I have seen many papers on "encrypted firmware" (for example, firmware on Apple Watch like devices) .

Can you tell me how it works? Is there a stub in the firmware which is not encrypted and can decrypt the encrypted firmware on runtime?

Where is the encryption key stored?

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    $\begingroup$ This usually requires a) a trusted processor and b) trusted, tamper-resistant data storage. Given these two parts, you can store a signed image of the bootloader which will decrypt the remaining firmware. $\endgroup$ – SEJPM Jun 6 '16 at 20:28
  • $\begingroup$ If I understand right, a bootloader is stored on the data storage (so the key too?) and can decrypt the firmware ? It will be so easy for a malicious person to steal the key if it so... $\endgroup$ – Duke Nukem Jun 7 '16 at 5:55

Loading and using encrypted firmware usually requires one of the following conditions to be met:

  1. You have a security (co-)processor and the processor is trusted to follow the instructions given by that (co-)processor

  2. You have a trusted processor and some tamper-resistant storage

Scenario 1

You have a security (co-)processor and the processor is trusted to listen to this coprocessor.
You now also have a firmware image, signed by the manufacturer and symmetrically encrypted using a key hard-coded in the coprocessor. The co-processor will now load, decrypt and verify the image by itself and then pass it on to the processor.
Note that the coprocessor is designed to be tamper-resistant and thus extracting the secret symmetric key or changing the public manufacturer key is impossible.

Scenario 2

In this case you trust your processor to do what it is supposed to do and also have some tamper-resistant storage.
This time, you store your bootloader unencrypted in the tamper-resistant storage which will prevent its exfiltration or manipulation and thus you can just hard-code the secret key needed to load and verify the firmware into the bootloader.


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