TinyI2C - A Protocol Stack for connecting Hardware Security Modules to IoT Devices

To enhance the security of devices in the Internet of Things, devices are augmented with Hardware Security Modules (HSMs). To connect HSMs to their hosting devices, serial interfaces, e.g. I2C, are used. On top of these interfaces, a protocol stack is utilized to establish a reliable communication channel. HSM vendors, such as Microchip, NXP, and Infineon, use protocols that differ in regard of provided features, complexity, and efficiency. These protocols are either complex to implement, or lack certain features. In the first case, this leads to significant system integration effort, in the latter, the HSM’s reliability and interchangeability suffers.In this paper, we perform an evaluation of state-of-the-art solutions, GlobalPlatform APDU Transfer over I2C, Microchip cryptoauthlib, and the Infineon I2C Protocol Stack. Based on this evaluation, we propose TinyI2C, a lightweight communication protocol stack. It is designed to allow simple implementations, while providing equivalent core features as state-of-the-art solutions, including reliability and packet fragmentation. Major design goals were to create a symmetric protocol, where code can be shared between both peers, which is not the case in state-of-the-art solutions. In addition, we add features, such as packet streaming support, to make the protocol suitable for Remote-Procedure-Call (RPC) based frameworks. Finally, we show a proof-of-concept and evaluate the achieved performance.