A Secure Key Sharing Algorithm Exploiting Phase Reciprocity in Wireless Channels
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This article presents a secure key exchange algorithm that exploits reciprocity in wireless channels to share a secret key between two nodes A and B. Reciprocity implies that the channel phases in the links A→ B and B→ A are the same. A number of such reciprocal phase values are measured at nodes A and B, called shared phase values hereafter. Each shared phase value is used to mask points of a Phase Shift Keying (PSK) constellation. Masking is achieved by rotating each PSK constellation with a shared phase value. Rotation of constellation is equivalent to adding phases modulo-2π, and as the channel phase is uniformly distributed in [0,2π), the result of summation conveys zero information about summands. To enlarge the key size over a static or slow fading channel, the Radio Frequency (RF) propagation path is perturbed to create several independent realizations of multi-path fading, each used to share a new phase value. To eavesdrop a phase value shared in this manner, the Eavesdropper (Eve) will always face an under-determined system of linear equations which will not reveal any useful information about its actual solution value. This property is used to establish a secure key between two legitimate users.
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