Encrypt message using RSA on ESP32 - rsa

Question

What I try to achieve here is to encrypt a message inside ESP32 app built using PlatformIO + Arduino framework.
After some searchings, I found this repo: https://github.com/espressif/arduino-esp32
There is a tool inside it seems able to help me achieve what I want https://github.com/espressif/arduino-esp32/blob/master/tools/sdk/include/mbedtls/mbedtls/rsa.h
I imported the library "mbedtls" at https://platformio.org/lib/show/10874/mbedtls to the PlatformIO project and start work from there.
Question: How to load private key file in the app and encrypt the message using the RSA tool?
What I have currently is:
int ret = 1;
char buf[1024];
mbedtls_pk_init(&pk);
memset(buf, 0, sizeof(buf));
mbedtls_mpi_init(&N);
mbedtls_mpi_init(&P);
mbedtls_mpi_init(&Q);
mbedtls_mpi_init(&D);
mbedtls_mpi_init(&E);
mbedtls_mpi_init(&DP);
mbedtls_mpi_init(&DQ);
mbedtls_mpi_init(&QP);
ret = mbedtls_pk_parse_key(&pk, vendorPrivateKey, sizeof(vendorPrivateKey), NULL, NULL);
if (ret != 0) {
Serial.print(" failed! mbedtls_pk_parse_key returned: ");
Serial.print(-ret);
Serial.println();
}
if (mbedtls_pk_get_type(&pk) == MBEDTLS_PK_RSA) {
mbedtls_rsa_context *rsa = mbedtls_pk_rsa(pk);
if ((ret = mbedtls_rsa_export(rsa, &N, &P, &Q, &D, &E)) != 0
|| (ret = mbedtls_rsa_export_crt(rsa, &DP, &DQ, &QP)) != 0) {
Serial.println(" failed! could not export RSA parameters.");
}
}
For now I import the private key content directly in char* form (I'm not sure how to import a pem key file into app.) through the header file:
const unsigned char *vendorPrivateKey = reinterpret_cast<const unsigned char *>(VENDOR_PRIVATE_KEY);
where the value is stored inside secrets.h
Then when I ran the program, it yields the following error message for me:
failed! mbedtls_pk_parse_key returned: 15616
According to the pk.h file description, this error code 15616 in hexa is 3D00 which indicates /**< Invalid key tag or value. */
Is there any website that provides format checking and see if my private key file fits the requirements of the mbedtls?