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424 Chapter 18
enterprise network. These two environments had distinct security requirements and
different infrastructure capacities to provide security. Therefore, 802.11i specified two
distinct security architectures. For the enterprise network, 802.11i specifies the use of
IEEE 802.1X for key establishment and authentication. As we will see in our discussion
in the next section, 802.1X requires the use of a backend authentication server. Deploying
a back end authentication server is not usually feasible in a home environment. Therefore,
for home deployments of 802.11, 802.11i allows the use of the “ out-of-band mechanism ”
(read manual configuration) for key establishment.
We look at the 802.1X architecture in the next section and see how it results in
the establishment of a Master Key (MK). In this section, we assume that the two
communicating end-points (the STA and the AP) already share a MK which has
either been configured manually at the two end-points (WEP architecture) or has been
established using the authentication process (802.1X architecture). This section looks at
how this MK is used in WPA.
Recall that a major loophole in WEP was the manner 15 in which this master key was
used which made it vulnerable to compromise. WPA solves this problem by reducing
the exposure of the master key, thus making it difficult for an attacker to discover the
master key. To achieve this, WPA adds an additional layer to the key hierarchy used in
WEP. Recall from Section 17.4 that WEP uses the master key for authentication and to
calculate the per-packet key. In effect there is a two-tier key hierarchy in WEP: the master
(preshared secret) key and the per-packet key.
WPA extends the two-tier key-hierarchy of WEP to a multitier hierarchy (See Figure
18.8 ). At the top level is still the master key, referred to as the Pair-wise Master Key
(PMK) in WPA. The next level in the key hierarchy is the PTK which is derived from the
PMK. The final level is the per-packet keys which are generated by feeding the PTK to a
key-mixing function. Compared with the two-tier WEP key hierarchy, the three-tier key
hierarchy of WPA avoids exposing the PMK in each packet by introducing the concept
of PTK.
15
The per-packet key is obtained by simply concatenating the IV with the preshared secret key.
Therefore, a compromised per-packet key exposes the preshared secret key.
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