Security and reliability are two terms used quite often in our industry. Though I have been in the control systems realm a short time, it appears that many people view the two subjects as opposing forces. I believe that is most cases security should be considered an aspect of reliability.
If a control network is compromised due to poor security policy (lack of patches, slow patch cycle, etc), the reliability of the network is decreased. If an attacker can perform INL’s favorite attack, man-in-the-middle, and send commands to a PLC that disrupts the grid, the reliability is decreased. Nearly all security threats should be seen as threats to the systems reliability.
I spoke about this subject with others and Martin Solum brought up the aspect of a backup network. In the case of failed patching or a compromise, the backup network is then used as the control network. This network would typically be on a delayed patch cycle and would therefore have more holes (in theory) than the production control network. Though an implementation with a backup network is going to add complexity to both the security and reliability aspects of a control network, I don’t feel that it invalidates the fact that security is an aspect of reliability.
This topic came about during a conversation I had with a former co-worker. Steve Hurd, of Sandia National Laboratories in California, was interested in writing a paper on the subject of how security affects reliability. Due to the fact that Steve never has enough time, he allowed me to write on the subject. If anybody has any hard numbers (which is almost never, it seems) that either agrees or disagrees with this view, I would love to see them.