IEEE 802.11 Wi-Fi LANs are designed to allow multiple devices to communicate. In practice, this means a group of mobile devices must share the radio channel and communicate with a single access point. Many LANs provide shared access. For example, conventional Ethernet LAN workstations share the wiring by transmitting one at a time and trying to avoid collisions. For efficiency, most shared LANs also provide the capability for one workstation to send data simultaneously to several others. Data sent between two workstations is called unicast and data sent from one to multiple workstations is called multicast; the case in which one workstation sends to all the others is a special case of multicast called broadcast. Multicast and unicast messages have different security characteristics.
Unicast data sent between two parties needs to be private to those two parties. This is best accomplished by using a specific key for each pair of devices communicating. We call this a pairwise key; usually it protects communication between a mobile device and the access point. This means that each mobile device needs to store one pairwise key, and the access point needs a set of pairwise keys one for each mobile device that is associated.
By contrast, broadcast (or multicast) data must be received by multiple parties who form a trusted group. Therefore, a key must be shared by all the members of that trusted group. This is called the group key. Each trusted mobile device and the access point need to know a single group key. The concept of pairwise and group keys is shown in Figure 10.1.
Figure 10.1. Pairwise and Group Keys
The methods of managing the pairwise keys and the group keys are somewhat different so we define each as a separate key hierarchy. We refer to the pairwise key hierarchy to describe all the keys used between a pair of devices (one of which is usually the access point) and the group key hierarchy to describe the various keys shared by all the devices.
The next important terms are preshared keys and server-based keys. As the name suggests, preshared keys are installed in the access point and in the mobile device by some method outside WPA/RSN. It could be that you phone up a user and tell him the password, or send him a letter that he has to eat after reading or whatever eccentric method you choose. Most WEP systems use preshared keys it is the responsibility of the user to get the keys delivered to the two parties who want to communicate. Preshared keys bypass the concept of upper-layer authentication completely; you are assumed to be authentic simply by proving possession of the key.
The alternative, server-based keys, requires an upper-layer authentication process that allows the mobile device and an authentication server to generate matching secret keys. The authentication sever arranges for the access point to get a copy for use in session protection. It has the major advantage that the operator can keep a single key database that can be used in conjunction with many access points. When a new employee joins, for example, the administrator has to update only one database.
Preshared keys are easier to implement in small networks. Most older WEP systems only supported preshared group keys.