSID is required for all communication
with an access point.
Association
• Before a client can communicate with the AP, a
relation or association must be established.
• There are 3 association states
– 1)Unauthorized and unassociated
– 2)Authenticated and unassociated
– 3)Authenticated and associated
• To move from state1 to state 2 in an open system,
a client sends a authentication request, and the AP
responds with an authorization success frame.
• Open systems may also employ MAC access lists
to determine if the AP responds with an
authorization success frame.
WEP Association
• WEP relies on a shared key
• The client sends a authentication request
indicating the use of a shared key
• The AP responds with a challenge containing 128
Octets generated with the WEP pseudo-random
number generator (PRNG) seeded with the shared
key and an initialization vector (IV).
• The client then encodes the message using the key,
a new IV and sends it to the AP.
• The AP decrypts and compares. This
authenticates the client to the AP
WEP
• Uses an RC4 stream cipher
– Creates a repeatable stream of “random” data
– The data is XORed with the data to get the encrypted
data
• Uses a shared key
– Can be “64 bit or 128 bit”
• Uses an initialization Vector (IV)
– 24 bit
– Actually used as part of the key so…
• The 64 bit key is actually a 40 bit shared key
• The 128 bit key is actually a 104 bit shared key.
– Increments with each frame (by convention)
– Sent as clear text in each frame.
IP Data
Seed RC4 with IV +
shared key, XOR output
with data
802.11 Hdr
IV
IV Sent in
clear text
ICV is a 32 bit CRC of
the plain IP data
Encrypted IP Data
ICV
Problems
• Because of the XOR and the static shared key, getting two
frames with the same IV one can statistically figure out the
message.
• Becomes even easier if you have multiple frames with the
same IV
• An AP sending out 1500byte packets at 11Mb/s will use
every IV in about 5 hours. But most packets are smaller so
the rollover will occur even sooner.
• Most wireless cards reset their IV to 0 on reset.
• Even worse…The 802.11 standard makes changing the IV
optional.
• The ICV uses CRC32 which is good at detecting
transmission problems, but easy to get a valid crc with
altered data
• Subject to replay attacks.
Attacks
• Passive
– Listen to the air waiting for a duplicate IV and apply
statistics.
• Active
– With knowledge of the contents (tcp/ip), the attacker
can guess where the destination is, and by flipping
some bits, the AP will send the packet to another
address.
• Once the attacker has the encrypted text and the decrypted text
the XOR is easily gotten
– Another active attack is to send a packet from the
internet to a destination on the WLAN. The AP will
encode the message, thus the attacker has both the
encrypted and plan text again.
Defense
• Stop the AP from broadcasting the SSID
• Change the SSID from the factory default
– Don’t use any identifying number
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Enable MAC authentication
Use 128 bit WEP
Change the default passwords on the AP itself
Set /change the shared key!!!
Place a firewall between the AP and the intranet
Use a higher level encryption such as SSL or VPN
Use WPA. This standard addresses these
problems.
WPA
• WPA – Wi-Fi Protected Access
• Designed to run with existing WEP access
points with just a firmware change
• Changes to WEP
– TKIP (Temporal Key Integral Protocol)
• 48 Bit IV (instead of 24 bit)
– At current 802.11a (54 Mb/s) it will take 645 years before
the IV will roll
– Initialized to 0 when the shared key is initialized
– Increments with every packet sent out
– AP rejects any packet with a IV less then the current count
– Traffic halts if the key gets to it’s max value
WPA
• Changes to WEP
– TKIP (Temporal Key Integral Protocol)
• Change the per packet key with every association (instead of
static)
– Employ 802.1X EAPOL key message (RADIUS)
• Use the transmitter’s MAC Address in the key (TA)
Shared Key
TA
IV
XOR
Shared Key
Phase 1
IV
RC4
Per Packet Key
WPA
• Changes to WEP
– Addition of a MIC (message Integrity Check)
• Often referred to as Michael
• A hash used to prevent message alteration
– Hash the shared key, source address, destination address,
and the data
WPA
IP Data
Seed RC4 with TKIP
XOR output with data
802.11 Hdr
IV
+Source MAC
+Dest MAC
+Per packet Key
+IV
Encrypted IP Data
IV Sent in
clear text
802.11 Hdr
IV
Encrypted IP Data
IV MIC
CRC32
MIC
ICV
WPA Problems
• If you know the type of packet (most likely ARP), the crc32
checksum will allow you to guess the bytes of the packet
• Once you have a guess, you submit the packet to the AP. If the
AP accepts the packet, you guessed the bytes correctly.
Continue guessing.
• WPA counter measure requires a 60 second delay between
guesses
• Using an ARP packet, you can generate your own ARP packets
in about 12 minutes.
• One you have ARP, you can do a man in the middle attack and
see both the clear text and the encrypted text, and get the key
stream, but…
• You must use the QoS channel (if enabled) and then you can
only send 15 packets before the IV changes the entire key
stream.
WPA2
• Fixes the WPA problems by only using a block cipher
– Impossible to get a “key stream” as they don’t exist.
– Problem: All packets are the increment of the block size!
(multiples of 128 bit).
• Uses AES –symmetric key, block cipher
– Personal mode
• Pre-shared key
– Enterprise mode
• Requires a pre-shared key
• Uses 802.1x EAPOL key message (RADIUS) to generate a
session key
• Called a Robust Security Network (RSN)
– Requires different hardware then WPA equipment
– RSN is incompatible with WPA