Tuesday 24 April 2018

Implementing Captchas

"Completely Automated Public Turing Tests to Tell Computers And Humans Apart (CAPTCHAs) are challenge-response tests used to determine whether or not the user is a human" -- Wikipedia

These are used to ensure that a human is indeed in front of the screen making a web request. Sensitive parts of application are protected with captchas. A form that adds data into a database may need to be protected from accepting automated postings. Without protection, a stream of automated posts can not only just swamp the application but can also fill the database/disk with valid junk. For example, gibberish in a feedback form. 

Contemporary captchas are usually one of the following

1) One image or a set of images with a challenge question
An image will have text in it. The user has to type the text into a box. Another type is a set of images and the user has to pick a subset of the images. Say you are shown 4 images and there are animals in only 2 of them. You have to select the correct ones with the animals.

2) A click request on a box
3) An audio playback 
4) Even a short video playback can be used

The last two have storage and bandwidth impact on a web application.


A pluggable captcha that can be used in any Django web application. A mechanism to add and configure captchas with challenges. Once a captcha is added, the system must pick it from there. Captchas have to be one level up in difficulty. i.e something more than just 'enter the text', although these can also be used. 


A single image captcha with additional semantic requirements is implemented. A reusable Django app 'captchas' holds the model, form etc to select and process captchas. The template can be included in any HTML form. The default display is as a Bootstrap4 card. How and where this card renders on a form is up to the page designer. Django views just need to send a form in responses for GET and process the submitted form in  a POST. The validation of the captcha is isolated in its form.

The add web page functionality in HUD application is protected with these captchas. This implementation can not only ask for just the text but can also ask for anything based on the image. This iteration includes captchas with challenges like 

- Type the text
- Type only the numbers seen in image
- Type only the letters
- Type the last four letters in reverse

Examples are

Or anything that can be inferred from the image i.e the challenge is configurable as shown. In this iteration basic colored images were used. Using strike throughs, blurs and other effects and on the images can further confuse models. It is also important to change the size of the image as it will slightly increase processing cost.


1) There is a one-to-many relation between the images and challenges. With many images and challenges this approach can mitigate the effect of a sweatshop. A captcha image will show up with a different challenge thus mitigating image signature based attacks. If an attacker is getting past the security then it has to be on expensive discipline.

2) There are online free captcha services that can be easily integrated  to sites. However, these tend to have one or another pattern. The popular services may have already been subjected to continuous automated machine learning to created models. Such models are posed with a custom unfamiliar challenge thus making it difficult.

3) Ability to change the challenge over time allows for reuse. This is because it is the challenge that can hold a semantic requirement on a static image.

4) Even if the captcha images are harvested from the application, the challenge remains unknown. The challenge on a harvested image can be changed to a more complicated question.

Saturday 7 April 2018

Migrating to Bootstrap 4

This post lists the effort in migrating HUD project to the latest stable version 4 of Bootstrap. Screens from mobile, tablet and wide screens are posted at the end.

Project details

# Web framework: Django 1.10
# HTML files / Templates: 66
# Custom css files: 10
# Effort: 4.5 days


Branch out and import Bootstrap 4 into the project. Things will look out of place. Go through official Migration Guide for each user interface component used and adopt changes. 


Notable points

Panels: There are no panels in bootstrap 4. If you have been using a lot of them, then you need to find and replace them with cards. Migration details link.

Cards: These can do all that panels did in the application and more. So moving was not fruitless. The most important advantage in the application is the better ux on a really small mobile, through the tablet to desktop. It was easier to create a tile like structure with images out of cards. Previously this was done using custom css. 

Navbar: A number of changes in the classes for navbar elements. Glyphicons are no longer part of bootstrap. Although this was not much of an impact, it needs to be brought in manually. The new approach can avoid nesting of ul li and a tags to create navbar items. In the new approach, a dropdown-menu class can be applied to a div. And a dropdown-item class make an a tag a drop down item. While a nesting with li is avoided. This can wreck havoc if there are css combinators applied to create custom navbars.  There was limited impact on the project and css was migrated. Migration details link

Display/hide: Some classes have been dropped altogether. For example, hidden-sm was used to hide a component on small screens and save space on the navbar. This has been replaced using a combination of screen size and values as .d-none .d-sm-block.  This is explained here. image-responsive is now image-fluid.

Forms: It is always important to keep forms simple. This is one part where both users and developers can make the layout complicated. Forms were kept simple in the application and migration was not much of a problem with changes listed here. Labels have new classes and also to control sizes. Again, help-block class for help text is now form-text class. This is a better option in a Django application. The error check in template can be moved outside the element.

Grids: Although this can appear daunting, a quick read can help to push through the changes. In reality the addition of a tier helps finer control of flexible display. For example, the application after migration was tested on a portrait mobile device (Lumia 630!), portrait + landscape of tablet and wide screen laptop. Screen widths at which the transition occurs has changed. The exact details are here

Margin and Padding: New formatted classes are in place. This helps to control the values for each break point. mb-4 is a simple one used where 4em is the bottom-margin. Another class mr-auto helps with right aligned navbar links.  

Flex layout: d-flex was used to create a container to hold the avatar icon and user name in the navbar. More about Flex here. This was used in conjunction with mr-auto the new margin classes to right align the avatar and user name.

The effort was worth it when viewed on multiple screens. Some screens mobiles, tablets and desktop.

Mobile 480 x 854 pixels

Samsung Galaxy Tab S2 1536 x 2048 (portrait)

4K Laptop