Cryptography Reference
In-Depth Information
12.2.7
WLAN design issues
The main cryptographic design issues concerning WLAN security are as follows:
Use of symmetric cryptography
. This is a sensible decision because WLANs
transfer bulk traffic between networked devices, hence speed of encryption is
important. For small networks, such as a home network, key establishment
is straightforward. Larger enterprise WLANs may optionally choose to use
public-key mechanisms as part of the initial authentication between a device
and a central authentication server, but the core WPA2 security protocol
CCMP uses only symmetric cryptography.
Use of recognised cryptographic mechanisms
. This was not adhered to in WEP,
where the cryptographic design was rather ad hoc. WEP thus provides a useful
lesson regarding the potential folly of adopting unconventional mechanisms.
In contrast, WPA2 adopts more widely accepted cryptographic mechanisms.
Flexibility, but only when appropriate
. While WLANs may be deployed in
quite different environments, they do not require the same cryptographic
flexibility as open applications such as SSL. Thus it makes sense to 'lock down'
the cryptographic mechanisms, where appropriate. WPA2 does this for the
confidentiality and data origin authentication services. However, WPA2 allows
for flexibility in choosing the initial entity authenticationmechanism (between
the device and a centralised authentication server), recognising that different
environments may well have different approaches to identifying network users.
The potential need to cater for migration
. When the flaws in WEP became
apparent, it was clear that due to the difficulty of upgrading a widely deployed
technology, any complete redesign of the WLAN security mechanisms could
not be rolled out quickly. It was thus necessary to design a 'fix' that was based on
the existing cryptographic mechanisms, which would provide 'good enough'
security. The 'fix' is WPA, which is based on RC4. The 'complete redesign' is
WPA2, which is based on AES.
We now look at a very different application of cryptography, but one that most
of us use almost every day. The aspect that makes it so different from the
previous two applications that we have studied is the nature of the operating
environment. Mobile telecommunication services are provided by companies
who have agreed on certain operational standards in order to make their services
compatible. Collectively they thus represent a sort of 'closed' environment, albeit
one distributed across a substantial number of different organisations. We will
see that this has influenced some of the cryptographic design decisions that have
been taken.
