Fingerprint Authentication with swaylock

This guide shows how to configure fingerprint authentication for swaylock on Alpine Linux, allowing you to unlock using fingerprint sensor with password as fallback.

Prerequisites

Enable Polkit

Installation

Install the fprintd package:

$ doas apk add fprintd

Configuration

To get the USB ID for your fingerprint reader, issue the command:

$ lsusb

Once onfigured this allows you to unlock using either:

<enter password> → <hit enter>
<hit enter> → <touch fingerprint sensor>

Configure policyKit permissions

Upon installation, standard users are not authorized to enroll fingerprints. Create a PolicyKit rule etc/polkit-1/rules.d/50-fingerprint.rules to allow members of the input group to manage fingerprints with contents as shown:

Contents of etc/polkit-1/rules.d/50-fingerprint.rules

polkit.addRule(function (action, subject) {
    if (action.id.indexOf("net.reactivated.fprint.") == 0) {
        if (subject.isInGroup("input")) {
            return polkit.Result.YES;
        }
    }
});

Ensure that correct permissions are set:

# chown root:root /etc/polkit-1/rules.d/50-fingerprint.rules # chmod 644 /etc/polkit-1/rules.d/50-fingerprint.rules

Add your user to the input group:

$ doas adduser $USER input

Note: You must log out and back in (or reboot) for the group membership & PolicyKit rule to take effect.

Enroll fingerprints

If you previously enrolled fingerprints as root (or want to start fresh), delete existing enrollments:

To delete fingerprints for current user:

$ fprintd-delete $(whoami)

If you accidentally enrolled as root, delete those too

$ doas fprintd-delete root

Enroll your fingerprint(s):

$ fprintd-enroll

Verify the enrollment works:

$ fprintd-verify

Configure PAM for swaylock

Create the PAM configuration file /etc/pam.d/swaylock for swaylock with contents as shown:

Contents of /etc/pam.d/swaylock

# Try password authentication first
auth sufficient pam_unix.so nullok
# If no password provided, try fingerprint
auth sufficient pam_fprintd.so ignore-empty-password
auth required pam_deny.so

# KWallet integration (optional)
-auth    optional        pam_kwallet.so
-auth    optional        pam_kwallet5.so
-session optional        pam_kwallet.so auto_start
-session optional        pam_kwallet5.so auto_start

Ensure that correct permissions are set:

# chown root:root /etc/pam.d/swaylock # chmod 644 /etc/pam.d/swaylock

Extending to other services

You can apply similar fingerprint authentication to other services by adding the same PAM configuration pattern to files in /etc/pam.d/ such as:

sudo
polkit-1
login
su

Usage

Once configured, swaylock will accept both authentication methods:

Password authentication: Type your password and press Enter
Fingerprint authentication: Press Enter without typing anything, then touch the fingerprint sensor

Troubleshooting

Permission denied during enrollment

If you receive Permission denied message during enrollment ensure you're in the input group and have logged out/in after adding the group.

Fingerprint recognized but doesn't unlock

If Fingerprint is recognized but doesn't unlock, check that fingerprints are enrolled for the correct user (not root)

No fallback to password

If there is no fallback to password when using fingerprint authentication verify the PAM configuration has pam_unix.so before pam_fprintd.so