Dear MbedTLS dev,

I am writing an opaque driver (equivalent of PSA_CRYPTO_TEST_DRIVER_LOCATION/PSA_KEY_PERSISTENCE_READ_ONLY in the test suite) with the PSA API compiled with MBEDTLS_PSA_CRYPTO_BUILTIN_KEYS.

But I am not sure if I have to use "psa_import_key" before "psa_driver_wrapper_sign_message" or if I have to "read/generate" the builtin key on the fly, at each psa_driver_wrapper_* operation ?

Would it be possible to add this example in the test suite ? Or explain how builtin key + opaque driver is supposed to be used with, for example, "psa_driver_wrapper_sign_message" ?

I put more details below.

Thank you in advance,

Best regards,

Rehan

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I would like to write an example doing :

I) psa_sign_message (ECDSA SECP256R1 SHA-256)

II) psa_export_public_key

III) psa_verify_message (with the public key from II)

I am following the examples provided by the test suite, especially :

1) sign_message transparent driver: calculate in driver ECDSA SECP256R1 SHA-256

using the API :

- psa_import_key

- psa_sign_message

with the attributes :

- key_type = PSA_KEY_TYPE_ECC_KEY_PAIR(PSA_ECC_FAMILY_SECP_R1)

2) PSA opaque driver builtin pubkey export: secp256r1

using the API :

- psa_export_public_key

with the attributes :

- key_id = MBEDTLS_PSA_KEY_ID_BUILTIN_MIN + 1

- key_type = PSA_KEY_TYPE_ECC_KEY_PAIR(PSA_ECC_FAMILY_SECP_R1)

and the driver code :

- platform_builtin_keys.c : mbedtls_psa_platform_get_builtin_key() (PSA_KEY_PERSISTENCE_READ_ONLY, PSA_CRYPTO_TEST_DRIVER_LOCATION)

- test_driver_key_management.c : mbedtls_test_opaque_export_public_key()

3) verify_message transparent driver: calculate in driver ECDSA SECP256R1 SHA-256

using the API :

- psa_import_key

- psa_verify_message

with the attributes :

- key_type = PSA_KEY_TYPE_ECC_KEY_PAIR(PSA_ECC_FAMILY_SECP_R1)

It seems to me that II)=2) and 3) should be pretty similar to III) because I assume that neither transparent vs opaque, nor PSA_KEY_TYPE_ECC_KEY_PAIR vs PSA_KEY_TYPE_ECC_PUBLIC_KEY_BASE, is gonna change much here.

But the signature I) is less clear... Has the built-in key feature been thought such that the read-only key is read each time we call a different PSA API function ?

Long story short, an "opaque driver + builtin keys" equivalent of the "sign_message transparent driver: calculate in driver ECDSA SECP256R1 SHA-256" example in the test-suite would be really helpful :)