RE: Hashing Key is TF-m

Amanda Hsieh — Fri, 14 Apr 2023 15:01:15 GMT

Hi,

SHA-256 is a deterministic hashing algorithm but the result of calling SHA-256 is a "digest" and I don't think it is common to refer to that as a key.

There are multiple options of keeping a hash-digest following the nRF9160. I will list out the options. You don't need to use the PSA Crypto APIs (psa_import_key) for this.

Use some data in the OTP region to keep the hash digest and implement an ARoT to give access to this in the S-world, or alternatively use the ARoT to do the whole operation they want to do (like a custom security service). Note that this may limit the number of upgrades you can do if downgrade protection is used in the DFU
Use a CPU-accessible slot in KMU (Don't lock it only for CryptoCell usage) and make the same type of ARoT to either give direct access to the digest or make a custom ARoT functionality to do the operation using this digest
Use PSA PS (Protected Storage) API provided by TF-M to store the hash-digest without permissions to make it erasable. Data in PSA PS will be accessible from the NS world
Use psa_import_key but give usage permissions to export the digest key (so NS world can get access to it)

Final notes: Only option 1 and 2 can be done in a production line without having any application running on the device. This is a huge benefit. But as you see, the OTP and the KMU is a secure-only peripheral so there needs to be a secure service when TrustZone is enabled (with TF-M)

Option 3 and 4 work without any specialized ARoT service and the business-logic of handling this SHA-256 digest will be in the NS image. This means that this hash for sure is readable in the NS image. So the concept of "key" is further diminished as everyone will have access to it.

-Amanda H.

RE: Hashing Key is TF-m

WhiteCloud2 — Thu, 13 Apr 2023 10:50:14 GMT

Thanks for the note.

The provisioning_image example demonstrates writing the identity key and the implementation id. However, I am looking for the key used for hashing. I did change the identity key and implementation id and get the same hashing value for a given string. That I think confirms that they are not the hashing key.

RE: Hashing Key is TF-m

Amanda Hsieh — Wed, 12 Apr 2023 21:00:34 GMT

Hi,

[quote user=""]Could someone shed some light into it? Where is the key stored?[/quote]

The key is managed and stored in the key management unit (KMU). See more information in KMU documentation.

[quote user=""]how do I provision it?[/quote]

Provisioning image sample and PSA template are good samples to start as you did.

Regards,
Amanda H.