How to Implement Secure Boot in Your Firmware

Understand Secure Boot

• Secure Boot is a security standard aiming to ensure that a device boots using only software that is trusted by the Original Equipment Manufacturer (OEM).

 

• The firmware checks the software's digital signature against a pre-defined key. If the signature is trusted, the system boots and the firmware hands over control to the operating system.

 

• This technology is crucial for preventing unauthorized software or infections prior to booting an operating system.

Prepare Your Development Environment

• Install necessary tools for building firmware, such as a cross-compiler toolchain appropriate for your system’s architecture.

 

• Ensure you have access to the firmware source code and relevant documentation from the hardware manufacturer.

 

• Set up a version control system to manage your changes to the firmware source code.

Enable Secure Boot in Firmware Configuration

• Access the firmware source code and locate the configuration files responsible for boot verification settings.

 

• Enable Secure Boot options within these files. This usually involves setting predefined macros or flags that instruct the firmware to perform signature verification during boot.

 

• Common configuration files could be named something similar to `config.h` or `secure_boot_config.h`.

// Example configuration
#define SECURE_BOOT_ENABLED 1
#define SECURE_BOOT_KEY "your-public-key"

Generate Keys for Secure Boot

• Create a pair of cryptographic keys; a private key for signing the firmware and a public key for verification.

 

• Use tools like OpenSSL to generate these keys. Ensure the private key is stored securely.

openssl genrsa -out private.pem 2048
openssl rsa -in private.pem -outform PEM -pubout -out public.pem

Sign Your Firmware Image

• Once the firmware is compiled, you need to sign the firmware image with the private key. This can be done using software that handles digital signatures.

 

• Ensure to replace paths and filenames with your actual firmware binary names.

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

Embed the Public Key into Firmware

• Modify the firmware’s source code to include the public key used for verification. Typically, this would be done in a header file.

 

• Ensure to format the public key correctly to be included as a string within the source code.

// Example public key inclusion
const char* secure_boot_public_key = 
    "-----BEGIN PUBLIC KEY-----\n"
    "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQ...\n"
    "-----END PUBLIC KEY-----\n";

Implement Verification Code

• Embed a verification function in the firmware source. This function will check the signature of the firmware image against the embedded public key.

 

• If the signature verification fails, configure the firmware to abort the boot process.

// Simplified verification function pseudocode
int verify_firmware_signature(const unsigned char* signature, const unsigned char* data) {
    // Implement verification logic using the public key
    // Return 0 on success, non-zero on failure
    return 0;
}

Test and Validate Your Implementation

• Flash the signed firmware image onto the device and observe the boot process.

 

• Ensure that the system only boots the images that are signed with the known private key.

 

• Test the failure scenario by attempting to boot unsigned or tampered images. The firmware should prevent the boot process if verification fails.

Maintain and Update Secure Boot Keys

• Plan a secure method for updating the keys and re-signing firmware images, as cryptographic keys can be compromised over time.

 

• Regularly audit your Secure Boot implementation to ensure compliance with the latest security standards and practices.