Hi Poppy,

Some rough ideas about secure Flash support in TF-M. IMHO, there can be two options:

1. Based on ITS service

Secure Flash acts as the flash driver at the backend of ITS service. The secure Flash driver can invoke Crypto secure service instead of mbedTLS to avoid building two duplicates of mbedTLS library.

The cryptographic operation and key management are implemented in secure Flash driver and can be transparent to ITS service while some initialization can be necessary.

But it cannot support persistent keys otherwise it will generate a circular dependency between ITS and Crypto services.

1. Based on PS service

With PS service, the cryptographic operations can be implemented by PS crypto steps, such as encryption and authentication. Secure Flash driver only covers communication with remote device.

We don’t have to worry about the circular dependency with Crypto service.

However, there are some limitations of PS service currently:

* PS service cannot support multiple asset keys for cryptography. * PS service cannot work alone without ITS service.

Best regards, Hu Ziji

From: TF-M <tf-m-bounces@lists.trustedfirmware.orgmailto:tf-m-bounces@lists.trustedfirmware.org> On Behalf Of Edward Yang via TF-M Sent: Monday, December 21, 2020 1:22 PM To: Jamie Fox <Jamie.Fox@arm.commailto:Jamie.Fox@arm.com> Cc: Julien Su <juliensu@mxic.com.twmailto:juliensu@mxic.com.tw>; tf-m@lists.trustedfirmware.orgmailto:tf-m@lists.trustedfirmware.org; Aaron Zhu <AaronZhu@mxic.com.cnmailto:AaronZhu@mxic.com.cn>; TF-M <tf-m-bounces@lists.trustedfirmware.orgmailto:tf-m-bounces@lists.trustedfirmware.org>; nd <nd@arm.commailto:nd@arm.com> Subject: [TF-M] Discuss how to support secure Flash in TF-M

Hi Jamie,

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.

The crypto keys you mentioned in your previous reply are the crypto keys used to encrypt the communication channel between host MCU with the secure Flash? Or the secrets such as (public/private)key pairs used in TLS protocol?

If the former,I think these keys may be stored in NVM memories such as OTP or internal flash of MCU,and these keys can be loaded and imported as transient keys in RAM with a key handle during such as secure boot, then we can call PSA Crypto service with these key handles, in this case, the dependency from Crypto to secure flash doesn't exist.

Best Regards, Poppy Wu 吴偏偏

Macronix Microelectronics (Suzhou) Co.,Ltd 旺宏微电子（苏州）有限公司 No. 55, Su Hong Xi Road ,SIP, Suzhou, PRC, 215021 苏州工业园区苏虹西路55号 Tel: 86-512-62580888 EXT: 3147 Fax: 86-512-62585399 ZIP: 215021 http://www.mxic.com.cnhttp://www.mxic.com.cn/ Poppy Wu/CHINA/MXIC

2020/12/16 14:20

To Edward Yang <EdwardYang@mxic.com.cnmailto:EdwardYang@mxic.com.cn> cc Subject Re: [TF-M] Discuss how to support secure Flash in TF-MLinkNotes://cnmail01/482583DD001E2E02/DABA975B9FB113EB852564B5001283EA/6BDCE39917970ADF4825863F00689523

Hi Jamie,

Thank you for your reply.

You are right,the secure Flash can be used to store sensitive information like TLS private keys,in this application, during TLS handshake,Crypto service depends on secure Flash driver to get these private keys,so the dependency from Crypto to the secure flash needs to be kept. And you mentioned implementation of crypto functions varies depending on the platform(some platforms have hardware crypto module while the others have to depend on software crypto library), platfrom without HW crypto module could use AES encrypt/decrypt functions from a software crypto library (e.g. Mbed TLS). As I understand, currently tfm Crypto service also uses Mbed TLS as the backend software crypto library by default. I am wondering if secure Flash driver could call this Mbed TLS library APIs (such as mbedtls_aes_encrypt（）) directly?

Best Regards, Poppy Wu 吴偏偏

Macronix Microelectronics (Suzhou) Co.,Ltd 旺宏微电子（苏州）有限公司 No. 55, Su Hong Xi Road ,SIP, Suzhou, PRC, 215021 苏州工业园区苏虹西路55号 Tel: 86-512-62580888 EXT: 3147 Fax: 86-512-62585399 ZIP: 215021 http://www.mxic.com.cnhttp://www.mxic.com.cn/ Jamie Fox via TF-M <tf-m@lists.trustedfirmware.orgmailto:tf-m@lists.trustedfirmware.org> Sent by: "TF-M" <tf-m-bounces@lists.trustedfirmware.orgmailto:tf-m-bounces@lists.trustedfirmware.org>

2020/12/16 03:02 Please respond to Jamie Fox <Jamie.Fox@arm.commailto:Jamie.Fox@arm.com> To Edward Yang <EdwardYang@mxic.com.cnmailto:EdwardYang@mxic.com.cn> cc nd <nd@arm.commailto:nd@arm.com>, "tf-m@lists.trustedfirmware.orgmailto:tf-m@lists.trustedfirmware.org" <tf-m@lists.trustedfirmware.orgmailto:tf-m@lists.trustedfirmware.org> Subject Re: [TF-M] Discuss how to support secure Flash in TF-M

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.orgmailto: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.orgmailto: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. [cid:image001.gif@01D6D91C.DAC89B20]

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

Macronix Microelectronics (Suzhou) Co.,Ltd Tel: 86-512-62580888 EXT: 3147 Fax: 86-512-62585399 ZIP: 215021 http://www.mxic.com.cnhttp://www.mxic.com.cn/

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