Hi Alex,
May check out this link https://www.intel.com/content/www/us/en/docs/programmable/683463/22-1/support-for-the-ieee-1735-encryption.html
Only file encryption with a third-party tool requires the public encryption key. File encryption with the Intel® Quartus® Prime Pro Edition software does not require the public encryption key.
Probably you have to get the public encryption key for encryption with third-party tool.
Best Regards,
Sheng
p/s: If any answer from community or Intel support are helpful, please feel free to mark as solution and give Kudos.