Signal listening

Question

Is there a way to know if someone listens to signal I produce (air signal as well as signal on network)? Is there a way to know if the signal was listened along the way until the signal is reached to the required listener? Can I distinguish between a signal that was listened in his way to non-listened signal?

Answer 1

Passive detection is called "passive" exactly for that reason: it is not detectable. That's by definition.

Now your question might be: is there a way to "emit a signal" (i.e. convey information) that cannot be passively detected ? In fact physicist have found a method, called quantum key distribution. This requires sending photons one by one, and security relies on some rather fundamental characteristics of quantum theory. This is Science: you can never be sure of having attained the Truth; you can only make theories that seem to work well, or not. However, breaking QKD would meaning proving wrong a century of the smartest physicists, so that's deemed rather improbable. Of course, any experimental setup can have shortcomings; e.g. some instances of QKD have been broken by crafty attackers who, instead of observing the photons going in the optic fibre, used the fibre to observe the state of the apparatus at the sending site.

Information security is the art of cheating with definitions. Thus, one can have something which is almost as good as detecting passive detection: encryption. Encrypted data, if done properly, ensures that the actual information cannot be observed by outsiders. It does not tell whether some people are trying; only that they are not expected to succeed.

Answer 2

Thomas is correct, as usual. I'm going to add a small observation related to the physics involved.

Radio waves have two regions of propagation (with the same underlying Maxwell's equations): near-field and far-field.

In the near field presence of listening devices can be "detected" from the way they interfere with the transmissions (with some hoop-jumping and practical limits).

In the far field this type of detection is impossible. Yet, to listen to what you transmit over the air, one has to employ an electronic device. When improperly implemented, such devices radiate their own weak signals which can alert you to the fact of possible interception.

_{TLDR: Passive interception does leave physical traces but you won't be able to detect those without equipment at prices in the 10^5 - 10^6 $ range, some training, being sufficiently close to the listener, and sheer luck.}

In practice, it is safe to assume that everything you transmit can be intercepted by someone and plan accordingly. There are satellites in space, automated radio scanners at listening posts, vast antenna fields, ships, planes, helicopters, mobile teams, your nosy neighbor, his 10-year old boy, and perhaps even the neighbor's dog that can listen to your messages.