Although it's really outside the scope of this book, it's important to remember that computing and communications equipment leaks a lot information that makes them hard to really secure. Many people are aware of TEMPEST requirements which deal with radio frequency emissions of computers, displays, keyboards, and other components which can be eavesdropped. Displays can also be eavesdropped, even if it's bounced off an office wall at great distance [Kuhn 2002]. Modem lights are also enough to determine the underlying communication.