This document covers the procedure for configurating a YubiKey as a GPG smartcard for SSH authentication. The benefit is a good model for two-factor authentication, something you have and something you know. In this example, there is a token and a passphrase.
The YubiKey 5 Series is used here. The YubiKey 4 or YubiKey NEO will also work, although the YubiKey NEO is limited to 2048-bit RSA keys, and the YubiKey 4 does not support ECC keys.
The same instructions should work on other GPG smart card implementations, although they were developed using the Yubikey implementation
Examples below are using a Fedora 33 x86_64 and Ubuntu 15.04 x86_64 fresh install. There are other tutorials for other operating systems and keys available online. See the CREDITS section below for alternate tutorials, examples, etc.
To configure authentication using the previously generated GnuPG key,
the GNOME-Shell needs some adjustments. With help from several
resources, configure the system to allow gpg-agent to take over SSH
authentication.
Certain software must be installed, including utilities for the YubiKey
libyubikey (libyubikey-dev on Ubuntu), gnupg2 (which is probably
already installed), and gnupg2-smime (gpgsm on Ubuntu).
Fedora:
sudo dnf install ykpers libyubikey gnupg gnupg2-smime
Ubuntu:
sudo apt-get install gnupg-agent gnupg2 pinentry-gtk2 scdaemon \
libccid pcscd libpcsclite1 gpgsm yubikey-personalization \
libyubikey-dev libykpers-1-dev
Optional: Install the YubiKey NEO Manager GUI. If running Ubuntu, you can install the YubiKey NEO manager and other YubiKey software from the Yubico PPA.
This will enable the smartcard portion of your YubiKey:: This is only required for the YubiKey NEO.
ykpersonalize -m82
If you have a dev key, Reboot your YubiKey (remove and reinsert) so that ykneomgr works.
Turn off ssh-agent inside gnome-keyring-daemon.
For Fedora this can be achieved by creating a new file /etc/X11/xinit/Xclients.d/Xclients.gnome-session.sh or appending to the existing one. Add the following code portion:
if [[ $(gconftool-2 --get /apps/gnome-keyring/daemon-components/ssh) != "false" ]]; then
gconftool-2 --type bool --set /apps/gnome-keyring/daemon-components/ssh false
fi
Configure GPG to use its agent (only for smartcard):
echo "use-agent" >> ~/.gnupg/gpg.conf
Enable ssh-agent drop in replacement support for gpg-agent:
echo "enable-ssh-support" >> ~/.gnupg/gpg-agent.conf
Disable pcscd to avoid conflicting with GPG/scdaemon's built in CCID:
systemctl disable pcscd.socket --now
systemctl disable pcscd.service --now
systemctl mask pcscd.socket
systemctl mask pcscd.service
Allow admin actions on your YubiKey (if your gnupg version is < 2.0.11):
echo "allow-admin" >> ~/.gnupg/scdaemon.conf
Open Startup Applications
Uncheck "GPG Password Agent" and "SSH Key Agent"
Edit /usr/share/upstart/sessions/gpg-agent.conf so that the pre-start
script contains the following:
eval "$(gpg-agent --daemon --enable-ssh-support --sh)" >/dev/null
initctl set-env --global GPG_AGENT_INFO=$GPG_AGENT_INFO
initctl set-env --global SSH_AUTH_SOCK=$SSH_AUTH_SOCK
initctl set-env --global SSH_AGENT_PID=$SSH_AGENT_PID
Add the following lines to the post-stop script section:
initctl unset-env --global SSH_AUTH_SOCK
initctl unset-env --global SSH_AGENT_PID
Disable the other system gpg-agent:
mv /etc/X11/Xsession.d/90gpg-agent ~/bak/90gpg-agent
Note: We could have used the Xsession gpg-agent and trashed the upstart one, but there is an open bug report for 90gpg-agent. Also, the upstart script has the capability of exporting the environment variables globally with initctl set-env --global.
If running gnome, this problem may be solved by running the following to disable gnome-keyring from autostarting its broken gpg-agent and ssh-agent implementation:
mv /etc/xdg/autostart/gnome-keyring-gpg.desktop /etc/xdg/autostart/gnome-keyring-gpg.desktop.inactive
mv /etc/xdg/autostart/gnome-keyring-ssh.desktop /etc/xdg/autostart/gnome-keyring-ssh.desktop.inactive
Next, place the following in ~/.bashrc to ensure gpg-agent starts with
--enable-ssh-support :
# Start gpg-agent if it's not running
if ! pidof gpg-agent > /dev/null; then
gpg-agent --homedir $HOME/.gnupg --daemon --sh --enable-ssh-support > $HOME/.gnupg/env
fi
if [ -f "$HOME/.gnupg/env" ]; then
source $HOME/.gnupg/env
fi
gpg-connect-agent updatestartuptty /bye > /dev/null 2>&1
Now go to next step (Reload GNOME-Shell) :)
Rebooting the machine works the best. After reboot, make sure that the output of the following command is false:
gconftool-2 --get /apps/gnome-keyring/daemon-components/ssh
There is a regular PIN, which is used to unlock the token for Signing, Encryption or Authentication. Additionally, there is an admin PIN, which is used to reset the PIN and/or the Reset Code for the key itself.
default pins are 123456 and 12345678 respectivly
gpg2 --card-edit
..snip..
gpg/card> admin
Admin commands are allowed
gpg/card> passwd
gpg: OpenPGP card no. D27600012401020000050000158A0000 detected
1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit
Your selection? 3
Enter the Current Admin PIN
Then enter the New Admin PIN twice
1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit
Your selection? 1
Enter the Current PIN
Then enter the New PIN twice
NOTE: If the Admin PIN has not been entered, it may be required before changes are applied.
There are several ways to generate an SSH Key using GnuPG. A common way is to link the new authentication key to an already existing key:
gpg2 --edit-key 8A8F1D53
gpg (GnuPG) 1.4.12; Copyright (C) 2012 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Secret key is available.
pub 3072R/8A8F1D53 created: 2012-10-06 expires: never usage: SC
trust: ultimate validity: ultimate
sub 3072R/2F15E06B created: 2012-11-23 expires: 2022-11-21 usage: S
sub 3072R/EB8B4EBD created: 2012-11-24 expires: 2022-11-22 usage: E
sub 3072R/6BB325E9 created: 2012-11-24 expires: 2022-11-22 usage: A
[ultimate] (1). Clint Savage <herlo1@gmail.com>
[ultimate] (2) Clint Savage <herlo@fedoraproject.org>
[ultimate] (3) Clint Savage <csavage@linuxfoundation.org>
gpg>
Once in the edit-key dialog, create a key on the card:
gpg> addcardkey
Signature key ....: 91BC 60CC B9EC 8E73 923A FC6D 58CD 88A6 2F15 E06B
Encryption key....: 0CC3 DC3E 0D17 6111 A62B F656 63C6 4DA9 EB8B 4EBD
Authentication key: 9EBF A9FE 8AE1 0FEB 1699 CE9A 779F 43D5 EC6F CC13
Please select the type of key to generate:
(1) Signature key
(2) Encryption key
(3) Authentication key
Your selection? 3
IT WILL PROMPT YOU TO ENTER THE ADMIN PIN, AND THEN THE REGULAR PIN. Don't fat finger this part!
gpg: WARNING: such a key has already been stored on the card!
Replace existing key? (y/N) y
What keysize do you want for the Authentication key? (3072)
Key is protected.
You need a passphrase to unlock the secret key for
user: "Clint Savage <herlo1@gmail.com>"
3072-bit RSA key, ID 8A8F1D53, created 2012-10-06
Please specify how long the key should be valid.
0 = key does not expire
<n> = key expires in n days
<n>w = key expires in n weeks
<n>m = key expires in n months
<n>y = key expires in n years
Key is valid for? (0) 10y
Key expires at Mon 21 Nov 2022 05:29:00 PM MST
Is this correct? (y/N) y
Really create? (y/N) y
gpg: Note that the key does not use the suggested creation date
pub 3072R/8A8F1D53 created: 2012-10-06 expires: never usage: SC
trust: ultimate validity: ultimate
sub 3072R/2F15E06B created: 2012-11-23 expires: 2022-11-21 usage: S
sub 3072R/EB8B4EBD created: 2012-11-24 expires: 2022-11-22 usage: E
sub 3072R/6BB325E9 created: 2012-11-24 expires: 2022-11-22 usage: A
[ultimate] (1). Clint Savage <herlo1@gmail.com>
[ultimate] (2) Clint Savage <herlo@fedoraproject.org>
[ultimate] (3) Clint Savage <csavage@linuxfoundation.org>
Upon completion of the key, be sure to save the record to the card and gpg key:
gpg> save
Assuming everything above is configured correctly, a simple test is performed with the SmartCard inserted:
ssh-add -L
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABgQDL/XmU......BL0luE= cardno:00050000158A
The github repository contains all the files to make the changes above. Please feel free to read through them.
A special thanks to the following people and/or links.