After successfully installing Linux Mint 19.1 Cinnamon for the first time, I went through the recommended steps to take after the installation. Here I also upgraded my system (after checking for and installing drivers). During this upgrade process however, following message popped up:
Your System has UEFI Secure boot enabled. UEFI secure boot is not compatible with the use of third party drivers. The System will assist you in disabling UEFI secure boot. To ensure that this change is being made by you as an authorized user, and not by an attacker, you must choose a password and then use the same password after reboot to confirm the change. If you choose to proceed but do not confirm the password upon reboot, Ubuntu will still be able to boot on your system, but these third party drivers will not be available for your hardware.
I then set a MOK PW and rebootedd the machine, signing the key. However, i don't really know what prompted this message, and what key I signed there. I am thinking it had to do with the third-party Nvidia-driver I had installed before, since I rolled back my system yesterday with timeshift to right after the driver installation (before the system update). Then I disabled the nvidia graphics card (for which the driver was for), and upon updating the system once again, no message which prompted me to sign a key popped up.
One of the currently signed keys, which i suspect it might be, has the following attributes, which sound bad:
X509v3 Basic Constraints critical CA: False
All in all my main questions are as follows: What does this all mean? What did I actually do with signing said key, and does this affect my system in any negative way? How can i find out what key i originally signed there and if said key is "safe"?