Department of Homeland Security: Control Systems Communications Encryption Primer [open pdf - 328KB]
"This primer addresses the use of encryption systems within control systems environments of the U.S. critical infrastructure. Control systems have operating parameters that differ significantly from traditional Information Technology systems. In control systems networks, security goals are normally prioritized in the following order: integrity (including safety), availability, and confidentiality. Proper application of encryption systems will assist control system security professionals to achieve these goals. However, applying encryption techniques to industrial control systems can introduce significant design challenges as they add complexities and operational limitations to the environment. If not implemented correctly, an encryption system will only provide an illusion of security and could introduce risks. The reader is informed of the basics of encryption systems (referred to herein as 'cryptosystems') and then shown two specific implementations: Internet Protocol Security and Transport Layer Security/Secure Sockets Layer. Understanding these implementation examples will help the reader decide when encryption techniques could serve as an appropriate security control within the industrial control systems environment."
United States- Computer Emergency Readiness Team (US- CERT): http://www.us-cert.gov/