B.4. Hard As You Try
Just as you thought it was safe to discuss TEMPEST countermeasures, another form of emanation eavesdropping has come onto the scene. "Optical TEMPEST" is the nickname given to compromising emanations based on the LED status indicators of data communication equipment. Apparently, under certain conditions, the LEDs that indicate the status of links and data flows can end up being modulated by the information passing through the device. Exposure to prying eyes may yield information about the traffic being passed. Most LEDs connect to regulated power supply rails, but if the equipment is constructed in a certain way, and if the operating voltages are such that voltage regulators are forced to operate in difficult regions, there may be enough correlation to deduce a message, or at least a class of message. One suggested countermeasure: put a small patch of black electrical tape over anything in the telecommunications room that glows.