Debian 9.0 Stretch.

[dkiestra]

Got it working for Debian 9 Stretch. However, had to make the following changes.
Needed some extra delay for slow computers to find the USB key file.
Needed nls_ascii module for newer kernels in order to read the USB key file.
No need to install dropbear package but only dropbear-initramfs package.
Fixed the path of the id_rsa.pub file.
Unlock: ssh -i id_rsa -o HostKeyAlgorithms=ssh-rsa root@192.168.1.13
After typing the passphrase the computer unlocks but the remote computer does no longer respond.

• Change the "MAX_SECONDS=2" line into "MAX_SECONDS=5" in the crypto-usb-key.sh file.

• Add the following line to the install.sh file.
  grep -q '^nls_ascii$' /etc/initramfs-tools/modules || echo 'nls_ascii' >> /etc/initramfs-tools/modules

• Change the following lines in the install.sh file.
  apt-get install -y dropbear initramfs-tools busybox
  apt-get install -y dropbear-initramfs initramfs-tools busybox

  cat /etc/dropbear-initramfs/id_rsa/id_rsa.pub >>/etc/dropbear-initramfs/authorized_keys
  cat /etc/dropbear-initramfs/id_rsa.pub >>/etc/dropbear-initramfs/authorized_keys