Repeated prompts for SSH key passphrase after upgrading to Ubuntu 18.04 LTS?

This was a tricky one (for me, anyway) so posting a note to help others.

The problem was that after upgrading to Ubuntu 18.04 LTS from 16.04 LTS, I had trouble with my SSH agent. I was always being asked for the passphrase again and again, even if I had just used the key. This wouldn't have been a showstopper otherwise, but it made using virt-manager over SSH impossible because it was asking for the passphrase tens of times.

I didn't find anything on the web, and I didn't find any legacy software or obsolete configs to remove to fix the problem. I only got a hint when I tried ssh-add -l, with which I got the error message ”error fetching identities: Invalid key length”. This lead me on the right track, since after a while I started suspecting my old keys in .ssh that I hadn't used for years. And right on: after I removed one id_dsa (!) key and one old RSA key from .ssh directory (with GNOME's Keyring app to be exact), ssh-add -l started working and at the same time the familiar SSH agent behavior resumed and I was able to use my remote VMs fine too!

Hope this helps.

ps. While at the topic, remember to upgrade your private keys' internal format to the new OpenSSH format from the ”worse than plaintext” format with the -o option: blog post – tl; dr; ssh-keygen -p -o -f id_rsa and retype your passphrase.

Converting an existing installation to LUKS using luksipc - 2018 notes

Time for a laptop upgrade. Encryption was still not the default for the new Dell XPS 13 Developer Edition (9370) that shipped with Ubuntu 16.04 LTS, so I followed my own notes from 3 years ago together with the official documentation to convert the unencrypted OEM Ubuntu installation to LUKS during the weekend. This only took under 1h altogether.

On this new laptop model, EFI boot was already in use, Secure Boot was enabled and the SSD had GPT from the beginning. The only thing I wanted to change thus was the / to be encrypted.

Some notes for 2018 to clarify what is needed and what is not needed:

• Before luksipc, remember to resize existing partitions to have 10 MB of free space at the end of the / partition, and also create a new partition of eg 1 GB size partition for /boot.
• To get the code and compile luksipc on Ubuntu 16.04.4 LTS live USB, just apt install git build-essential is needed. cryptsetup package is already installed.
• After luksipc finishes and you've added your own passphrase and removed the initial key (slot 0), it's useful to cryptsetup luksOpen it and mount it still under the live session - however, when using ext4, the mounting fails due to a size mismatch in ext4 metadata! This is simple to correct: sudo resize2fs /dev/mapper/root. Nothing else is needed.
• I mounted both the newly encrypted volume (to /mnt) and the new /boot volume (to /mnt2 which I created), and moved /boot/* from the former to latter.
• I edited /etc/fstab of the encrypted volume to add the /boot partition
• Mounted as following in /mnt:
• mount -o bind /dev dev
• mount -o bind /sys sys
• mount -t proc proc proc
• Then:
• chroot /mnt
• mount -a # (to mount /boot and /boot/efi)
• Edited files /etc/crypttab (added one line: root UUID none luks) and /etc/grub/default (I copied over my overkill configuration that specifies all of cryptopts and cryptdevice some of which may be obsolete, but at least one of them and root=/dev/mapper/root is probably needed).
• Ran grub-install ; update-grub ; mkinitramfs -k all -c (notably no other parameters were needed)
  
• Rebooted.
• What I did not need to do:
• Modify anything in /etc/initramfs-tools.

If the passphrase input shows on your next boot, but your correct passphrase isn't accepted, it's likely that the initramfs wasn't properly updated yet. I first forgot to run the mkinitramfs command and faced this.