Body Area Networks (BAN) can play a major role in monitoring the health of soldiers in a battlefield. Securing BANs is essential to ensure safety of the soldiers. This paper presents a novel key agreement protocol called Photoplethysmogram PPG-based based Key Agreement (PKA) which allows sensors in a BAN to agree to a common key using PPG values obtained from the subject (soldier) they are deployed on. Using the stimuli which the sensors are designed to monitor directly for cryptographic purposes, enables administrators to provide security for BANs with minimal initial setup. The principal contributions of this paper are: 1) demonstration of the viability of the PPG signals for agreeing upon common symmetric cryptographic keys between two nodes in BAN, and 2) analysis of the security, performance and quality of the keys produced by PKA.