Re: [T2TRG] T2TRG interim meeting follow-ups - draft-irtf-t2trg-taxonomy-manufacturer-anchors-00

[Hannes Tschofenig <[e-mail]>]

Hi Mohit,


Am 15.06.2023 um 11:34 schrieb Mohit Sethi:
>
> * Generally, reading and writing to TPM requires some minimal OS? How
> is that signed/verified/booted?
>

A typical implementation of a TPM chip is some form of Cortex M
processor (with crypto hardware) running a firmware to support the TPM
specification functionality.


The code calling the TPM features is run on a separate processor. You
could call it the "application processor" since it will run the actual
application code. Typically, TPM chips are used with higher-end
processors (like Cortex A-class processors or similar) and they often
run embedded Linux or similar.

I have not seen TPM chips being used alongside low-end microcontrollers.

Secure boot is also a simple concept: At every layer of the boot chain
you verify the next layer (before you hand over control to that next
layer). Since there are multiple layers in the boot process, the
hand-over happens several times.

With secure boot the idea is that you stop the boot process when you
fail to successfully verify software/firmware. With measured boot, you
continue but report what you "measured". Measured typically means
computing a hash over the software/firmware plus some meta data.

See also
https://media.fidoalliance.org/wp-content/uploads/2022/12/FIDO-Device-Onboard-The-Device-Key-White-Paper.pdf
for more discussion about measured and secure boot.


> * When manufacturing my IoT device, how is the primary stage
> bootloader signed and verified?


The concept is simple: there is a root of trust, which is used to get
started. The root of trust includes a "minimum" amount of software,
hardware, keys and configuration which everything else relies on. For
the first stage bootloader you have to assume an immutable bootloader
code (typically stored in ROM) and immutable public keys (serving as
trust anchors). From a terminology point of view you have to decide
whether you call the first stage the immutable part  or the bootloader
that is started by the immutable part. When you read the data sheets of
manufacturers then you will realize that they mix the terminology  (and
even invent their own terms). Hence, you have to read carefully.


Ciao
Hannes