  Over the past decade, focus on the security and privacy aspects of
implantable medical devices (IMDs) has intensified, driven by the multitude of
cybersecurity vulnerabilities found in various existing devices. However, due
to their strict computational, energy and physical constraints, conventional
security protocols are not directly applicable to IMDs. Custom-tailored schemes
have been proposed instead which, however, fail to cover the full spectrum of
security features that modern IMDs and their ecosystems so critically require.
In this paper we propose IMDfence, a security protocol for IMD ecosystems that
provides a comprehensive yet practical security portfolio, which includes
availability, non-repudiation, access control, entity authentication, remote
monitoring and system scalability. The protocol also allows emergency access
that results in the graceful degradation of offered services without
compromising security and patient safety. The performance of the security
protocol as well as its feasibility and impact on modern IMDs are extensively
analyzed and evaluated. We find that IMDfence achieves the above security
requirements at a mere less than 7% increase in total IMD energy consumption,
and less than 14 ms and 9 kB increase in system delay and memory footprint,
respectively.
