Hi
Poppy,
Some feedback from my understanding on the first question.
In the version of PSA crypto spec in which psa_open_key is supported, the persistent key can be opened after the reset with the key_id which is specified in the key attributes in key creation. This is not very friendly. The reason
is that each time the user calls the psa_open_key, the crypto service loads the key material from slot to running area(ram or flash) thus a new associated resources is allocated. The application must eventually call psa_close_key to release the allocated associated
resources. It can happen that multiple applications call psa_open_key multile times as they may do not know whether the key is opened by other applications. So it can happen that multiple copies of associated resources are allocated for the same key. This
is not user friendly.
In the updated PSA crypto spec, the key material management strategy is changed. The user does not need to call the psa_open_key to load the key from slot to ram/flash. See the section of “9.4 Key identifies” of the spec:
```
Key identifiers are output from a successful call to one of the key creation functions.
For persistent keys,
this is the same identifier as the one specified in the key attributes used to create the key.
The key
indenter remains valid until it is invalidated by passing it to
psa_destroy_key(). A volatile key identifier
must not be used after it has been invalidated.
```
So I think the user can use the key_id directly to call crypto operations, such as
means encrypt, decrypt.
Regards,
Sherry Zhang
From: TF-M <tf-m-bounces@lists.trustedfirmware.org>
On Behalf Of Edward Yang via TF-M
Sent: Wednesday, June 2, 2021 1:30 PM
To: tf-m@lists.trustedfirmware.org
Subject: [TF-M] Questions about psa crypto persistent key
Hi Experts,
I have some questions about crypto persisten keys.
1. psa_open_key() is removed in psa crypto spec,so it is impossible to import a persistent key into key slot with key_id,which means encrypt/decrypt data directly with a persistent key is not allowed,these
persistent keys can only be
used to derive volatile keys which will be used for encryption/decryption, I am not sure if I understand correctly.
2. Besides,HUK can be used to derive the other crypto keys,such as ps crypto key.HUK may be stored in OTP area of MCU(without crypto element such as cc312),then what's intended flow to derive crypto
keys from HUK via calling PSA crypto service?There is no reference implementation in tf-m code.
tfm_plat_get_huk_derived_key(){
get HUK from OTP
||
||
\/
how to derive crypto key from HUK with calling crypto service?
}
3. BTW,HUK has a persistent key id
TFM_CRYPTO_KEY_ID_HUK defined in tfm_crypto_defs.h,but I haven't seen any reference to this macro. What's the intended
use of this key id?And what's the key owner of HUK?
Best Regards,
Poppy Wu
Macronix Microelectronics (Suzhou) Co.,Ltd
http://www.mxic.com.cn
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