secure boot, mkimage with external signing server

[Rasmus Villemoes]

On Mon, Jan 20 2025, "Rosenschild, Klaus" <Klaus.Rosenschild at hilti.com> wrote:

> Hello,
> I have a question regarding the signing of a FIT image using mkimage. I already contacted DENX, they referred me to this mailing list.
>
> mkimage supports the creation of a signed FIT image. To do this, we need to have an appropriate .its file and pass the private key as a parameter to the mkimage command:
> mkimage -f fitImage-sign.its -k keys/  fitImage-signed
>
> However, this approach does not work in our setup, as we do not have access to the private key.
> The private key resides on an HSM (Hardware security module) that is not directly accessible for us. We can invoke signing related functions via an external signing server that takes a sha256 hash as input and returns the signed hash.
> Then we need to add the signed hash to the FIT image.
>

You may want to look into using an openssl pkcs11 module interfacing
with that HSM. Then use appropriate openssl configuration (set
OPENSSL_CONF env variable) and pass "-N pkcs11" and "-G <some pkcs11
URI>" to mkimage. This is something we've done in a number of cases with
a Yubi HSM.

Rasmus