Mbedtls - Using keys handled by PSA Crypto

Question

Hello, 
 
 I need to implement the code that calculates the ECDSA signature, using the mbedtls library. Development environment: 
 
 nRF53DK 
 nRF Connect SDK 2.3.0 
 
 Currently, the process is implemented as follows: 
 
 The private key is generated using openssl, and stored to flash memory in DER format. 
 The signature is calculated by loading the mentioned key in mbedtls_pk_context using mbedtls_pk_parse_key , and passing such context to the mbedtls_ecdsa_write_signature function. 
 
 As a next step, it is necessary to implement key generation within the firmware and use it for signature so that the code itself does not need access to the key content. 
 I was able to generate the key using psa_generate_key . The plan is to load the generated key to the corresponding mbedtls_pk_context using mbedtls_pk_setup_opaque as a next step. This should be the only change from the current state (after configuring the mbedtls_pk_context , mbedtls_ecdsa_write_signature is called as before). 
 However, the problem is that, despite researching similar forum posts, and corresponding mbedtls configuration requests, I am still not able to use the mbedtls_pk_setup_opaque function because it looks like MBEDTLS_USE_PSA_CRYPTO is not defined. 
 The following security modules are included in the prj file (here there are certainly those that are not needed for the mentioned ECDSA signing, but remained in the project for the needs of tests of various mbedtls functionalities): 
 
CONFIG_NRF_SECURITY=y

CONFIG_MBEDTLS_LIBRARY_NRF_SECURITY=y
CONFIG_MBEDTLS_USE_PSA_CRYPTO=y
CONFIG_MBEDTLS_PSA_CRYPTO_C=y
CONFIG_MBEDTLS_PSA_CRYPTO_STORAGE_C=y

CONFIG_MBEDTLS_PK_C=y
CONFIG_MBEDTLS_RSA_C=y
CONFIG_MBEDTLS_PKCS1_V15=y
CONFIG_MBEDTLS_SHA256_C=y
CONFIG_MBEDTLS_ECP_C=y
CONFIG_MBEDTLS_ECDSA_C=y
CONFIG_MBEDTLS_ECDH_C=y
CONFIG_MBEDTLS_CTR_DRBG_C=y
CONFIG_MBEDTLS_ENTROPY_C=y
CONFIG_MBEDTLS_PK_PARSE_C=y

CONFIG_MBEDTLS_TLS_LIBRARY=y
CONFIG_MBEDTLS_X509_LIBRARY=y
CONFIG_MBEDTLS_PK_WRITE_C=y
CONFIG_MBEDTLS_X509_CREATE_C=y
CONFIG_MBEDTLS_X509_CSR_WRITE_C=y
CONFIG_MBEDTLS_LEGACY_CRYPTO_C=y
CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=n 
 
 Do you have any idea what could be the cause of this behavior? I s this a regular approach for implementing not exposing secret key material to the users of mbedtls API? 
 Thanks.

Sigurd Hellesvik · Accepted Answer

Here are what I got: 
 " 
 There are multiple issues here, one of them being the lack of ability to convert CSR related configs in 2.3 Ask them to remove MBEDTLS_LEGACY_CRYPTO_C in their prj.conf and add MBEDTLS_X509_CREATE_C and MBEDTS_X509_CSR_WRITE_C here: https://github.com/nrfconnect/sdk-nrfxlib/blob/6d0f58448fae164cfa4d28c494d6bddf5d0d0224/nrf_security/configs/nrf-config.h#L396 
 
 This file gets copied during build when the system is using PSA crypto APIs 
 Additionally they are using legacy crypto APIs for RNG for the CSR. I've removed quite a bit (entropy and ctr_drbg). 
 " 
 And here is an example for some pointers: 
 
 I hope this can be useful for you to dig into this issue.

Sigurd Hellesvik · Answer

They say: 
 " 
 Hey. It was commented out as it is not needed in TF-M build. Previously there was a false dependency on TLS/DTLS and X.509 libraries inside TF-M. But when we started optimizing we finally removed that dependency (TF-M just tried to consume Mbed TLS as a whole thing, while they really only needed the crypto toolbox functionality) 
 
 The easiest thing to do is to just add it into the template files, but I would guard it with #if !defined(MBEDTLS_PSA_CRYPTO_SPM) to ensure that it isn't enabled inside the TF-M image. 
 Likely the user-configurable header file was easier to get to work when we were using the builtin Mbed TLS. We have gravitated away from that with PSA core from Oberon. We have no way of converting header file into CMake, so we won't pull in the right files in the compilation... (or set up the right linking rules) We need to remove this chapter from the documentation (of which we appologize). 
 "

Mbedtls - Using keys handled by PSA Crypto

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