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:

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 and socket. If you want the app to launch on startup, just enable the service like so:

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

If you want the service to be started only when there is a listener on Unix socket, enable the socket instead like so:

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

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:

Prequisites for building locally

gpgme

sudo apt install libgpgme-dev

For Go <1.17

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

For Go >1.17:

$ go install github.com/maximbaz/yubikey-touch-detector@latest

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_STDOUT`	`--stdout`
`YUBIKEY_TOUCH_DETECTOR_NOSOCKET`	`--no-socket`

You can configure the systemd service by defining any of these environment variables in $XDG_CONFIG_HOME/yubikey-touch-detector/service.conf - see service.conf.example for a configuration example.

Integrating with other UI components

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

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)

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, define $GNUPGHOME environment variable, globally or in $XDG_CONFIG_HOME/yubikey-touch-detector/service.conf.

Since v1.11.0 we started using gpgme to perform some operations above:

we are now using Assuan protocol to query card status, instead of spawning gpg --card-status processes.
we are now querying path to pubring.kbx from gpgme.

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

If you are going to frequently use OpenPGP operations, cached or cached-fixed may be better for you. See more details here.

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