Norms 1

• different norms: 802.11a, 802.11b, 802.11g (DRAFT), ...
• 802.11a: up to 54Mbits, 5GHz freq band.
• 802.11b: up to 11Mbits, 2.4GHz freq band.
• 802.11g: up to 54Mbits, 2.4GHz freq band.
• the most spread norm: 802.11b aka Wifi. Frequency range 2.4Ghz, 14 avail. channels, 11Mbits
• law: actually hightly regulated in France by the ART (authorized emission power, authorized channels, use in public area, possible conflict with military stuff...).

Norms 2

• 2 modes: AdHoc and Infrastructure
• advantages: vendor neutral (maintained by IEEE), similar to ethernet, high range (300 meters), very easy to install.
• drawbacks: weak data encryption algorithm (WEP) and use of unadapted crypto methods.

Wifi privacy

• SSID: Access point (AP) specific identifier. Possibly broadcasted in the air. Else can be read in connection packets. In clear text. Can be sniffed with passive channel listening.
• Channel: Some cards can listen simultaneously on multiple channels. Different channels used to set up several AP
• WEP off: Traffic is transmitted in clear text. No security.
• WEP 64/128 bits: a fixed key is shared between AP and clients.

configuration at CREATIS lab

• 3Com AP, acting as an ethernet bridge.
• AP installed on a separate external subnet on our router. Wifi clients are not on our local network.
• dedicated to occasional roaming visitors.
• WEP 128 is in use.
• extra features disabled on the AP : DHCP
• Wifi is of marginal use compared to wired network : the 128 bits WEP key is very unpractical, especially on PCMCIA cards coming from a different manufacturer than the AP (key needs to be entered in hexadecimal).

WEP

• a shared 40 or 104 bits length key
• a 24 bits length initialization vector (IV) *randomly* chosen for each emitted packet.
• a RC4 algorithm applyed on these 64/128 bits generates a stream 1
• to the clear text packet is added a CRC32, and we obtain a stream 2
• we emit IV followed with (stream 1 XOR stream 2).
• the receiver generates stream 1 on his own, and finds back the packet in clear text by XORing the crypted stream with stream 1

WEP : possible attacks

attacks based on weak IV

• Fluhrer, Mantin and Shamir (the S in RSA)
• a case where IV takes specific values.
• collect the first byte of each packet emitted with a weak IV.
• this algorithm is used for example in AirSnort.
• requires to *see* a large amount of traffic (~1GB). But could be sollicited by a machine on the wired network belonging to the attacker (eg by pinging).
• cracking duration is proportional to the number of bits of the key.
• can be patched : AP firmwares upgrades, wireless PCMCIA cards drivers.

other attacks

• DoS : force the deconnection between client and AP.
• DoS : connect to an AP, without knowing the WEP key.
• DoS : fake AP (well placed, and higher emission power than the legitimate AP).
• Arbitrary modification of the content of a crypted frame, without knowing the WEP key (because of the linearity of the CRC).
• some algorithms that generate a WEP hedecimal values from a given string can be cracked.

Experiments at lab

• Part 1
• Part 2
• Part 3

Experiments - Part 1

• the hardware choice: Behind the vendor of the wifi card exists the chipset manufacturer : Intersil (Prism chipsets), Atheros, Atmel, Proxim (orinico chipsets)...
• drawbacks of the 3COM AP, interoperability problems with cards from other vendors.
• finding linux drivers. HostAP driver, how to convert a wifi card into an AP.
• first try to crack WEP
• recommended linux tools

Experiments - Part 2

• configuration of a wifi enabled laptop as an AP, with the host AP driver.
• configuration of another machine in monitor mode, to sniff the traffic between the AP and a regular wireless client.
• ethereal is able to handle gracefully IEEE 802.11b frames, and to analyse the protocol until the higher level (eg HTTP), when encryption if off on the AP. Unsecure mode.
• Example of the sniffing of the crypted session : beacon frames, and crypted frames (IV available).
• Example of a tool dedicated to collect weak IV : wepcrack

Experiments - Part 3

• example of a program using brute force analysis to discover weak 40 bits WEP keys.

Wardriving

Using a wifi enabled laptop, or a smaller device like a PDA, a GPS to record coordinates, and you're ready to go wardriving. Softwares, like Net Stumbler are dedicated to this job. Scanning all channel sequencialy, and when available, getting and storing information from AP :

• ESSID

• channel

• Ethernet address

• kind of encryption

• GPS location of the strongest signal

In Lyon, France, an association also provides access points to its registered members.

Solution?

• Radius authentication server ?
• IPSec ? Few client support it.
• Application level crypto ?
• Wait for next protocol revision including better secured data transfer ? Political pressure to use hotest technology..

Tools

Note: Table partly taken from MISC magazine, issue 6, March 2003.