For the past few weeks, I’ve been working on a research project that includes radio frequency (RF) nodes with a proprietary protocol running on top of Nordic Semiconductor (Nordic)[1] chips, specifically nrf52840. While it’s been quite challenging (no strings at all and of course no symbols), it’s been interesting and satisfying at the same time. As part of this work, I uncovered the code that handles encryption and decryption of RF packets. I wanted to share my findings in the hope that it will serve as a guide to anyone working on a similar project.
Let’s jump straight to the technical part. These chips use a couple of different crypto peripherals depending on the protocol they implement. In this case, the code used the AESCCM[2][3] engine, which is quite simple. It just takes three main parameters, key /nonce, input buffer, and output buffer. Once these are configured, the system can encrypt or decrypt data.
One nice feature of these chips is the ability to chain peripherals using the Programmable Peripheral Interconnect (PPI)[4] subsystem. This allows embedded developers to connect the RADIO peripheral and the crypto engine, causing the AESCCM to immediately trigger encryption or decryption, as can be seen in Figure 1 and Figure 2.
With this information in mind, let’s jump into the disassembled binary and see what the code looks like. All of the function names, variables, and registers are my best guess based on the reverse-engineered functionality.
