In the U.S., there are laws in every state that make it a felony to illicitly observe signals being sent over wires used for, or devices connected to, interstate communication equipment. Thus anything connected to the telephone system is protected from wiretapping, and any tapper found guilty of such an activity is liable to end up in jail for several years. Catching a tapper may be very difficult without the aide of a professional surveillance expert with the proper equipment and knowledge. A recent addition to this law which is currently in congress prohibits illicit observation of any electromagnetic signaling including radio waves, this law being designed primarily to protect the cable TV companies from those with satellite dishes and decoders, and the telephone company from those who listen in on transmissions from microwave dishes. The major line into Washington D.C. passes over the Soviet embassy, and they are widely assumed to be listening in, but this law does not apply to that particular case since the equipment is not on US territory.
There are exceptions to the wiretap laws for criminal investigations where a warrant has been legally attained from a federal judge, and there is an exception for the telephone company to allow them to test their lines. Abuses have often been found in both of these cases, but for the most part the system seems effective. Current technology makes other forms of attack quite easy, especially in the case of computer systems. An example is the availability of components with a total cost under $100 which can be used to listen in on a CRT terminal from over 100 meters away [vanEck85] . The new wiretap law would also make this an illegal activity.