Hi George,

Some comments from my side:

Internal trusted storage is part of the Root of Trust domain. If ITS storage device is attacked, then the code flash where the PSA Rot SP locates may also be attacked. Does the thread model of your system require the encryption in ITS?
The ITS service is intended to be used to interface to a small piece of storage. Encryption would increase the context for each ITS file. For example, similarly to PS object context, the `IV` which is used in encryption as well as the generated `tag` should be added into each file context. They total together can be about more than 20 bytes.
If the encryption is mandatory/ necessary required by the thread model of your system, as discussed, the PSA crypto service should not be called to avoid the circular. I think a HAL API for encryption may be created in ITS for platform implementation defined encryption/decryption.

Regards,

Sherry Zhang

From: TF-M <tf-m-bounces@lists.trustedfirmware.org> On Behalf Of Vasilakis, Georgios via TF-M
Sent: Thursday, September 23, 2021 10:47 PM
To: Gyorgy Szing <Gyorgy.Szing@arm.com>; Fabian Schmidt <fabian.schmidt@nxp.com>
Cc: nd <nd@arm.com>; tf-m@lists.trustedfirmware.org
Subject: Re: [TF-M] Supporting encryption with ITS

Hey Gyorgy,

These are very valuable comments! I am aware of the circular dependency issue because the PSA apis are using the ITS as a storage backend. This, as you said, can be circumvented by using a software crypto library or an implementation specific API. So, for the encryption a flexible API can be used which can allow externals to use their own function calls.

Regarding the key storage, this is what I had in mind as well, using derived keys from the HUK. So that we don't need to store anything but the crypto metadata. Adding another layer of storage will raise more issues, I think.

From: Gyorgy Szing <Gyorgy.Szing@arm.com>
Sent: Thursday, September 23, 2021 4:30 PM
To: Fabian Schmidt <fabian.schmidt@nxp.com>; Vasilakis, Georgios <georgios.vasilakis@nordicsemi.no>
Cc: tf-m@lists.trustedfirmware.org <tf-m@lists.trustedfirmware.org>; nd <nd@arm.com>
Subject: RE: [TF-M] Supporting encryption with ITS

Hi,

AFAIK the main reason for ITS not using encryption is the problem of circular dependency. ITS is used by crypto SP for key storage, so how will crypto fetch the key from ITS to decrypt ITS? You could use a software crypto implementation (another mbed-tls instance) in ITS, but where would you safely store the keys? If you have a two layer ITS, one for only storing the keys for the second instance, and a second, encrypted one, then you end up with something like ITS and PS.

You may not need a full blown on-chip FLASH device for ITS. If you have a HUK available, you can derive the same SP specific keys from that at each boot, and store these in RAM backed ITS. You won’t be able to store other keys in ITS in a persistent way of course, but for that you can use PS. Well, something along these lines.

Perhaps the TF-M team could help better if you could share some details on why your customer would need encrypted ITS. (A PSA for Cortex-A (TS) maintainer chiming in to a “not his business” discussion here 😉 )

/George

From: TF-M <tf-m-bounces@lists.trustedfirmware.org> On Behalf Of Fabian Schmidt via TF-M
Sent: September 23, 2021 15:51
To: Vasilakis, Georgios <georgios.vasilakis@nordicsemi.no>
Cc: tf-m@lists.trustedfirmware.org
Subject: Re: [TF-M] Supporting encryption with ITS

Hi George,

I’m wondering if that would add value. To my understanding, ITS was never designed to be encrypted because of the way it’s supposed to be set up. (It’s Internal Trusted Storage.) I believe best practice is to place it in a “trusted” location, one that is ideally only accessible from Secure world, and also ideally on-die. If you then restrict outside access to the internal flash (JTAG, flash programmer ports,…), you’re pretty golden, in that no unauthorized party should be able to read from or write to the ITS.*

Let me know if I misunderstand anything about ITS or TrustZone, but that’s my view. Maybe I’m painting an idealized picture.

Greetings,

Fabian Schmidt

* at least short of a sophisticated physical attack or finding some loophole in TrustZone…

From: TF-M <tf-m-bounces@lists.trustedfirmware.org> On Behalf Of Vasilakis, Georgios via TF-M
Sent: Donnerstag, 23. September 2021 15:28
To: tf-m@lists.trustedfirmware.org
Subject: [EXT] [TF-M] Supporting encryption with ITS

Caution: EXT Email

Hey all,

Lately the requirement for an encrypted ITS solution is being asked from our customers and I would like to have a discussion here on how we can design this in a reasonable way. The first thought that came to my mind was to add the functionality to the ITS flash-fs layer. This layer contains file metadata in the its_file_meta_t structure and it should be possible to expand this to include additional crypto metadata (conditionally). This seems to be the less invasive change to me, even though it will introduce some increased memory usage since supporting encryption will mean that we cannot read the data in chunks anymore, we will have to use static buffers.

At the same time, I looked at the PS partition since I knew that it has support for encryption. I believe that some core concepts of both solutions have similarities even though the code is quite different. For example, a file in ITS is similar to an object in PS and the (linear) list of file metadata in ITS is similar to the concept of the object table in PS. So, I think that it should be possible to design some generic-enough APIs that we can use for both the ITS and PS. Even though this will require some major refactoring in both partitions, it will decrease the code of these services which will probably decrease maintenance later.

What are your thoughts on this?

Regards,

George