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

64 CHAPTER 4 Wireless Networking
The shared-key authentication process is a four-step process that begins when
the AP receives the validated request for association. The four steps break down
in the following manner:
1. The requestor (the client) sends a request for association.
2. The authenticator (the AP) receives the request, and responds by
producing a random challenge text and transmitting it back to the requestor.
3. The requestor receives the transmission, encrypts the challenge with the
secret key, and transmits the encrypted challenge back to the authenticator.
4. The authenticator decrypts the challenge text and compares the values
against the original. If they match, the requestor is authenticated. However,
if the requestor does not have the shared key, the cipher stream cannot be
reproduced, therefore the plaintext cannot be discovered, and theoretically
the transmission is secured.
One of the greatest weaknesses in shared-key authentication is that it provides
an attacker with enough information to try to crack the WEP secret key. The
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 clear text challenge and
the encrypted cipher text 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. In
cryptography, this attack is termed a known-plaintext attack and is the primary
reason why shared-key authentication is actually considered slightly weaker than
open authentication.
802.11I AUTHENTICATION
The IEEE 802.11i 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:
1. There is a significant decrease in hardware cost and complexity.
2. There are more options, allowing administrators to pick and choose their
security solutions.
3. The latest and greatest security technology can be installed and should still
work with the existing infrastructure.
4. You can respond quickly to security issues as they arise.
When a client device connects to a port on an 802.11i-capable AP, the AP port
determines the authenticity of the devices. Before we discuss the workings of the
802.11i standard, the following terminology must be defined: