A smart phone and a Body Area Network (BAN) can be used to monitor the elderly or patients with chronic diseases. Securing data and commands between smart phones and BANs is essential for preserving the privacy of health data and for ensuring the safety of the patient. We propose a new type of key establishment protocols for a body area network. The protocols takes the error margin that can be found in sensed data into account. We provide an example of this new type of key establishment protocol. The protocol can establish a key between two devices without the need for any form of initialization or a priori knowledge. Previous protocols that used sensed data to establish keys required wired electrode patches to gather ECG data. We show how photoplethysmographic data obtained from the video camera on a smart phone can be used as a secure environmental value. Our initial experiments show that we can successfully create secure communication after discovering a single inter-pulse interval. We also show preliminary results of photoplethysmographic measurements from an Arduino device and demonstrate that they do not overlap with existing measurements, which are obtained from a smart phone.