So I'm trying to modify initrd.img which is used on Raspberry Pi 2(Ubuntu 18.04) but when I repack it again and start Raspberry Pi with new initrd.img I can't connect via SSH(dropbear is installed) to enter passphrase for unlocking LUKS encrypted drive: Permission denied (publickey).Same situation when I don't make any changes: just extracting initrd.img and repacking it again - can't connect via SSH. I'm getting almost same file size after repacking comparing with original file(difference is around ~1-2KB). So maybe my repacking method is wrong or maybe there is some protection for SSH keys inside initramfs?

Commands that I used for extraction:

gzip -dc < ~/initrd.img | cpio -i

Repacking:

find . | cpio -H newc -o | gzip -9 > ~/initrdNEW.img

EDIT

After extracting initrd.img there is root-sJD1OZ/.ssh/ directory with authorized_keys file inside it. When I update-initramfs on my Rasperry Pi and then extract contents again, that directory name changes(e.g. root-XXXXXX).

If you have any questions, please ask.