Secure Boot on the nRF53

[NickinFrance]

Hello 

I would like to know if there is a capability of a truly secured boot with the nRF53, starting with a secure bootloader in some locked or immutable memory - i.e. secure even against someone gaining physical access to a device and being able to wipte/reflash it. Or indeed an nRF52840.

Thanks

Nick

[haakonsh]

The nRF5340 is more secure than the nRF52840. It does not suffer from the same issues as the nRF52 series with regards to the recent fault-injection attack that can disable the APPROTECT (read-back protection / debugger access). It also has a KMU — Key management unit that allows it to store cryptographic keys that are only accessible through the ARM Cryptocell 312. That means that the CPU does not have R/W access to the keys and no malicious code can ever read them out or overwrite existing keys. This KMU peripheral along with the CryptoCell is used to establish a root-of-trust for a secure bootloader.  
The KMU unit allows you to use a list of keys, where in the event of a single key getting discovered, you can invalidate that key in your device and use the next one in the list. This allows you to re-gain a secure root-of-trust in all deployed devices in the event that a single device has been successfully attacked and a key is revealed. 

There is no such thing as a truly secure device, only more or less secure. Any device can be successfully attacked given enough resources, it all about increasing the amount of resources required for a successful attack, and that's what we've done with the nRF5340 compared to the nRF52 series. 

[NickinFrance]

Hi Haakonsh, 

Thanks for your reply, and I appreciate that security is never perfect and always a trade off. My specific question was whether a device could be sufficient locked that it would always start booting up from a read only location, so it could verify the signature of the next level boot loader (using locked keys). So effectively, even if you gained physical access and could wipe the Flash entirely and reload some other software, you couldn't the boot up the device without signed code. 

I understand the keys are locked, but if you can get the device to boot up without verifying them then that won't assist you. 

Thanks

Nick

[haakonsh]

If you have the ability to erase and program the device then you have full control over it. The ARM Cortex M-33 application core will start to execute the instruction at address 0x0000 0000, there is no other 'secure bootloader' peripheral that prevents that execution.