I installed Lubuntu 17.10 to a new laptop, let's say that went smoothly (there were some hoops, but in the end it was booting and working fine). The UEFI BIOS is in secure boot mode. I have a 200MB EFI partition followed by the big root partition. No swap partition because of SSD best practices (the /tmp will use tmpfs as well once I get this to work).
At the last minute and changed my mind and want to go with Devuan: something which is not systemd based. Yet again I faced with having to shovel firmware files for the wifi to work (this haunts me all the time https://askubuntu.com/questions/794218/getting-killer-wireless-ac-1535-working-for-installing-ubuntu-16-04). During the first install attempt I just thought I gonna continue with off-line install and bring up the wifi and add the network mirrors a-posteriori. Unfortunately the Devuan installer didn't even install GRUB in the end.
Second install attempt I found the right versions of the wifi firmware (4th time the charm with various versions!), the installer was in a better mood because of that, and it did install the GRUB at the end. I have a feeling it might have not touched the EFI partition though.
What's happening now is that during boot the UEFI thinks to see two bootable assets. One is the old Ubuntu (lubuntu) and the other is the Devuan. The Devuan does not boot because the UEFI does not approve it's keys. The Ubuntu is approved, but it stops at the grub prompt, since everything else is gone now.
Should I try to install the EFI partition with some rescue mode? Will that repair the keys stored in the UEFI BIOS? How can I make the BIOS forget the old installation and also it's key?
I got to the point when th UEFI refuses to boot my pen drives as well for the same reasoning about the key. My plan is to obtain a USB DVD drive and boot a CD from there. Then wipe the SSD. If the UEFI does not forget the previous installations after that then this will be a rough battle.
I start to believe that this is Devuan specific. Got the USB DVD, and I can boot Lubuntu or other install or live DVDs, but the Devuan displays this message only. I think they f-ed up something with their signature.