In this paper, we propose a cross body signal pairing (CBSPCR) protocol to enable key generation between wearable devices with cardiac and respiratory sensors. Most previous pairing protocols require both wearable devices to access the same biometric signal for pairing. The restriction can greatly limit communication between wearable devices and the development of new applications on the devices. We implemented CBSPCR for pairing wearable devices based on respiratory and cardiac signals. Two pairing modes are proposed for the pairing: (1) The model-based pairing is based on mathematical models between respiratory and cardiac signals. (2) The extraction-based pairing is based on the extraction of respiratory signals from cardiac signals. The pairing performance of these two modes varies with exercise intensity. To take full advantage of both pairing modes, we developed a mode switching based on the respiration rate, a reliable indicator of exercise intensity level. CBSPCR uses Lloyd-Max quantization and a BCH-based mismatch correction method to digitize the signals and correct mismatches between bit-strings. Our extensive experiments with 30 participants show that CBSPCR can generate about 0.0125 keys per second with high resistance to impersonation attacks and negligible battery consumption.

We propose Breathe-to-Pair (B2P), a protocol for pairing and shared-key generation for wearable devices that leverages the wearer's respiration activity to ensure that the devices are part of the same body-area network. We assume that the devices exploit different types of sensors to extract and process the respiration signal. We illustrate B2P for the case of two devices that use respiratory inductance plethysmography (RIP) and accelerometer sensors, respectively. Allowing for different types of sensors in pairing allows us to include wearable devices that use a variety of different sensors. In practice, this form of sensor variety creates a number of challenges that limit the ability of the shared-key establishment algorithm to generate matching keys. The two main obstacles are the lack of synchronization across the devices and the need for correct noise-induced mismatches between the generated key bit-strings.