Any way to prove that two devices are in the same physical space, for authentication?

I need to come up with digital proof that two physical devices are in the same (approximate) location whenever that transaction occurs.

This could be:

• A digital ID and the authenticator


• A consumer and a merchant


• A digital key for a door, and a lock

Is there any way to prove that the devices are in the same location?

One way I was thinking of making this work, is to read environmental data (RF noise, bluetooth signal distance) and include this data in the verification channel. Since the devices are somewhat near each other, this environmental data should be almost the same.

Question

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  If it makes sense to include environmental data to prove two devices are in the same location, what data should be used?

  
  


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  Since the environmental data will likely never be identical, but very similar, how can this be included in an authentication protocol? (this is the crypto aspect of the question)