Using Yubikey

We will be using a GPG key as a SSH one with the Yubikey. I guess that the Yubikey already contains a sign, an encrypt and an authorize subkeys. See the Yubikey official site for further details.

Install some packages:

sudo apt install gnupg2 pcscd scdaemon

Edit ~/.gnupg/gpg-agent.conf:

enable-ssh-support
default-cache-ttl-ssh 10800
max-cache-ttl-ssh 10800

Edit ~/.bashrc:

# Set SSH to use gpg-agent
unset SSH_AGENT_PID
if [ "${gnupg_SSH_AUTH_SOCK_by:-0}" -ne $$ ]; then
  export SSH_AUTH_SOCK="$(gpgconf --list-dirs agent-ssh-socket)"
fi

# Set GPG TTY
export GPG_TTY=$(tty)

# Refresh gpg-agent tty in case user switches into an X session
gpg-connect-agent updatestartuptty /bye >/dev/null

To check if it works:

ssh-add -L
ssh-rsa AAAAB3NzaC ... pdqtlwX6m1 cardno:000123457915 # Output