YubiKey touch detector

This is a tool that can detect when YubiKey is waiting for your touch. It is designed to be integrated with other UI components to display a visible indicator.

For example, an integration with i3wm and py3status looks like this:

See also: FAQ: Which UI components are already integrated with this app?

Installation

This tool only works on Linux. If you want to help implementing (at least partial) support for other OS, pull requests are very welcome!

On Arch Linux, you can install it with pacman -S yubikey-touch-detector

The package also installs a systemd service, make sure to start and enable it:

$ systemctl --user daemon-reload
$ systemctl --user enable --now yubikey-touch-detector.service

Alternatively you can download the latest release from the GitHub releases page. All releases are signed with my PGP key.

Finally you can install the app with go:

$ go get -u github.com/maximbaz/yubikey-touch-detector

This places the binary in your $GOPATH/bin folder, as well as the sources in $GOPATH/src for you to use the detection functions in your own code.

Usage

Command line

To test how the app works, run it in verbose mode to print every event on STDERR:

$ yubikey-touch-detector -v

Now try different commands that require a physical touch and see if the app can successfully detect them.

Desktop notifications

You can make the app show desktop notifications using libnotify if you run it with corresponding flag:

$ yubikey-touch-detector --libnotify

Configuring the app

The app supports the following environment variables and CLI arguments (CLI args take precedence):

Environment var	CLI arg
YUBIKEY_TOUCH_DETECTOR_VERBOSE	-v
YUBIKEY_TOUCH_DETECTOR_LIBNOTIFY	--libnotify
YUBIKEY_TOUCH_DETECTOR_GPG_PUBRING_PATH	--gpg-pubring-path

You can configure the systemd service by defining any of these environment variables in $XDG_CONFIG_HOME/yubikey-touch-detector/service.conf, e.g. like so:

YUBIKEY_TOUCH_DETECTOR_LIBNOTIFY=true
YUBIKEY_TOUCH_DETECTOR_GPG_PUBRING_PATH=/path/to/gpg/pubring.kbx

Integrating with other UI components

First of all, make sure the app is always running (e.g. start a provided systemd user service).

Next, in order to integrate the app with other UI components to display a visible indicator, use any of the available notifiers in the notifier subpackage.

notifier/unix_socket

unix_socket notifier allows anyone to connect to the socket $XDG_RUNTIME_DIR/yubikey-touch-detector.socket and receive the following events:

event	description
GPG_1	when a gpg operation started waiting for a touch
GPG_0	when a gpg operation stopped waiting for a touch
U2F_1	when a u2f operation started waiting for a touch
U2F_0	when a u2f operation stopped waiting for a touch

All messages have a fixed length of 5 bytes to simplify the code on the receiving side.

How it works

Your YubiKey may require a physical touch to confirm these operations:

• sudo request (via pam-u2f)
• WebAuthn
• gpg --sign
• gpg --decrypt
• ssh to a remote host (and related operations, such as scp, rsync, etc.)
• ssh on a remote host to a different remote host (via forwarded ssh-agent)

See also: FAQ: How do I configure my YubiKey to require a physical touch?

Detecting u2f operations

In order to detect whether a U2F/FIDO2 operation requests a touch on YubiKey, the app is listening on the appropriate /dev/hidraw* device for corresponding messages as per FIDO spec.

See detector/u2f.go for more info on implementation details, the source code is documented and contains relevant links to the spec.

Detecting gpg operations

This detection is based on a "busy check" - when the card is busy (i.e. gpg --card-status hangs), it is assumed that it is waiting on a touch. This of course leads to false positives, when the card is busy for other reasons, but it is a good guess anyway.

In order to not run the gpg --card-status indefinitely (which leads to YubiKey be constantly blinking), the check is being performed only after $GNUPGHOME/pubring.kbx (or $HOME/.gnupg/pubring.kbx) file is opened (the app is thus watching for OPEN events on that file).

If the path to your pubring.kbx file differs, provide it via --gpg-pubring-path CLI argument.

Detecting ssh operations

The requests performed on a local host will be captured by the gpg detector. However, in order to detect the use of forwarded ssh-agent on a remote host, an additional detector was introduced.

This detector runs as a proxy on the $SSH_AUTH_SOCK, it listens to all communications with that socket and starts a gpg --card-status check in case an event was captured.

FAQ

How do I configure my YubiKey to require a physical touch?

For sudo requests with pam-u2f, please refer to the documentation on Yubico/pam-u2f and online guides (e.g. official one).

For gpg and ssh operations, install ykman and use the following commands:

$ ykman openpgp touch sig on   # For sign operations
$ ykman openpgp touch enc on   # For decrypt operations
$ ykman openpgp touch aut on   # For ssh operations

Make sure to unplug and plug back in your YubiKey after changing any of the options above.

Which UI components are already integrated with this app?

• waybar can display an indicator via custom yubikey module.
• py3status can display an indicator via yubikey module.
• barista can display an indicator via yubikey module.