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Captcha API

This project contains a simple Captcha API that returns an encoded image containing letters a-Z and numbers 1-9 via a GET endpoint. The POST endpoint can be used to validate the captcha.

A Dockerfile is provided for containerization.

Running with Docker

Build the Docker image

docker build . -t captcha-api

Run the Docker image

docker run -d --name captcha -p 8080:8080 -e CAPTCHA_API_CONFIG=captcha.cfg captcha-api

Navigate to http://localhost:8080/swagger-ui

Running locally

This guide is written for users of Debian-based systems, the package names might differ on other operating systems such as CentOS.

For usage on Windows, please use the WSL2 distribution in order to build and run the application.

Installing the required libraries

Unfortunately we need some binary dependencies, since PIL and the audio CAPTCHA generation require them. Run the following command:

sudo apt-get install libtiff5-dev libopenjp2-7-dev zlib1g-dev \
    libfreetype6-dev liblcms2-dev libwebp-dev tcl8.6-dev tk8.6-dev python3-tk \
    libharfbuzz-dev libfribidi-dev libxcb1-dev ffmpeg espeak python3-dev

Installing and running the application

Run

pip install -e '.[dev]'

in order to install all the required dependencies for development.

To start the server locally, you need to create a captcha.cfg file. Just cp captcha_api/captcha.cfg.example captcha_api/captcha.cfg to get started.

Running the server is done by running:

flask run

or

gunicorn --bind 0.0.0.0:5000 wsgi:app

Audio CAPTCHA

For accessibility reasons, one might one to listen to the CAPTCHA message. In order to do that, you can point to the following endpoint:

/api/v1.0/captcha/audio/$CAPTCHA_ID

The file returned is in the mp3 format and can be easily loaded into an HTML form as such:

<div>
  <audio controls="controls" className="audio-element">
    <source src="http://localhost:8080/api/v1.0/captcha/audio/$CAPTCHA_ID" } />
  </audio>
</div>

Running migrations

Make sure you installed the dependencies using pip install -e ..

Afterwards, run flask db upgrade to bring your DB to the latest level. By default it will use a test.db SQLite file in the captcha_api folder.

Embedded captcha.js

The Captcha API includes a static Javascript file to help with simple web form integrations. To use or test this script:

  1. Configure the captchaApiUrl variable in captcha_api/static/captcha.js.
  2. Run the Captcha API.
  3. Include the script in your page, and the HTML element <div id="cern-captcha"></div> in your form (see the example in captcha_api/static/demo.html).
  4. When processing your form, validate captchaAnswer and captchaId with the POST endpoint of the Captcha API.

Example for the production Captcha API:

<!DOCTYPE html>
<html>
    <script type="module" src="https://captcha.web.cern.ch/static/captcha.js"></script>
<body>
    <form>
        <div id="cern-captcha"></div>
        <input type="submit" value="Submit">
    </form>
</body>
</html>