Hi Poppy,
One more thing about IAK, it's defined as one of the immutable PSA parameters in PSA security model document.
And
"To prevent cloning or spoofing, the IAK must be secret and accessible only to the Initial Attestation Service." 

In that case, for the product, you may need to consider how to ensure confidentiality/integrity and it's immutable based on the embedded flash solution.

Regards,

David Wang

ARM Electronic Technology (Shanghai) Co., Ltd

Phone: +86-21-6154 9142 (ext. 59142)



From: TF-M <tf-m-bounces@lists.trustedfirmware.org> on behalf of Raef Coles via TF-M <tf-m@lists.trustedfirmware.org>
Sent: 17 May 2021 16:50
To: tf-m@lists.trustedfirmware.org <tf-m@lists.trustedfirmware.org>; Edward Yang <EdwardYang@mxic.com.cn>
Subject: Re: [TF-M] Factory provisioning with tf-m
 
Hi Poppy.

The current code we use for provisioning the CC312 OTP on the Musca reference platforms is under platform/ext/accelerator/cc312/cc312_provisioning.c, and the code for using keys from OTP is at platform/ext/target/musca_b1/sse_200/crypto_keys.c

Note that because the Musca S1 has volatile OTP memory (I think it's a design choice that makes sense on a dev-board) a workaround is used, for the reference implementation it's best to look at the Musca B1.

The basic flow is:
* Build TF-M with -DCRYPTO_HW_ACCELERATOR_OTP_STATE=PROVISIONING, which will cause an ifdef in the bootloader to jump to the provisioning code instead of TF-M
* TF-M then generates a 256 bit AES HUK and an ECC_P256 private key using the code in cc312_provisioning.c, and program these key into the OTP memory of the CC312
* Programming these keys into OTP causes the lifecycle state to be changed, so a reboot is needed between programming the keys to allow the new LCS to be used
* The board is now provisioned - building TF-M with
* -DCRYPTO_HW_ACCELERATOR_OTP_STATE=ON will cause TF-M to use the HUK, IAK and ROTPK from OTP, and will derive the SST/PS keys from the HUK on boot

The general idea is that a platform can provide it's own implementation of the platform/ext/common/template/crypto_keys.c file, which could then load the keys from wherever they've been provisioned. I'd say it's reasonable to program keys into any NV storage region that is internal to the chip itself (and hence safe from tampering), which would include embedded flash. You'd then just need to load that data from flash in crypto_keys.c. Note that it would also be necessary to prevent secure partitions / non-secure code from directly accessing the flash where the keys are stored, probably by using an mpc/ppc to make it only accessible by secure-privileged code.

Let me know if you have any other questions about the current CC312 provisioning flow.

Raef

________________________________________
From: TF-M <tf-m-bounces@lists.trustedfirmware.org> on behalf of Edward Yang via TF-M <tf-m@lists.trustedfirmware.org>
Sent: 14 May 2021 10:04
To: tf-m@lists.trustedfirmware.org
Subject: [TF-M] Factory provisioning with tf-m


Hi,

I would like to know about how to use tf-m with factory provisioning,for example,HUK,IAK,ROTPK these ROT assets should be injected to device via factory provisioning,besides,application specific certificates and private keys can also be injected during this procedure,in tf-m roadmap,there is a provision service,what's the functionality of this service?


In initial attestation service, attest_register_initial_attestation_key() calls tfm_plat_get_symmetric_iak() to get IAK, I found implementation template of tfm_plat_get_symmetric_iak() just copys hardcoded key value(for developer mode).
I think this is not allowed in production mode.Is there any recommended implementation of this API?

What if I stored IAK in a specific address of MCU embedded flash during factory provisioning, such as #define IAK_REGION  IAK_base_addr  in flash_layout.h,and then use flash.read get the key value？

tfm_plat_get_symmetric_iak( )
{

  TFM_HAL_ITS_FLASH_DRIVER.ReadData(IAK_REGION, buff, size);
}



Best Regards,
Poppy Wu

Macronix Microelectronics (Suzhou) Co.,Ltd
Http: //www.mxic.com.cn

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