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I've been asked to improve the security of my company's iot device, and encrypt a storage partition. It's a general purpose arm based device, and currently can boot from a variety of sources (USB, network, sd card, etc).

The company wants to add a TPM chip to perform the partition encryption.

Am I right in thinking that a system lacking secure boot, that can boot whatever OS the user wants via network boot for example, could never be secure? I imagine the network booted OS could ask the TPM chip to decrypt the encrypted partition, showing that there's little actual security on this system.

Finally, if there's a uboot variant that uses a TPM chip to implement secure boot such as sboot, is that actually secure? I can't help but think that a system that allows any bootloader to run, would allow any security mechanism to be reverse engineered given enough time.

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  • AFAIK secure boot is still secure as long as the first bootloader is secure - even if you can load alternative bootloaders, the TPM won't release its keys
    – user253751
    Apr 8 at 11:41
  • No the TPM 2.0 chip (using a 2.0 enabled driver) wouldn't release the keys, but I imagine I could still decrypt the encrypted partition? If the key is randomised on each device, the key isn't hugely important.
    – Alex
    Apr 8 at 12:39
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    How would you decrypt the partition without the encryption key???
    – user253751
    Apr 8 at 12:45
  • @Alex What you're saying doesn't make a whole lot of sense. You need the key in order to decrypt the data.
    – MechMK1
    Apr 8 at 15:55
  • The key, which is stored in the TPM and isn't supposedly retrievable, is used by the TPM to decrypt the partition. I assume that if there's no method of securing the system communicating with the TPM, then a TPM can be instructed to do whatever is asked of it, including decrypting a partition. Looking into this further, the TPM chip should be setup to measure the boot process, to verify the system that it's communicating with hasn't been modified.
    – Alex
    Apr 8 at 22:02

1 Answer 1

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Without authenticated boot, a TPM which blindly decrypts a partition can be commanded to do so and the partitions contents dumped by the custom bootloader to disk.

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