Implement Secure Boot on STM32F4 with RSA & ECC Using OpenSSL and CubeMX

Implementing Secure Boot on STM32F4 with RSA and ECC for Firmware Authentication Using OpenSSL and CubeMX

Secure Boot is a critical security feature that ensures that only authenticated firmware runs on a microcontroller. This tutorial will guide you through the steps to implement Secure Boot on the STM32F4 microcontroller using RSA and ECC for firmware authentication. We will utilize OpenSSL for cryptographic operations and STM32CubeMX for project setup.

Prerequisites

• Basic understanding of embedded systems and STM32 architecture.
• STM32F4 development board.
• STM32CubeMX installed on your computer.
• OpenSSL installed on your computer.
• IDE for STM32 development (e.g., STM32CubeIDE).

Parts/Tools

• STM32F4 Development Board
• USB Cable
• Computer with STM32CubeMX and OpenSSL
• STM32CubeIDE or any compatible IDE

Steps

1. Setting Up the Project
   1. Open STM32CubeMX and create a new project.
   2. Select the STM32F4 microcontroller you are using.
   3. Configure the peripherals as needed (e.g., GPIO, UART).
   4. Enable the FreeRTOS middleware if required for your application.
   5. Generate the project files for your IDE.
2. Generate Keys for RSA and ECC
   1. Open a terminal and navigate to the directory where you want to store your keys.
   2. Generate RSA private and public keys using OpenSSL:

      openssl genrsa -out private_key.pem 2048

   3. Extract the public key:

      openssl rsa -in private_key.pem -outform PEM -pubout -out public_key.pem

   4. Generate ECC keys:

      openssl ecparam -name prime256v1 -genkey -noout -out ecc_private_key.pem

   5. Extract ECC public key:

      openssl ec -in ecc_private_key.pem -pubout -out ecc_public_key.pem

3. Integrating RSA and ECC in Your STM32 Application
   1. Include the OpenSSL library in your STM32 project.
   2. Write code to initialize the cryptographic functions. Here’s a basic example:

      #include 
      #include 
      #include 
      
      void init_openssl() {
          ERR_load_crypto_strings();
          OpenSSL_add_all_algorithms();
          // Additional initialization if required
      }

   3. Load the RSA public key for firmware verification:

      RSA *rsa_pubkey = NULL;
      FILE *pubkey_file = fopen("public_key.pem", "r");
      rsa_pubkey = PEM_read_RSA_PUBKEY(pubkey_file, NULL, NULL, NULL);
      fclose(pubkey_file);

4. Implementing Firmware Authentication
   1. Sign the firmware binary with the private RSA key:

      openssl dgst -sha256 -sign private_key.pem -out firmware.sig firmware.bin

   2. Load the signed firmware and signature into your STM32 application.
   3. Verify the signature during boot:

      int verify_firmware(const unsigned char *firmware, size_t firmware_len, const unsigned char *sig, size_t sig_len) {
          return RSA_verify(NID_sha256, firmware, firmware_len, sig, sig_len, rsa_pubkey);
      }

5. Testing Secure Boot
   1. Upload the firmware to your STM32F4 board.
   2. Reset the board and observe the boot process.
   3. Check if the firmware verifies successfully before execution.

Troubleshooting

• Signature Verification Fails: Ensure that the firmware binary and the signature match. Use the correct public key for verification.
• Key Format Issues: Ensure that the keys are in the correct PEM format. Use OpenSSL commands to convert if necessary.
• Memory Issues: Check that your STM32 has enough memory to handle the RSA or ECC operations. Consider optimizing memory usage.

Conclusion

Implementing Secure Boot on the STM32F4 using RSA and ECC provides a robust method for ensuring firmware authenticity. By following this tutorial, you should be able to integrate secure boot functionality into your projects effectively. Remember to test thoroughly and adapt the code to fit your specific application needs.