Cyber-physical approach to securing Body Area Networks (BANs) provides solutions that are plug-n-play and transparent to its users. These Cyber-Physical Security Solutions (CPSS) actively involve characteristics from the physical environment. As a result they require a combination of signal processing (to extract features from the physical environment) and security primitives to function. In this paper we outline some of our experiences while implementing Plethys-mogram based Key Agreement (PKA) - a CPSS - that uses Photoplethysmogram (PPG) based features for key agreement. Given the limited capabilities (computation, memory, power) of individual sensor nodes in a BAN, implementing CPSS for them is challenging. We therefore design Field Programmable Gate Array (FPGA) based hardware add-on for sensor nodes in a BAN, dedicated to execute PKA. The main contributions of this work are: 1) description of our experiences in implementation of PKA on FPGA platform; 2) identification of the design goals and trade-offs for validating implementation of CPSS; and 3) a discussion on the feasibility of a software implementation of PKA based on our experience gained from the FPGA prototyping.