Hi,

The trouble with calling the PSA Crypto functions from your secure flash driver is that it will create a circular dependency between the ITS service and the Crypto service. Crypto uses ITS to store its keys, but then the secure flash driver calls the Crypto service again to encrypt the data. These circular dependencies between partitions are forbidden in TF-M.

As I understand it, one of the uses of the secure flash is to store crypto keys, so the dependency from Crypto to the secure flash needs to be kept. That means the dependency on the Crypto service in the secure flash driver needs to be avoided. I think ideally the best way to do this would be to use hardware crypto functions available in the platform instead of PSA Crypto APIs inside the flash driver, but if no crypto hardware is available then you could use AES encrypt/decrypt functions from a software crypto library (e.g. Mbed TLS).

Hope that helps.

Kind regards,

Jamie

From: TF-M <tf-m-bounces@lists.trustedfirmware.org> On Behalf Of Edward Yang via TF-M
Sent: 15 December 2020 10:08
To: tf-m@lists.trustedfirmware.org
Subject: [TF-M] Discuss how to support secure Flash in TF-M

Dear all,

I would like to discuss how to add secure Flash support in TF-M.

Firstly, I want to describe the motivation of adding secure Flash support in TF-M.

Currently ARM TF-M provides protected storage service(PS service for short) to implement security protection on external normal storage, however this type external normal storage is still vulnerable to unauthorized physical modifications/erasing and cloning.Macronix and other Flash memory suppliers have developed secure Flash products to enhance the security in external flash devices. Secure Flash enables mutual authentication between itself and host MCU/SoC and only permits the authorised host to perform access, besides, the communication channel between host MCU/SoC and secure Flash is protected by encryption, authentication, data scrambling, and frame sequencing with monotonic counters as shown below, so the secure Flash provides dependable defense against unauthorised access, man-in-the-middle, replay, sniffing and other security threats.

If we port TF-M to a platform which uses secure Flash as external flash,then secure Flash driver should be added to TF-M.However,compared with nomal external flash driver,secure Flash driver needs extra crypto functions(such as calling AES crypto functions to encrypt/decrypt data), if the secure Flash driver is placed in platform folder in TF-M code structure as a backend of ITS service, I don't know whether secure Flash driver is allowed to call Crypto service(such as psa_aead_encrypt(), psa_aead_decrypt() )directly.If not, are there any other solutions to perform crypto operations in secure Flash driver?




Best Regards,
Poppy Wu

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