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.