Dancer Two-Factor Authentication Demo

This demo shows how Two-Factor Authentication (2FA) can be implemented with Dancer2, Dancer2::Plugin::Auth::Extensible and a couple of other Perl modules.

It also shows how Dancer2::Plugin::Auth::Extensible enhances your Dancer application with user management using very little code.

Get the source code from GitHub

~# git clone

Start the app

~# cd dancer2-tfa-demo
~# plackup -o bin/app.psgi
HTTP::Server::PSGI: Accepting connections at

Login and Setup 2FA

Go to the browser and enter the login URL

The default credentials are dancer2 as username and 2fanow as password. You can change or add users in the configuration file.

Now use an 2FA app like Authy, Google Authenticator or FreeOTP+ to scan the QR code and confirm the token.

Finally log out and test your token.

Two-Factor Authentication

Creating the secret

The demo application creates the secret when the user is logged in and goes to the 2FA setup page for the first time. The secret is created with Data::SimplePassword, default length is 30 characters.

Generating image with QR code

First we create an object which is going to generate an image with the QR code.

my $qr = Imager::QRCode->new(
   size => 6,
   margin => 2,
   version => 1,
   level => 'M',
   casesensitive => 1,

Now we construct the label that is going to be used by the authentication app. It consists of the fixed string and the user name in parenthesis.

my $instance = $self->qr_code_label;
my $user_link = uri_escape("$instance (" . $username . ')');

my $data;
my $img = $qr->plot("otpauth://totp/$user_link?secret=" . encode_base32($secret));
$img->write(data => \$data, type => 'png');

We send this back with:

$self->plugin->app->send_file (\$data, content_type => 'image/png');

Storing the secret

The secret is stored on the server and in the authentication app of the user. The demo keeps the secret in memory.


The authentication code is split into two modules, a role with the logic for the specific routes and a demo provider which consumes that role and takes care of the secret's generation and the "storage".


We intercept the standard authentication of Dancer2::Plugin::Auth::Extensible from the Demo provider using around:

around authenticate_user => sub {
  my ($orig, $self, $username, $password, @args) = @_;
  my $ret = $orig->($self, $username, $password, @args);

  return unless $ret;
  if ($self->check_tfa($username,
                       $self->plugin->app->request->param('token'))) {
      return $ret;
  else {

So we first call the original authenticate_user method and only if that is successful we are checking the token.

We determine the token that is valid at the current time and compare that with the token passed by the user:

my $expected = Authen::OATH->new->totp($secret);

if ($token eq $expected) {
else {


We are using a fixed set of credentials in the configuration file config.yml.

       provider: Demo
       username_key_name: user
         - user: dancer2
           pass: 2fanow


Dancer2::Plugin::Auth::Extensible supplements the demo with routes for login and logout, so we only need a few more routes specific to 2FA.


GET /tfa/setup/

Shows the form for 2FA setup with the QR code.

GET /tfa/qrcode.png

Produces QR code.

POST /tfa/setup/

Verifies token from 2FA setup form.

From plugin

The routes for 2FA are established by the plugin, e.g.

    method => 'get',
    regexp => '/tfa/setup',
    code => sub { $self->tfa_setup }

Use cases

We are using Two-Factor Authentication for a couple of websites for ECA and an online shop in Germany.


As the secrets are stored into memory, this demo should be run only as a single instance. Maybe Dancer2::Plugin::Cache::CHI could help here.


This article has been written by Stefan Hornburg (Racke) for the Perl Dancer Advent Calendar 2020.