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188 Chapter 4 • Communication Security: Wireless

challenge, which is sent from authenticator to requestor, is sent in the clear.The
requesting client then transmits the same challenge, encrypted using the WEP
secret key, back to the authenticator.An attacker who captures both of these
packets now has two pieces of a three-piece puzzle: the cleartext challenge and the
encrypted ciphertext of that challenge.The algorithm RC4 is also known.All that
is missing is the secret key.To determine the key, the attacker may simply try a
brute force search of the potential key space using a dictionary attack.At each step,
the attacker tries to decrypt the encrypted challenge with a dictionary word as the
secret key.The result is then compared against the authenticator’s challenge. If the
two match, then the secret key has been determined. In cryptography, this attack is
termed a known-plaintext attack and is the primary reason why shared-key authenti-
cation is actually considered slightly weaker than open authentication.

TEST DAY TIP
While the Security+ exam does not cover the authentication process in
great detail, it is important to remember the two authentication mecha-
nisms in the 802.11 standard: open and shared-key.

802.1x Authentication
The current IEEE 802.11b standard is severely limited because it is available only
for the current open and shared-key authentication scheme which is non-exten-
sible.To address the weaknesses in the authentication mechanisms discussed above,
several vendors (including Cisco and Microsoft) adopted the IEEE 802.1x authen-
tication mechanism for wireless networks.The IEEE 802.1x standard was created
for the purpose of providing a security framework for port-based access control
that resides in the upper layers of the protocol stack.The most common method
for port-based access control is to enable new authentication and key management
methods without changing current network devices.The benefits that are the end
result of this work include the following:

■ There is a significant decrease in hardware cost and complexity.
■ There are more options, allowing administrators to pick and choose their
security solutions.

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