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

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.

Install the nuget package Kaptcha.NET.

• 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" />

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.

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.