Yubikey

This article describes how you can integrate Yubico's YubiKey with NixOS.

GPG and SSH

Based on a guide by @drduh:

services.udev.packages = [ pkgs.yubikey-personalization ];

# Depending on the details of your configuration, this section might not be necessary
# on NixOS 22.11 setting this is no longer required because it is already done by the gnupg module
environment.shellInit = ''
  export GPG_TTY="$(tty)"
  gpg-connect-agent /bye
  export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"
'';

programs.gnupg.agent = {
  enable = true;
  enableSSHSupport = true;
};

If you don't have a graphical user interface, you'll have to adjust the pinentry program (it's the program launched by operating system to ask for YubiKey's PIN):

programs.gnupg.agent.pinentryFlavor = "curses";

Logging-in

To use your yubikey as a user login or for sudo access you'll have to install a PAM (Pluggable Authentication Module) for your yubikey.

pam_u2f

The `pam_u2f` module implements the U2F (universal second factor) protocol. The protocol was initially developed by Yubico, Google and NXP and is nowadays hosted as an open-standard by the FIDO Alliance. All current and most legacy Yubikeys support the U2F protocol making this the preferred way to use Yubikeys for user login.

Use this page to check whether your Yubikey supports FIDO U2F before starting: https://www.yubico.com/products/identifying-your-yubikey/

1. Connect your Yubikey

2. Create an authorization mapping file for your user. The authorization mapping file is like `~/.ssh/known_hosts` but for Yubikeys.

1. nix-shell -p pam_u2f
2. mkdir -p ~/.config/Yubico
3. pamu2fcfg > ~/.config/Yubico/u2f_keys

3. Verify that `~/.config/Yubico/u2f_keys` contains one line in the following style:

<username>:<KeyHandle1>,<UserKey1>,<CoseType1>,<Options1>:<KeyHandle2>,<UserKey2>,<CoseType2>,<Options2>:...

4. Enable the u2f PAM module for login and sudo requests

security.pam.services = {
  login.u2fAuth = true;
  sudo.u2fAuth = true;
};

PAM U2F Docs: https://developers.yubico.com/pam-u2f/

yubico-pam

The `yubico-pam` module uses a OTP (one time password) challenge response to authenticate users.

Use this page to check whether your Yubikey supports Yubico OTP before starting: https://www.yubico.com/products/identifying-your-yubikey/

You can enable challenge-response logins with:

security.pam.yubico = {
   enable = true;
   debug = true;
   mode = "challenge-response";
};

You'll also need to program the Yubikey for challenge-response on slot 2 and setup the current user for logon:

1. nix-shell -p yubico-pam -p yubikey-manager
2. ykman otp chalresp --touch --generate 2
3. ykpamcfg -2 -v

To automatically login, without having to touch the key, omit the --touch option.

Having that, you should be able to use your Yubikey to login and for sudo. You can also set security.pam.yubico.control to "required" in order to have multi-factor authentication.

See also: https://developers.yubico.com/yubico-pam/Authentication_Using_Challenge-Response.html.

Smartcard mode

To use the smart card mode (CCID) of Yubikey, you will need the PCSC-Lite daemon:

services.pcscd.enable = true;

Please note that the PCSC-Lite daemon sometimes conflicts with gpg-agent.

OTP

In order to manage OTP keys, you should install the yubioath-desktop package in your profile. This application will also require both the udev rules as well as pcscd enabled.

Key generation

It is best practice to create the keys on a system without network connection to avoid leakages. This guide explains in depth the steps needed for that. There is also a nix expression that creates a nixos live image with all necessary dependencies pre-installed. The image can be created with the nixos-generator tool and depending on the image copied onto a usb stick or executed directly using kexec

Multiple keys

If you want to use GPG with multiple keys, containing the same subkeys, you have to do this routine when swapping the key

1. killall gpg-agent
2. rm -r ~/.gnupg/private-keys-v1.d/
3. Plug in the new YubiKey
4. gpg --card-status (optional, to see if key is visibile)

Links

• Yubikey_based_Full_Disk_Encryption_(FDE)_on_NixOS