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202 Chapter 4 • Communication Security: Wireless

(GRE) and transport layer protocols to encapsulate and transport data. HTTP-S
and Secure Shell (SSH) are application layer (layer 7) protocols for encrypting data.
Because of the complexity of the 802.11 MAC and the amount of processing
power it requires, the 802.11 standard made 40-bit WEP an optional implementa-
tion only.

Vulnerability to Plaintext Attacks
From the outset, knowledgeable people warned that WEP was vulnerable because
of the way it was implemented. In October 2000, Jesse Walker, a member of the
IEEE 802.11 working group, published his now famous paper,“Unsafe at Any Key
Size:An Analysis of WEP Encapsulation.”The paper points out a number of serious
shortcomings of WEP and recommends that WEP be redesigned. For example,
WEP is vulnerable to plaintext attacks because it is implemented at the data link
layer, meaning that it encrypts IP datagrams. Each encrypted frame on a wireless
network contains a high proportion of well-known TCP/IP information, which
can be revealed fairly accurately through traffic analysis, even if the traffic is
encrypted. If a hacker can compare the ciphertext (the WEP-encrypted data) to the
plaintext equivalent (the raw TCP/IP data), they have a powerful clue for cracking
the encryption used on the network.All they would have to do is plug the two
values (plaintext and ciphertext) into the RC4 algorithm used by WEP to uncover
the keystream used to encrypt the data.

Vulnerability of RC4 Algorithm
As discussed in the previous paragraph, another vulnerability of WEP is that it uses
RC4, a stream cipher developed by RSA to encrypt data. In 1994, an anonymous
user posted the RC4 algorithm to a cipherpunk mailing list, which was subse-
quently re-posted to a number of Usenet newsgroups with the title “RC4
Algorithm Revealed.” Until August 2001, it was thought that the underlying algo-
rithm used by RC4 was well designed and robust, so even though the algorithm
was no longer a trade secret, it was still thought to be an acceptable cipher to use.
However, Scott Fluhrer, Itsik Mantin, and Adi Shamir published a paper entitled,
“Weaknesses in the Key Scheduling Algorithm of RC4” that demonstrated that a
number of keys used in RC4 were weak and vulnerable to compromise.The paper
designed a theoretical attack that could take advantage of these weak keys. Because
the algorithm for RC4 is no longer a secret and because there were a number of
weak keys used in RC4, it is possible to construct software that is designed to break
RC4 encryption relatively quickly using the weak keys in RC4. Not surprisingly, a

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