I’ve spent the last few weeks working with the security architecture of the nRF54L series from Nordic Semiconductor (in case you missed it, I recently joined Nordic !). While doing so, I have engaged my typical low-level learning technique of eschewing writing firmware for manually poking at registers using the debugger. A few nights ago I found myself observing unexpected values in memory when working with the key management unit. It turned out to be a familiar issue, but one that requires an understanding of the internal system on chip (SoC) components, and how the debugger interacts with them, to diagnose.

For a bit of background, the nRF54L series has a fairly advanced set of security capabilities, headlined by Arm TrustZone support in the Cortex-M33 core , a CRACEN cryptographic accelerator , and a Key Management Unit (KMU) . The KMU is used for storing sensitive data, such as key seeds and associated metadata, in the Secure Information Configuration Region (SICR) .

The SICR is divided into slots.…