Calculate CMAC using KMU and nrfx library

[GiulianoFranchetto]

Hi,

Is it possible to calculate a CMAC using the nrfx security library?

Our keys are stored in the KMU of a nRF9160.

For now, we are using mbedtls function mbedtls_aes_setkey_enc_shadow_key( ) for encryption for example, but I've not found a way to calculate a 4-bytes CMAC.

Regards

Giuliano

[GiulianoFranchetto]

Hi,

Sigurd Hellesvik said:

This is a great idea! I will see if I can make such a "guide". I can not make any guarantees for when this is done, but thanks for the suggestion.

Glad you love the idea

Can't wait for that guide, before I'll need to rework all of my security/crypto code!

Regarding security, I'd prefer to be as secure and efficient as possible.

If I can be of any help, feel free to ask.

I'll keep the ticket open if you don't mind, so we can keep track of the progress on either the guide or suggestions/ideas coming from the community.

Regards

Giuliano

[GiulianoFranchetto]

Hi Sigurd,

Was there any progress on this subject?

Regards

Giuliano

[Sigurd Hellesvik]

Hi

I am still working on a blog post on secure key storage, but I am unfortunally not done yet.
Thank you for checking on it!

Just a couple of points I learned while working on this which could be relevant to you from your previous questions:

If you want to store keys in ITS, just use the PSA Crypto API.
For example as it is done in the Persistent key storage Sample.

ITS is for saving data securely to internal flash.

While Protected Storage is intended for saving data to external flash.
This is showcased in the Zephyr Protected Storage Sample.

Regards,
Sigurd Hellesvik

[Sigurd Hellesvik]

Hi again GiulianoFranchetto 

The blog-post on secure storage is not ready yet.
But one of the per-cursors for many of the types of secure storage is Trusted Firmware-M.
Recently I posted a blog on An Introduction to Trusted Firmware-M (TF-M). Maybe you find this useful.

Regards,
Sigurd Hellesvik

[lseg]

Hi, is there any progress on this secure storage blog post?

I am wondering what I can do if I want to store ECC keys. Could I use KMU for this? Is it correct that I could use KMU to store any key, but the biggest problem is that KMU can only pass symmetric keys through HW to the cryptocell (I am using the nRF53)?

So my only option for ECC keys would be to use ITS (Internal Trusted Storage) using the Platform Security Architecture (PSA) APIs?

Is the use of ITS by PSA still under development as you noted 1 year ago?

Is it also correct to say that this ITS is a part of flash storage where things will be stored encrypted (AES?). So for example if I want to store my ECC key there, it will be encrypted and stored in flash. If I want to use it, it will be read from flash, decrypted to a variable, and this variable would be used as an input for my ecc encryption or decryption function? So it will reside in memory for a short amount of time. There is no way to direct it to the cryptocell HW immediately?

Kind regards,