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Advanced wireless network connection properties
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Configuring Security for 802.11 Wireless Network Connectivity
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The most basic type of security for 802.11 wireless networks is Wired Equivalent Privacy, or WEP. WEP provides for authentication and data transmission security for wireless clients to protect against unauthorized access and eavesdropping. Unlike Windows XP and Windows Server 2003, Windows 2000 does not have integrated wireless network management features. In Windows XP and Windows Server 2003, you can configure the network key that is used for WEP. The key is used for authentication to the wireless network. In addition, the data encryption is enabled, which means a shared encryption key is generated to encrypt the data transmission between the computer and the wireless access point. In Windows 2000, 802.11 configuration must be done in the application provided by the wireless network interface vendor.
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Configuring 802.11 Security with WEP
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The 802.11 standard supports two subtypes of network authentication service: open system and shared key. When open system authentication is used, any computer or device can request authentication for the access point, and consequently, any computer or device can connect to the network. Using open system authentication does not prevent data transmission encryption. Unlike open system authentication, shared key authentication requires that the client computer or device have knowledge of a secret key that is shared by the wireless access point and all other wireless clients. When using shared key authentication, the access point generates a random 64-bit or 128-bit number that is used as a challenge. The wireless client returns the challenge, which is encrypted with the WEP shared key. The encryption process involves using the RC4 stream cipher to perform an exclusive or (XOR) binary operation on the plaintext payload. The RC4 keystream is generated by using a random number generator (RNG). The seed of the RNG is the result of concatenating the 40-bit or 104-bit WEP key with a 24-bit initialization vector. The encrypted payload and the initialization vector are sent to the access point. The access point concatenates the WEP key with the initialization vector to seed the keystream for RC4 to perform an XOR binary operation on the encrypted payload to reveal the plaintext payload. Unfortunately, an attacker who captures these frames possesses the plaintext challenge, the ciphertext challenge, and the initialization vector. Because of the way that XOR operations work, the attacker would then know the keystream that was used, which is the concatenated initialization vector and the WEP key. Although the attacker still does not know the WEP key, she can attempt to authenticate to the access point and use the keystream derived from the captured packets to encrypt the challenge and retransmit the captured initialization vector. Note Several utilities available on the Internet automate this process of compromising shared key authentication.
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How XOR Operations Work
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To understand how an attacker can compromise WEP security, you must know how the binary XOR operation works. An XOR takes two binary numbers of equal lengths and performs a comparison of each bit, yielding a result of bits that is equal to the two numbers. The following list shows the result of XOR operations:
0 0 1 1 XOR XOR XOR XOR 0 1 0 1 = = = = 0 1 1 0
The XOR is frequently used by stream ciphers to encrypt data. For example, the name BEN can be represented in ASCII as 0x42 0x45 0x4E and converted to the binary for transmission or storage. The RC4 algorithm might generate the keystream shown next. You then perform an XOR on the plaintext with the keystream. The result is the ciphertext.
Plaintext Keystream Ciphertext XOR 01000010 01101100 00101110 01000101 00010111 01010010 01001110 01101111 00100001
If you convert the ciphertext back to ASCII characters, you get the following: - . The problem with using XOR for encryption is that if you know any two of the three elements, you can determine the one you do not know. For example, if you can intercept the plaintext and the ciphertext, you can determine the keystream by performing an XOR on the plaintext with the ciphertext:
Plaintext Ciphertext Keystream XOR 01000010 00101110 01101100 01000101 01010010 00010111 01001110 00100001 01101111
Although shared key authentication is not completely secure, it does provide more protection than open system authentication. Thus, when combined with Media Access Control (MAC) address filtering, implementing shared key authentication provides a base level of security for wireless networks against novice attackers. If your organization issues laptops to users with wireless network cards, the users will likely install a home wireless network. To ensure that employees do not expose information contained on their laptops to potential attackers, you should create guidelines for installing home wireless networks, and these guidelines should include enabling shared key authentication. WEP also provides data encrypted services by using the same process as defined for shared key authentication. Because only 2^24 (roughly 16 million) initialization vectors exist, if you assume that each packet uses a new initialization vector, the probability is over 50% that one initialization vector will be repeated after about 4500 packets have been transmitted. This is an example of a birthday attack on a cryptography algorithm. Thus, if an attacker can get the access point to send known plaintext (such as ping packets) and then capture all encrypted
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