Emissions and Immunity
The prevention of "compromising emissions" is an important requirement for secure hardware. All electronic devices generate electromagnetic inference (EMI) in one form or another. Emission security can be traced back as early as World War I, when field telephone lines could be monitored using the phenomenon of "cross talk," interference caused by the energy from one telephone conversation invading another by electrostatic or electromagnetic coupling. Due to the sensitive nature of the subject, most research was classified and it soon disappeared from public literature. Much of the government shielding requirements (known as "TEMPEST") remains to be classified by the U.S. government. However, many unclassified documents are available on John Young's TEMPEST Documents web site. 
van Eck's 1985 "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?"  was the first academic article on the subject, which described the results of research into eavesdropping on video display units by picking up and decoding the electromagnetic interference. More recently, Kuhn and Anderson's "Soft Tempest: Hidden Data Transmission Using Electromagnetic Emanations"  showed how compromising emissions from a PC could be made better or worse by using specific font styles and colors. Loughry and Umphress's "Information Leakage from Optical Emanations"  describes how light emitting diodes (LEDs) in certain communications equipment leak the data contents that are being transmitted, and details a number of design changes to reduce such risk.
Rao and Rohatgi's "EMPowering Side-Channel Attacks"  provides preliminary results of attacks by measuring EMI. These types of focused EMI and power attacks are useful on small, possibly portable devices, such as smart cards or authentication tokens. Larger devices, such as desktop computers and network appliances, might generate too much EMI to be able to measure specific, minute changes. Well-filtered power supplies and physical shielding can make attacks infeasible.
Eavesdropping on EMI emissions is referred to as a passive attack. An active attack consists of directing high-energy RF (HERF) signals or directing electrostatic discharge (ESD) at the product in order to cause failures. Properly designing a product to meet specific EMI and RF emissions conditions is part of many specifications, including those of the FCC, EU, IEC, and FDA. Essentially the inverse of emissions testing, immunity testing subjects a product to various RF phenomena to see whether they affect the product's performance. ESD protection components are often designed into external connectors and contacts to reduce the chance of failure. One attack uses an ESD simulator tool to generate a high voltage spike and inject it into a device's external interface or keypad in hopes of causing an unexpected or unintended condition (for example, by causing the program counter of the microprocessor to jump to a different code portion or change the values on the address or data bus). However, unless the injection of HERF or ESD can be reproduced in a controlled manner, the results may be too unpredictable to be useful.
At the enclosure level, EMI shielding can easily be designed in or retrofitted to a design in the form of coatings, sprays, tapes, or housings in order to decrease emissions and increase immunity. If the enclosure is metal, EMI tape can be used to seal any slots or seams in the case. If the case is plastic, EMI spray (a conductive paint) can be used to coat the inside of the case. EMI tapes can come loose, causing short circuits, and EMI paint can flake or chip if the surface is not cleaned prior to application. Also, be aware of any changes in thermal characteristics that EMI shielding may cause. EMI shielding solutions are provided by a large number of manufacturers, including W.L. Gore, Schlegel Systems, and GC/Waldom Electronics. The Thomas Register Directory also provides a large listing of EMI/RFI shielding manufacturers and suppliers.