Kaptcha.NET

Kaptcha.NET is simple captcha library for .NET Core projects.

Motivation

With Google's reCAPTCHA beeing the de facto standard solution for captchas nowadays, it is pretty much the opposite of a privacy respecting solution. It collects substantial information which can be used to track users and makes complying with GDPR much more complicated. Therefore I wanted to create a minimalistic, privacy respecting solution, which is simple to implement, yet sufficiently difficult to solve.

Examples

Install

Install the nuget package Kaptcha.NET.

Getting Started

Add the required captcha services via the service collection extension. You have to use a Cache Provider. You can use SQL Server Cache, In Memory Cache or Distributed Cache (like a Redis).

public void ConfigureServices(IServiceCollection services)
{
	//...
	services.AddDistributedMemoryCache(); // Required for the specific default implementation of ICaptchaStorageService
	services.AddCatpcha(Configuration);
	//...
}

In case you prefer to create and use your own, custom implementation of any service, replace '.AddCatpcha(Configuration)' with the following and use your own classes:

public void ConfigureServices(IServiceCollection services)
{
	//...
	services.AddCatpchaOptions(configuration);
			.AddCaptchaFontGenerator<FontGeneratorService>();
			.AddCaptchaEffect<EffectGeneratorService>();
			.AddCaptchaKeyGenerator<KeyGeneratorService>();
			.AddCatpchaGenerator<CaptchaGeneratorService>();
			.AddCaptchaStorage<CaptchaStorageService>();
			.AddCaptchaValidator<CaptchaValidationService>();
			.AddScoped(typeof(ValidateCaptchaFilter));
	//...
}

Add the TagHelper to the _ViewImports.cshtml

@addTagHelper *, Kaptcha.NET

Actually adding the captcha into the ViewPage afterwards is as simple as

<captcha />

If you want to provide a direkt link to a captcha, add a new controller and inherit from Controllers\CaptchaController.cs, which provides all requied methods.

@{
	var captcha = await generator.CreateCaptchaAsync();
}
<captcha captcha-link="/captcha/@captcha.Id" captcha-id="@captcha.Id" />

Or you can use controller and actions instead of a direct link and let the TagHelper generate the captcha on it'S own.

<captcha asp-controller="Home" asp-action="GetCaptcha" />

Configuration

You can configure the following options:

Image height (default: 250)
Image Width (default: 100)
Scaling (default: 2)
Background color (default: Color.White)
Foreground color (default: Color.Black)
Number of characters (default: 5 - 8)
Charset (default: "abcdefghijklmnopqrstuvwxyz")
Rotation angle (default: -30 - 30)
Imageformt (default: PNG)
Validation Timeout (default: 5min)
Font family (default: Arial)
Font size (default: 22 - 30)
Font style (default: regular)

To change the default values you need to set the appropriate properties in eigher the CaptchaOptions or FontOptions. Moreover you can enable different effects within the EffectOptions.

BlobEffect
BoxEffect
LineEffect
NoiseEffect
RippleEffect
WaveEffect

In ASP.NET Core projects, you can do this via the appsettings.json file, an example is provided under test\Kaptcha.NET.Web.

TODO

Currently, System.Drawing.Common is used. I plan on replacing it in the future. To use it on Linux, add the following commands to your docker file

sudo apt install libc6-dev 
sudo apt install libgdiplus

and consider checking out this blog post by Scott Hanselman.

License

twsl/Kaptcha.NET

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