As Matthew described in an earlier blog post:
I've taken Suse's code for key management and merged it into my own shim tree with a few changes. The significant difference is a second stage bootloader signed with an untrusted key will cause a UI to appear, rather than simply refusing to boot. This will permit the user to then navigate the available filesystems, choose a key and indicate that they want to enrol it. From then on, the bootloader will trust binaries signed with that key.Today -- on his final day officially working for Red Hat -- Matthew shares in a new blog post that the signed shim boot-loader has been published. It's available for download in this directory (along with the unsigned copy of the source-code) and is quite easy for distributors to utilize:
Distributions are then able to take an existing signed copy of shim and put it on their install media, along with a file containing their key. If a user attempts to boot then the boot will fail because the second stage bootloader isn't signed with a trusted key, but the user can then use the navigator and select the distribution's key file. After providing confirmation and rebooting, the second stage bootloader's signature will now be recognised and the installer will boot.
This has the advantage over the first two options that the UI is consistent, making it easier to document the install process. The primary disadvantage is that the distribution won't be able to rebuild shim and will have to ship a pre-compiled binary. This may well be unacceptable to distributions like Debian, but should still provide a viable approach for other distributions who are either unwilling or unable to deal with Microsoft themselves.
I'm pleased to say that a usable version of shim is now available for download. As I discussed here, this is intended for distributions that want to support secure boot but don't want to deal with Microsoft. To use it, rename shim.efi to bootx64.efi and put it in /EFI/BOOT on your UEFI install media. Drop MokManager.efi in there as well. Finally, make sure your bootloader binary is called grubx64.efi and put it in the same directory.
Now generate a certificate and put the public half as a binary DER file somewhere on your install media. On boot, the end-user will be prompted with a 10-second countdown and a menu. Choose "Enroll key from disk" and then browse the filesystem to select the key and follow the enrolment prompts. Any bootloader signed with that key will then be trusted by shim, so you probably want to make sure that your grubx64.efi image is signed with it.