Eliminating Reconciliation Cost in Secret Key Generation for Body-Worn Health Monitoring Devices

Medical data collected by wearable wireless sensor devices must be adequately secured. A prerequisite for mass deployment of these secure systems is the ability to periodically renew cryptographic keys without user involvement. Recent work has shown that two communicating devices can generate secret keys directly from measurements of their common wireless channel, which is symmetric but cannot be inferred in detail by an eavesdropper. These schemes may, however, yield mismatching keys at the two ends, requiring reconciliation mechanisms with high implementation and energy costs, unsuitable for resource-poor body-worn devices. In this work, we demonstrate a scheme for secret-key generation able to construct shared keys with near-perfect agreement, thereby avoiding reconciliation costs. Our specific contributions are: (1) we identify non-simultaneous probing of the channel by the link end-points as the dominant cause of channel measurement disagreement; (2) we develop a practical filtering scheme to reduce this disagreement, dramatically improving signal correlation between the two ends without affecting key entropy; and (3) we show that by restricting key generation to periods of significant channel fluctuation, we achieve near-perfect key agreement. We demonstrate in several representative body-worn settings that our scheme can generate secret bits with 99.8% agreement, and so yield near-perfect matching 128-bit keys approximately every half hour.