Saving root key on battery powered devices

[bperrin2]

Hi,

On battery powered devices, how is it possible to retain a root key when changing batteries? 

The crypto cell has an Always On power domain for retaining device secrets but it seems to work when crypto cell is disabled for power saving reasons. However, it doesn’t seems to retain keys when not powered. Or are the keys stored in flash zone only readable by the crypto cell?

If not what will be the proposed solution by Nordic?

Thanks in advance for your support.

Bernard

[PBT]

How does storing the "root key in flash" relate to "secure boot"?
Ie. what key is used for "secure boot" and how is that key stored? 

Best

Peter

[Einar Thorsrud]

Hi Peter,

There is no secure boot example in the lastest nRF5 SDK (15.2), so this depend on your firmware design. For secure boot you would typically want to store a key in a secure and tamper proof way so that it can be used to verify the application from the bootloader before the application is started. My initial reply describes how you can protect the key on the nRF52840.

[DamianV]

Hi Einar,

I have a follow-up question. Would it be possible to use the platform retail key (K_PRTL in the spec, I think?) to protect the root key in the flash (using a suitable mod of operation)? If I understood correctly, it is possible to use it for cryptographic operations, if you do not disable it, correct? I think, this would be no less secure than just having the root key in flash in plain.

Can you please comment on this?

Damian

[Blake]

Hi Einar - Thanks for covering this, it is appreciated. As a follow up, the CryptoCell as described in the nRF52840 reference manual can have a device root key (Kdr) set. Is it possible to store the asymmetric key pair to flash as you described, but encrypted to the CC310 Kdr? This would ensure it can only exist in RAM after being decrypted by the CC310, even if for some reason it was recovered from flash.

[Einar Thorsrud]

Hi,

Unfortunately it will not work, as the Device Root Key also has to be configured after every reset. This is because the CryptoCell peripheral does not have any persistent (flash) memory.