Comment 6 for bug 1619921

Conclusion thus far: What I am seeing might be a new breakage of yubikey-luks in U18.10/Cosmic.

I dug up an older Ubuntu system (U16.04.3 LTS) that is known-working with Yubikey LUKS + Plymouth, and `/conf/conf.d/cryptroot` is present in initramfs. Contents is one line, a target specification for the root FS. The line referencing it in `/scripts/local-top/yubikey-luks` looks substantially similar to the one in U18.10 / Cosmic.

Digging through various versions of cryptsetup imported from upstream (Debian), I see that between ubuntu/bionic and ubuntu/cosmic, generation of `/conf/conf.d/cryptroot` seems to have been retired. Specifically, as part of refactoring at:

debian/changelog
   cryptsetup (2:2.0.3-2) unstable; urgency=medium

     [ Guilhem Moulin ]
     * debian/initramfs/*-hook: complete refactoring. Common functions are now in
       /lib/cryptsetup/functions (source-able from shell scripts).
       (Closes: #784881.)
       https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=784881

… subsequent to which, debian/initramfs/cryptroot-hook add_device() (the function which was responsible for constructing `/conf/conf.d/cryptroot`) disappears.

Also of note: I fully removed yubikey-luks (`apt-get --purge remove yubikey-luks`), and only the first of the two cryptroot errors I mentioned above has disappeared. -- Looks like the "line 218" one is unrelated to yubikey-luks.