Physically Secure Code and Data Storage in Autonomously Booting Systems

Abstract

Today, full disk encryption is a common practice to protect data on desktop computers and notebooks from unauthorized physical access. For embedded systems, however, the situation is different and they often lack physical protection. Usually no user or remotely connected system is involved during the boot phase which requires autonomously booting systems. For this paper an entire software stack for secure code and data storage in embedded systems has been designed, implemented and evaluated regarding security aspects and performance. For the security evaluation, physical attacks on the flash chip and RAM access have been taken into account. The system is a combined hardware and software solution and provides a considerable amount of security without a second party involved that could participate in a trust bootstrapping protocol. A symmetric key hierarchy enables the use of applications from different vendors which are not able to decrypt each others software. For code, a signature chain ensures the authenticity of the code being run. For data, integrity is ensured on a per sector basis such that targeted manipulations are not only mitigated but can be detected as well. This is a novel technique that is currently not known from any publicly available full disk encryption system. We show that the confidentiality, integrity and authenticity of code and data protected with our system can be ensured provided that small parts of the hardware are considered trusted.