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̈ Chapter 11: Wireless LAN – WLAN LANCOM Reference Manual LCOS 3.50
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Wireless LAN – WLAN
case, so-called assymetric encryption methods such as RSA can be used, that
is, to decrypt the data, a different key is used than the one used to encrypt it.
Such methods are, however, much slower than symmetric encryption
methods, which leads to a two-phase solution: one side possesses an
asymmetric key pair and transmits the encryption key to the other side,
generally as a part of a certificate. The other side chooses an arbitrary
symmetric key, and encrypts this symmetric key with the asymmetric key
previously received. The owner of the asymmetric key pair can now decrypt it,
but a potential eavesdropper cannot—the aim of the secure key exchange is
achieved.
In the following sections, we will see these methods again, sometimes in
modified form.
11.2.2 WEP
WEP is an abbreviation for Wired Equivalent Privacy. The primary goal of WEP
is the confidentiality of data. In contrast to signals which are transmitted over
cables, radio waves spread out in all directions—even into the street in front
of the house and other places where they really aren't desired. The problem
of undesired interception is particularly obvious in wireless data transmission,
even though it can also arise in larger installations with wired networks—
however, access to cables is far more easily restricted than is the case with
radio waves.
During the development of the WLAN security standard, the IEEE Committee
did not intend to develop a "perfect" encryption method. Such high-security
encryption methods are, for instance, required and also used in electronic
banking—in this case, however, the applications themselves use high-quality
encryption methods, and it would be unnecessary to repeat this effort at the
radio transmission level. With the new security standards, only those
applications which normally work without encryption in wired LANs should be
provided with sufficient security against eavesdropping by unauthorised third
parties.
Figure 1 shows the process of WEP encryption—decryption runs in precisely
the opposite manner. WEP is therefore a symmetrical encryption method. WEP
uses RC4 algorithm as its basic encryption technology, a process already well-
known in other areas and considered highly secure. RC4 uses a key between
8 and 2048 bits in length, which is used to generate a pseudo-random series
of bytes using a predetermined process. The data packet is then XOR'd byte
by byte with this byte stream. The receiver simply repeats this process with the
same key and thus with the same sequence, in order to retrieve the original