On a wired network, authentication is implicitly provided by physical access; if you're close enough to the network to plug in a cable, you must have gotten by the receptionist at the front door. While this is a weak definition of authentication, and one that is clearly inappropriate for high-security environments, it works reasonably well as long as the physical access control procedures are strong. Wireless networks are attractive in large part because physical access is not required to use network resources. Therefore, a major component of maintaining network security is ensuring that stations attempting to associate with the network are allowed to do so. Two major approaches are specified by 802.11: open-system authentication and shared-key authentication. Shared-key authentication is based on WEP and requires that both stations implement WEP.
802.11 does not restrict authentication to any particular scenario. Any station can authenticate with any other station. In practice, authentication is most useful in infrastructure networks. The usefulness of authentication for infrastructure networks is due in part to the design of the authentication methods, which do not really result in mutual authentication. As a matter of design, the authentication process really only proves the identity of one station. 802.11 implicitly assumes that access points are in a privileged position by virtue of the fact that they are typically under control of network administrators. Network administrators may wish to authenticate mobile stations to ensure that only authorized users access the 802.11 network, but mobile stations can't authenticate the access point. For this reason, the examples in this section assume that a mobile station such as an 802.11-equipped PC is attempting to authenticate to an access point. The standard, however, does not restrict authentication to infrastructure networks.
802.11 authentication is currently a one-way street. Stations wishing to join a network must authenticate to it, but networks are under no obligation to authenticate themselves to a station. The designers of 802.11 probably felt that access points were part of the network infrastructure and thus in a more privileged position, but this curious omission makes a man-in-the-middle attack possible. A rogue access point could certainly send Beacon frames for a network it is not a part of and attempt to steal authentication credentials.
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