Secure Boot is a Unified Extensible Firmware Interface (UEFI) specification that prevents the loading of drivers or OS loaders that are not signed with an authorised signature, and whose public key is stored in the device’s firmware.
Garrett previously wrote that three options exist to allow non-Microsoft operating systems to boot. The first option is to disable Secure Boot entirely, which negates the benefits of Secure Boot’s trusted boot process. The second option is to remove an existing Platform Key, presumably Microsoft’s key, to allow the user the ability to add a new key. The last option that Garrett highlighted would be to ship a bootloader signed with Microsoft’s public key, which contains its own key store, MokManager, which allows a signed second-stage bootloader to boot.
Garrett’s new shim takes advantage of the third option, all the gory details of which are detailed here.
From a user’s perspective, they would see a 10-second countdown with a menu that contains an option to enroll a key from disk. The user could then browse the filesystem to select the appropriate key and add it into the device’s firmware.
Any operating system that is able to be booted from a GRUB environment can take advantage of this shim.
The one caveat with this system, though, is that MokManager is signed with a key that users and distributions will have to trust Garrett deleted immediately after the build was completed.
“You’ll need to accept my assurance that the private key was deleted immediately after the build was completed. Other than that, it will only trust any keys that are either present in the system db or installed by the end user,” wrote Garrett.
The shim can be downloaded from here