Hi Sherry,
Import persistent key with a specified key_id is indeed supported in the
updated crypto spec..
I mean,if a key created by derivation
functions,and this key is imported as a persistent key with s specific
key id such as KEY_ID_EXAMPLE,then this key is stored in internal Flash
with its service.
After the reset,this persistent key
still exists,but this persisten key cannot be used to do encryption directly,for
example,call psa_aead_encrypt(KEY_ID_EXAMPLE,alg,nonce,...),unless import
this key with psa_import() once again.
Call psa_aead_encrypt(SSKEY_ID,alg,nonce,...)
without calling psa_import_key() in advance is not allowed,because in tfm_crypto_check_handle_owner()(why
this API is needed?)it will check this key_id with handle_owner[] array.
So I am wondering what's the intended
use of persistent keys? After creation, this persistent keys can only be
used to derive other encryption keys,rather than used to encrypt data with
themselves?
Best Regards,
Poppy Wu
Macronix Microelectronics (Suzhou) Co.,Ltd
http://www.mxic.com.cn
Sherry Zhang via TF-M <tf-m@lists.trustedfirmware.org>
Sent by: "TF-M" <tf-m-bounces@lists.trustedfirmware.org>
2021/06/02 17:37
|
Please respond to
Sherry Zhang <Sherry.Zhang2@arm.com> |
|
|
To
| "tf-m@lists.trustedfirmware.org"
<tf-m@lists.trustedfirmware.org>
|
|
cc
| nd <nd@arm.com>
|
|
Subject
| Re: [TF-M] Questions about psa crypto
persistent key |
|
Hi Poppy,
Some feedback from my understanding on the
first question.
1. Import
persistent key with a key_id is still supported in the updated PSA crypto
spec.
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 ˇ°9.4 Key identifiesˇ± 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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