Code Signing and Intel SOF Binaries
===================================

Code signing is a mandatory feature on all Intel audio DSPs from Skylake
architecture onwards. Code that is not signed by Intel, OEM or the OTC public
key will not therefore be able to run (signing key used depends on hardware).

Most hardware vendors will use the default Intel signing key. This means only
Intel can sign firmware for these devices i.e. every SOF release and snapshot.

Some hardware vendors will install the public key so that developers can build
and sign their own firmware. Examples of such hardware are all recent
Chromebooks using Intel processors and development boards like the "UP^2" and
"UP Extreme". These devices are recommended for SOF development using Intel
hardware.

Older Intel audio DSPs like Merrifield, Baytrail, Braswell, Cherrytrail,
Haswell and Broadwell don't have the code signing feature. Code signing is also
not enabled on the Intel S1000 aka Sue Creek DSP.

Intel firmware directories are partitioned to reflect the signing authority
for each file i.e.

toplevel version --+--> intel-signed ----> All files under here signed by Intel
                   |
                   +--> public-signed ---> All files here signed using public
                   |                       development key.
                   |
                   +--> Unsigned binaries for platforms that dont have code
                        signing enabled.