Is it possible to use the KMU (or other secure area??) in the nRF9160 to store encryption keys?

[Spi]

I recently had a question about AES encryption on the nRF9160 and about how to store 32-byte encryption keys.   The ticket was closed after I got the following answer: "we don't have support for keys stored in the KMU using PSA Crypto on 91-series".  I should have followed-up to that with this more specific question:

Is it possible to store keys in the KMU (or similar?), then read them into RAM, then use psa_import_key() for subsequent use with psa_aead_encrypt()?

If it is possible, what are the API functions I should use to

a) write key(s) to KMU (presumably from RAM)

b) read key(s) from KMU into RAM

If that's not possible, what is the recommended way to store 32-byte encryption keys on the nRF9160?

[Michal]

Hello,

So in general, KMU has only support for 128-bit keys, which is 16-byte.

Also, you should be able to reopen that ticket and continue there I think?

Best regards,

Michal

[Spi]

Thank you for your response.  How many KMU keys are there? I can combine two 16-byte keys to make one 32-byte key.  What are the API functions to read/write the KMU keys?

[Spi]

Thanks. I think I might have seen that link before, but I'll take another look.

[Spi]

Thanks. I looked at the persistent_key_usage sample and it appears that it might only work for 128-bit keys because the psa_set_key_id() call can only specify a single KMU slot.  And the hw_unique_key API doesn't appear to support the ability to read keys.  I will take a closer look at manually driving the KMU registers.

[Michal]

Thank you for the information.

I will look more into the issue after the weekend.

Best regards,

Michal

[Michal]

Spi said:

I looked at the persistent_key_usage sample and it appears that it might only work for 128-bit keys because the psa_set_key_id() call can only specify a single KMU slot.  And the hw_unique_key API doesn't appear to support the ability to read keys.  I

This sample shouldn't use KMU, but Internal Trusted Storage though?

https://docs.nordicsemi.com/bundle/ncs-3.2.2/page/nrf/samples/crypto/persistent_key_usage/README.html

Are you sure that you are looking at the correct thing?

Best regards,

Michal

[Spi]

Thanks for your answer. I looked at the SDK V2.6 version of persistent_key_usage sample and I didn't understand it. What is the use of a random encryption key? To me it makes decryption very difficult when you don't know what the original encryption key was.  Either way, we gave up on this very complicated way of doing things and found a much simpler solution that we understand and control and most importantly, know the encryption key so that we can also decrypt the encrypted data.