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Hack 86 Making the Best of WEP


While not the answer to every wireless security
need, WEP can still be effective if used properly.

The 802.11b specification provides a
form of encryption called Wired
Equivalent Privacy (WEP). It operates on the
Media Access
Control (MAC) layer, which is part of the Data Link layer of the OSI
model. When using WEP, only clients that know the
"secret key" can associate with an
Access Point or Peer-to-Peer Group. Anyone without the key
may be able to see network traffic, but every frame is encrypted. The
specification employs a 40-bit shared-key RC4 PRNG algorithm from
RSA Data Security. Virtually all cards
that speak 802.11b support this encryption standard.


Although hardware encryption sounds like a
good idea, the implementation in 802.11b is far from perfect. First
of all, the encryption provided happens at the link layer, not at the
application layer. This means that your communications are protected
up to the gateway, but no further. Once it hits the wire, your
packets are sent in the clear. Worse than that, every other
legitimate wireless client that has the key can read your packets
with impunity, since the key is shared across all clients. You can
try it for yourself. On a network using WEP, simply run a packet
sniffer such as tcpdump [Hack #37] or
Ethereal [Hack #38] on your laptop and
watch your neighbor's packets just fly by.


40- Versus 64- Versus 104- Versus 128-bit WEP



Why do the various card manufacturers quote
so many different key lengths? The original 802.11b specification
defined a 40-bit user-specified key. This key is combined with a
24-bit Initialization Vector (the IV), a random
number that is part of the WEP algorithm. Together, this yields 64
bits of "key," although the IV is
actually sent in the clear!

Likewise, a 104-bit WEP is used with the IV to yield 128 bits of
"key." This is why user-defined
keys are 5 characters long (5 characters x 8
bits/character = 40 bits) or 13 characters long (13 characters
x 8 bits/character = 104 bits). The user
doesn't define the IV; it is part of the WEP
algorithm (and is generally implemented as 24 random bits.)

Naturally, more bits sounds more secure to the consumer, so some
manufacturers choose to list the larger number as the
"key length." Unfortunately for
WEP, more bits do not necessarily mean significantly greater
security.

Many manufacturers have implemented their own proprietary extensions
to WEP, including 104-bit keys and dynamic key management.
Unfortunately, as they are not defined by the 802.11b standard, there
is no guarantee that cards from different manufacturers that use
these extensions will interoperate.

To throw more kerosene on the burning WEP tire mound, a team of
cryptographers at the University of California at Berkeley (among
others; see the references at the end of this section) has identified
weaknesses in the way WEP is implemented, effectively making the
number of bits used in the encryption key irrelevant. With all of
these problems, why is WEP still supported by manufacturers? And what
good is it for securing your network?

WEP was not designed to be the ultimate
"killer" security feature (nor can
anything seriously claim to be). Its acronym makes the intention
clear: wired equivalent protection. In other
words, the aim behind WEP is to provide no greater protection than
you would have when you physically plug into your Ethernet network.
Keep in mind that in a wired Ethernet setting, there is no encryption
provided by the protocol at all.


WEP provides an easy, generally effective,
interoperable deterrent to unauthorized access. Given the choice
between an open access point with all of the defaults in place and a
network running 40-bit WEP, the casual user running NetStumbler
[Hack #21] will choose to
investigate the open network every time. While definitely not beyond
the reach of a determined network cracker, a well-chosen WEP key is
still just too much of a pain for the average War Driver to deal
with. To make the best use of WEP, consider the following guidelines:

Use a nonobvious key. Dictionary attacks against a WEP key are executed much
more quickly and easily than a full-blown AirSnort session. Make sure
that your key doesn't use a simple word, even if you
obfuscate it further with l33t h4x0r sP33k. Believe me, network
crackers know how to speak it better than you do. Throw in a couple
of symbols, or better yet, use a Hex key with nonprintable
characters.

Use the longest key that your hardware supports. If all of your wireless network hardware supports 104-bit WEP, use
it. But keep in mind that many devices do not support 104-bit WEP,
and those that do may not interoperate well.

Change keys often. Current WEP key attacks depend on either a dictionary attack or the
collection of large amounts of data to deduce the key. The more often
you change the key being used, the more difficult a potential
cracker's job will be. Unfortunately, this might not
be feasible for a network with a large user base, as you would be
faced with the classic key distribution problem.

Use WEP in combination with other security features. If you happen to have a network that uses hardware of the same
manufacturer, you might be able to take advantage of proprietary
extensions to shore up WEP. For example, Cisco equipment supports
rapid WEP key rotation and dynamic keying using 802.1x. If all of
your clients can take advantage of these extensions, then use them.
Unfortunately, as we will see in [Hack #87], using other standard features
like "closed" networks and MAC
filters really does little to improve network security.

Consider WEP a deterrent, not a guarantee. Remember that it is unlikely that WEP alone will keep out the most
determined attackers. When building a security policy, be sure to
consider your likeliest threats, and weigh them against the benefits
and restrictions of your implementation. The threat model for a
wireless network on dial-up in a house in the middle of the woods
looks very different from that of an AP on the internal LAN at a law
firm downtown. Consider the risks and benefits of your wireless
network, and configure it accordingly.

Consider not using WEP at all. This chapter is full of practical implementations that neatly
sidestep the whole question of WEP security by introducing strong
application-layer encryption. Consider doing away with WEP altogether
in favor of strong authentication and encryption.



See Also


Your 802.11 Wireless Network has No Clothes (http://www.cs.umd.edu/~waa/wireless.pdf) by
Arbaugh, Shankar, and Wan, University of Maryland, March 30, 2001.

Weaknesses in the Key Scheduling Algorithm of RC4 (http://www.crypto.com/papers/others/rc4_ksaproc.ps)
by Fluhrer, Mantin and Shamir, July 25, 2001.

Using the Fluhrer, Mantin, and Shamir Attack to Break WEP
(http://www.cs.rice.edu/~astubble/wep/)
AT&T Labs Technical Report by Stubblefield, Ioannidis, and Rubin,
August 21, 2001.

Security of the WEP algorithm (http://www.isaac.cs.berkeley.edu/isaac/wep-faql)
by Borisov, Goldberg, and Wagner, UC Berkeley, April 1,
2001.



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