Conventional security protocols such as IP Security, Secure
Socket Layer and VPN are not practical for use in low level
industrial automation networks due to their lack of support for
multicast- and broadcast transmissions [53]. Key distribution
is also problematic in the use of cryptographic algorithms in
industrial networks, as cryptographic keys may be needed by
thousands of devices. Various approaches to key distribution
have been discussed, for example loading keys onto physical
storage and installing them at each device [48], or distributing
keys electronically at install time when other configuration
settings are loaded onto an instrument [54]. Many of the key
distribution methods envisioned involve a high level of manual
intervention during the commissioning of the equipment and
fail to consider the lifetime of the keys. The length of the
key and the algorithm in use determine the length of time it
would require to decrypt sensitive information, and the two
are normally matched to the expected lifetime of the data to
be protected