The OnlyKey can be used with OpenSSH to provide multifactor authentication on SSH keys

OpenSSH v8.2

This document describes how to use the OnlyKey as a second factor authentication device with traditional SSH Keys.

The OnlyKey currently only supports ecdsa keys with OpenSSH.

Quickstart Guide

  1. You must have OpenSSH v8.2 or higher and the necessary prerequisites installed.

  2. You may now generate your SSH keys using ssh-keygen. Provide any desired optional arguments and you will be prompted to press your OnlyKey and provide an optional passphrase.

$ ssh-keygen -t ecdsa-sk
You may need to touch your authenticator to authorize key generation.
Enter file in which to save the key (/home/user/.ssh/id_ecdsa_sk):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/user/.ssh/id_ecdsa_sk
Your public key has been saved in /home/user/.ssh/id_ecdsa_sk.pub
The key fingerprint is:
SHA256:ECFmaoLZENpq0rLem8HC1F6vTwH1pjsnR6X8l/r54rQ [email protected]
The key's randomart image is:
+-[ECDSA-SK 256]--+
|o.  + oo         |
|o= + ....        |
|= =. ... o .     |
| =.   ..+ o      |
|+o.. . oS+       |
|=oo . . + .   .  |
|.o +   * o . +   |
|. o o o o = +.o  |
| . +....   .oEo. |
+----[SHA256]-----+
  1. Then copy the new public key to your remote hosts.
$ ssh-copy-id -i ~/.ssh/id_ecdsa_sk [email protected]
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "id_ecdsa_sk.pub"
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys

Number of key(s) added: 1

Now try logging into the machine, with:   "ssh '[email protected]'"
and check to make sure that only the key(s) you wanted were added.
  1. And then log in your remote host. You will be prompted to enter your passphrase (if entered during key generation) and asked to press your OnlyKey.
$ ssh -i ~/.ssh/id_ecdsa_sk [email protected]
Enter passphrase for key 'id_ecdsa_sk':
Confirm user presence for key ECDSA-SK SHA256:ECFmaoLZENpq0rLem8HC1F6vTwH1pjsnR6X8l/r54rQ
  1. Success!

Prerequisites

Void Linux

$ xbps-install -S openssh openssh-sk-helper

Arch Linux

$ pacman -S openssh libfido2

Ubuntu (20.10 Groovy Gorilla) & Debian (bullseye)

$ apt install openssh-client
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