RSU: How to handle FACTORY_IMAGE signing and trust validation?

Hi,

I have a technical question based on a customer's experiments with Remote System Update (RSU).

For RSU, a SOF file is required for the FACTORY_IMAGE, as quartus_pfg rejects the RBF file.

However, quartus_sign does not support SOF files for signing.

Is it possible to use a signed image as the FACTORY_IMAGE?
If not, what is the recommended method to ensure the integrity and authenticity of the FACTORY_IMAGE?

How can the FACTORY_IMAGE be trusted in a secure RSU design?

Thank you.