Comment 3 for bug 1528345

This appears to have been caused by the publication of a grub2 security update on 12/15 without the corresponding grub2-signed package being published at the same time.

We have had similar issues in the past with publication of grub2 and grub2-signed to the -updates pockets; the proposed solution there won't help for the security pocket, however, since those publications don't go via -proposed.

Subscribing the security team for comment.

To fix your current problem, you should install the grub-efi-amd64-signed package.