Public Key Re-Use for Signing both Debug and Production Code

A common usage of public-key cryptography is to verify the integrity and authenticity of another entity (for example a firmware binary). If a company wants to ensure that its firmware runs only on its own hardware, before the firmware runs, an encrypted hash of the firmware image will be decrypted with the public key and then verified against the now-computed hash of the firmware image. This means that the public key forms the root of trust, which necessitates that the public key itself must be protected and used properly.

During the development phase, debug firmware enables many hardware debug hooks, debug modes, and debug messages for testing. Those debug facilities provide significant, additional views about the firmware's capability and, in some cases, additional capability into the chip or SoC. If compromised, these capabilities could be exploited by an attacker to take full control of the system.

Once the product exits the manufacturing stage and enters production, it is good practice to use a different public key. Debug firmware images are known to leak. With the debug key being reused as the production key, the debug image will also work on the production image. Thus, it will open all the internal, debug capabilities to the attacker.

If a different public key is used for the production image, even if the attacker gains access to the debug firmware image, they will not be able to run it on a production machine. Thus, damage will be limited to the intellectual property leakage resulting from the debug image.