Security threats evolve at the rate of the latest technology
and older equipment often lacks the capacity to implement
current best-practice security algorithms within real-time constraints.
Factors such as key length and algorithm complexity
are limited by processing power when attempting to implement
any form of cryptography. In addition, other aspects
of low level industrial protocols make implementation of
security difficult. The low data transfer rate of many protocols
means that they would be adversely affected by the additional
overhead required for secure communication. Conventional
cryptographic mechanisms are also very sensitive to all levels
of electronic noise [42].
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.