Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

Wireless Network Concepts 63
mixing, a message integrity check, an extended initialization vector (IV), and
dynamic rekeying.
Creating Privacy with WEP
WEP provides for three implementations: no encryption, 64-bit encryption, and
128-bit encryption. No encryption means no privacy. In 64-bit and 128-bit varieties,
the greater the number of characters (bits), the stronger the encryption. The
initial configuration of the AP includes the setup of the shared key, which can be
in the form of either alphanumeric or hexadecimal strings, and must be matched
on the client.
WEP uses the RC4 encryption algorithm, a stream cipher, where both the sender
and the receiver use the stream cipher to create identical pseudorandom strings
from a known shared key. The receiver takes the shared key and identical stream
and reverses the process to gain the plaintext transmission.
The steps in the process are as follows:
1. The plaintext message is run through an integrity check algorithm (the
802.11 standard specifies the use of CRC-32) to produce an integrity check
value (ICV).
2. This value is appended to the end of the original plaintext message.
3. A“random”24-bitIVisgeneratedandprependedto(addedtothebeginning
of ) the secret key (which is distributed through an out-of-band method)
that is then input to the RC4 Key Scheduling Algorithm (KSA) to generate
a seed value for the WEP pseudorandom number generator (PRNG).
4. The WEP PRNG outputs the encrypting cipher-stream.
5. This cipher-stream is then XOR’d with the plaintext/ICV message to
produce the WEP ciphertext.
6. The ciphertext is then prepended with the IV (in plaintext), encapsulated,
and transmitted. A new IV is used for each frame to prevent the reuse of the
keyfromweakeningtheencryption.WEPincorporatesachecksumintoeach
frame. Any frame not found to be valid through the checksum is discarded.
AUTHENTICATION
There are two authentication methods in the 802.11 standard: open and sharedkey.
Open authentication is more precisely described as device-oriented authentication
and can be considered a null authentication – all requests are granted.
Without WEP, open authentication leaves the WLAN wide open to any client who
knows the SSID. With WEP enabled, the WEP secret key becomes the indirect
authenticator.
EXAM WARNING
Open authentication can also require the use of a WEP key. Do not assume that just
because the Network+ exam discusses open authentication that a WEP key should not
be set.