Encrypt message using RSA on ESP32

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?

1) "I imported the library "mbedtls" at ...", that is for mbed OS (an RTOS), not for Arduino platform. 2) For Arduino, you should take a look at this [tutorial](https://techtutorialsx.com/2018/05/10/esp32-arduino-mbed-tls-using-the-sha-256-algorithm/) on how to use mbedtls in ESP32 Arduino Core environment. — hcheung, Sep 25 '20 at 13:40
basically you use the library by adding `#include "mbedtls/rsa.h"` to your ESP32 Arduino sketch, and the rest refer to the [documentation](https://tls.mbed.org/api/rsa_8h.html) for the APIs. — hcheung, Sep 25 '20 at 13:48
Hi hcheung, do you have any example codes as a reference for me? I undertand there is a documentation for this RSA stuff, but there is no clear instruction says what API should I use first, then what's next, where should I store the key file or store the key in string form etc. — Daniel Tang, Sep 28 '20 at 01:39
Regarding the SHA256, I had it working with the exactly same tutorial, I found it last week. Now the problem I encounter is implemenet RSA encryption. — Daniel Tang, Sep 28 '20 at 01:43

Encrypt message using RSA on ESP32

0 Answers0