Secret Key Generation Between Ambient Backscatter Devices.

We analyze secret key generation between ambient backscatter devices where the channel between an ambient transmitter and the backscatter devices is used as a source of randomness. The devices do not need to estimate or measure the channel between themselves, which greatly simplifies the gathering of raw key material. We analyze the eavesdropper's mutual information based on fundamental principles and apply privacy amplification to remove any information that the eavesdropper overheard during the error correction phase. We show how the legitimate users can estimate the eavesdropper's knowledge and trade off between achievable key rate and the eavesdropper's knowledge of the final key. When modeling the channel between the ambient transmitter and the backscatter devices using state-of-the-art 3GPP channel models we show that even in non-line-of-sight channels the distance from legitimate users to an eavesdropper being larger than a few wavelengths is not alone a sufficient security guarantee. This is in contrast with previous secret key generation methods where distance is assumed to prevent the eavesdropper from having any information about the key prior to error correction. Our simulations show that a distance based approach is too optimistic and there is a possibility that the eavesdropper still knows a substantial part of the final key.