Dmitry Kasatkin is the developer principally responsible for the initramfs digital signature patches. The patches can be found on the mailing list and he's hoping for comments from other kernel developers.
The second patch explains how this digital signature process is handled for the Linux initramfs:
Often initramfs is (re)fabricated on the machine on which it runs. In such cases it is impossible to sign initramfs image, because private key is not supposed to be available.
This patch adds support for additional digitaly signed initramfs images. Digitaly signed initramfs image can be loaded from conventional initramfs image or from rootfs and '/pre-init' is executed prior 'init' from initramfs or root file system. If 'pre-init' fails, kernel panics. Signed initramfs image must be located in '/initramfs-sig.img'.
Digitally signed initramfs can be used to provide protected user-space environment for initialization purpose. For example, LSM, IMA/EVM can be securely initialized using such approach.
Signing is done using scripts/sign-file from kernel source code and uses module signature format and module verification API. Important to note again that signing of such images should be done on the build machine.
Bellow is an example, how to sign compressed initrd (cpio) image:
scripts/sign-file -v signing_key.priv signing_key.x509 initrd.gz initramfs-sig.img
When using initramfs-tools, initramfs-sig.img can be easily included into the conventional initramfs using initramfs-tools hooks, for example, by creating /etc/initramfs-tools/hooks/initramfs_sig.sh, and adding following lines there: