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According to Wikipedia Near Field Communication is intended for short-distance communication - something like less than a meter. I wonder how could this limitation be enforced?

Usually there're these factors that limit radio communication range:

  • radio waves being absorbed and/or diffused by obstacles
  • other radiation being present
  • initial signal strength
  • receiver sensitivity

At short distances the latter two factors are the key. Usually specifications will regulate the maximum transmitter power and expected receiver sensitivity and this effectively sets up the communication range.

Now suppose I want to create some fraudulent device that would communicate with devices without owners' consent. The specification will say the transmitter must have no more than some power, but I could easily make it times more powerful. The specification will say some level of sensitivity is required, but I could make a more sensitive receiver. I suppose with such adjustments I could make a device that would communicate with ordinary devices over several meters distance and no layman would notice that.

How does NFC protect from such abuse leading to unwanted communication over distance longer than expected? How could maximum communication distance of less than a meter be enforced?

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up vote 0 down vote accepted

Do you intend to sell this device? If so, prevention is guaranteed by the fact that you'll never get FCC approval for it.

If you're approaching it from a security standpoint, imagining a target which is relying on NFC restrictions of 1 meter vs. several meters to protect valuable data, you could create such a device. It would be expensive, and thoroughly illegal. If your target has data that's worth more than the cost to build such a device, then they're probably not going to keep it in a format where it could be accessed via NFC.

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Well, if it's a fraudulent device I won't sell it legally and I guess I won't need certification in this case. – sharptooth Sep 16 '11 at 13:47

I don't really want to help some dirtbag steal other people's data, so this answer just discusses near field theory.

Far field is where the energy from the transmitter has organized itself into a propagating wave that keeps going on its own. Both the E and B fields have to be envolved and interact in a specific way for this to happen. The point of a antenna is to excite both the E and B fields in its vicinity the right way to maximize the portion of the energy dumped into the antenna that will eventually radiate outwards.

Since it requires both a E and B field to propagate RF energy, transmissions intended to stay in the near field use one or the other by try not to create both. This means near field uses either capacitive coupling (E field) or transformer coupling (B field). Since it is impossilbe to create a varying E field wihout some B field and vice versa(*), some of the energy will escape the near field and propagate outwards as RF. This could be detected with a sensitive enough receiver if interference is low enough.

(*) Creating a varying E field takes changing voltage, which due to capacitance requires current, which creates a B field. To create a varying B field requires changing current, which due to resistance requires voltage, which creates a E field.

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It can't really be enforced 100% even when regulations are met. Unless maybe you placed both devices in a shielded box. Radio signals generated may still be picked up with a sensitive receiver a few meters away.
The near field only works up to about 20cm, the two devices combining to act like an air core transformer. As Olin mentions the intention is to keep the energy in the near field, but some RF is unavoidable.

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IMHO the only secure way to limit the distance is via the propagation delay caused by the finite speed of light. This is called a distance bounding protocol.

In practice it seems it is not (yet?) done, because it is hard and expensive to implement something that can reply in a few nanoseconds.

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NFC is a radio link, so you should never assume that it is secure, regardless of distance. An attacker does not need a device capable of observing or manipulating the field from metres away because they can probably stick an antenna close by.

If you want a radio link to be secure, you must use strong encryption.

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