WEP Shared Key Authentication

This process is illustrated below.

Figure 2-2

 

The following steps occur when two devices use Shared Key Authentication:

The station sends an authentication request to the access point.
The access point sends challenge text to the station.
The station uses its configured 64-bit or 128-bit default key to encrypt the challenge text, and it
sends the encrypted text to the access point.
The access point decrypts the encrypted text using its configured WEP key that corresponds to the
station's default key. The access point compares the decrypted text with the original challenge text.
If the decrypted text matches the original challenge text, then the access point and the station
share the same WEP key, and the access point authenticates the station.
The station connects to the network.

If the decrypted text does not match the original challenge text (that is, the access point and
station do not share the same WEP key), then the access point will refuse to authenticate the
station, and the station will be unable to communicate with either the 802.11 network or Ethernet
network.

Key Size and Configuration

The IEEE 802.11 standard supports two types of WEP encryption: 40-bit and 128-bit.

The 64-bit WEP data encryption method allows for a five-character (40-bit) input. Additionally, 24
factory-set bits are added to the forty-bit input to generate a 64-bit encryption key. (The 24 factory
-set bits are not user-configurable). This encryption key will be used to encrypt/decrypt all data
transmitted via the wireless interface. Some vendors refer to the 64-bit WEP data encryption as
40-bit WEP data encryption because the user-configurable portion of the encryption key is 40 bits
wide.

The 128-bit WEP data encryption method consists of 104 user-configurable bits. Similar to the
40-bit WEP data encryption method, the remaining 24 bits are factory-set and not user-
configurable. Some vendors allow passphrases to be entered instead of the cryptic hexadecimal
characters to ease encryption key entry.

The 128-bit encryption is stronger than 40-bit encryption, but 128-bit encryption may not be
available outside the United States due to U.S. export regulations.

When configured for 40-bit encryption, 802.11 products typically support up to four WEP keys.
Each 40-bit WEP key is expressed as five sets of two hexadecimal digits (0-9 and A-F). For example,
"12 34 56 78 90" is a 40-bit WEP key.

When configured for 128-bit encryption, 802.11g products typically support four WEP keys, but
some manufacturers support only one 128-bit key. The 128-bit WEP Key is expressed as 13 sets
of two hexadecimal digits (0-9 and A-F). For example, "12 34 56 78 90 AB CD EF 12 34 56 78 90"
is a 128-bitWEP key.

Typically, 802.11 access points can store up to four 128-bit WEP keys, but some 802.11 client
adapters can only store one. Therefore, make sure that your 802.11 access and client adapters'
configurations match.

Whatever keys you enter for an access point, you must also enter the same keys for the client
adapter in the same order. In other words, WEP key 1 on the AP must match WEP key 1 on the
client adapter, WEP key 2 on the AP must match WEP key 2 on the client adapter, etc.

------------------------------------------------PS--------------------------------------------

加密可以分别和Open system、Shared key链路使用。

1.采用Open system authentication方式：此时WEP密钥只做加密，即使密钥配的不一致，用户也是可以上线，但上线后传输的数据会因为密钥不一致被接收端丢弃。

2.采用Shared key authentication方式：此时如果双方密钥不一致，客户端就不能通过Shared key认证，无法上线。也就是说，当WEP和Shared key认证方式配合使用时，WEP也可以作为一种认证方法