The medical implant communication system is a critical cyber-physical system with significant security concerns due to its crucial role in patient health and safety. Implant devices integrate wireless technology, exposing them to potential cyber threats. This paper presents a secure data transmission protocol incorporating cryptographic primitives with a lightweight authentication scheme. This is specifically designed for secure communication in an Implantable Cardioverter Defibrillators (ICD) environment. We present a set of practical attacks and their countermeasures in the ICD environment using the proposed authentication scheme.