DNTCaptcha.Core is a captcha generator and validator for ASP.NET Core applications.

To install DNTCaptcha.Core, run the following command in the Package Manager Console:

PM> Install-Package DNTCaptcha.Core

You can also view the package page on NuGet.

• After installing the DNTCaptcha.Core package, add the following definition to the _ViewImports.cshtml file:

@addTagHelper *, DNTCaptcha.Core

• Then to use it, add its new tag-helper to your view:

For bootstrap-3:

<dnt-captcha asp-captcha-generator-max="9000"
             asp-captcha-generator-min="1"
             asp-captcha-generator-language="English"
             asp-captcha-generator-display-mode="NumberToWord"
             asp-placeholder="Security code as a number"
             asp-validation-error-message="Please enter the security code as a number."
             asp-font-name="Tahoma"
             asp-font-size="20"
             asp-fore-color="#333333"
             asp-back-color="#ccc"
             asp-text-box-class="text-box single-line form-control col-md-4"
             asp-text-box-template="<div class='input-group col-md-4'><span class='input-group-addon'><span class='glyphicon glyphicon-lock'></span></span>{0}</div>"
             asp-validation-message-class="text-danger"
             asp-refresh-button-class="glyphicon glyphicon-refresh btn-sm"
             />

For bootstrap-4 (you will need to npm install components-font-awesome for the missing font-glyphs too):

<dnt-captcha asp-captcha-generator-max="9000"
             asp-captcha-generator-min="1"
             asp-captcha-generator-language="English"
             asp-captcha-generator-display-mode="NumberToWord"
             asp-placeholder="Security code as a number"
             asp-validation-error-message="Please enter the security code as a number."
             asp-font-name="Tahoma"
             asp-font-size="20"
             asp-fore-color="#333333"
             asp-back-color="#ccc"
             asp-text-box-class="text-box form-control"
             asp-text-box-template="<div class='input-group'><span class='input-group-prepend'><span class='input-group-text'><i class='fas fa-lock'></i></span></span>{0}</div>"
             asp-validation-message-class="text-danger"
             asp-refresh-button-class="fas fa-redo btn-sm"
             />

• To register its default providers, call services.AddDNTCaptcha(); method in your Startup class.

using DNTCaptcha.Core;

namespace DNTCaptcha.TestWebApp.V3x
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDNTCaptcha(options =>
                // options.UseSessionStorageProvider() // -> It doesn't rely on the server or client's times. Also it's the safest one.
                // options.UseMemoryCacheStorageProvider() // -> It relies on the server's times. It's safer than the CookieStorageProvider.
                options.UseCookieStorageProvider() // -> It relies on the server and client's times. It's ideal for scalability, because it doesn't save anything in the server's memory.
                );
        }

• Now you can add the ValidateDNTCaptcha attribute to your action method to verify the entered security code:

[HttpPost, ValidateAntiForgeryToken]
[ValidateDNTCaptcha(ErrorMessage = "Please enter the security code as a number.",
                    IsNumericErrorMessage = "The input value should be a number.",
                    CaptchaGeneratorLanguage = Language.English,
                    CaptchaGeneratorDisplayMode = DisplayMode.NumberToWord)]
public IActionResult Index([FromForm]AccountViewModel data)
{
    if (ModelState.IsValid) // If `ValidateDNTCaptcha` fails, it will set a `ModelState.AddModelError`.
    {
        //TODO: Save data
        return RedirectToAction(nameof(Thanks), new { name = data.Username });
    }
    return View();
}

• This library uses unobtrusive Ajax library for the refresh button. Make sure you have included its related scripts too:
  • package.json
  • bundleconfig.json
  • Or you can download it from: https://github.com/aspnet/jquery-ajax-unobtrusive/tree/master/dist
  • Or include it from a CDN (It requires jQuery 1.4.4+):

  <script type="text/javascript" src="http://ajax.aspnetcdn.com/ajax/mvc/3.0/jquery.unobtrusive-ajax.min.js"></script>

Please follow the DNTCaptcha.TestWebApp.V3x sample for more details.

It's possible to use this captcha with Angular 4.3+ apps too. Here is a sample to demonstrate it:

• The server side controller
• The Angular 4.3+ component
• A sample Angular 4.3+ login page

To run this project on non-Windows-based operating systems, you will need to install libgdiplus too:

• Ubuntu 16.04 and above:
  • apt-get install libgdiplus
  • cd /usr/lib
  • ln -s libgdiplus.so gdiplus.dll
• Fedora 23 and above:
  • dnf install libgdiplus
  • cd /usr/lib64/
  • ln -s libgdiplus.so.0 gdiplus.dll
• CentOS 7 and above:
  • yum install autoconf automake libtool
  • yum install freetype-devel fontconfig libXft-devel
  • yum install libjpeg-turbo-devel libpng-devel giflib-devel libtiff-devel libexif-devel
  • yum install glib2-devel cairo-devel
  • git clone https://github.com/mono/libgdiplus
  • cd libgdiplus
  • ./autogen.sh
  • make
  • make install
  • cd /usr/lib64/
  • ln -s /usr/local/lib/libgdiplus.so libgdiplus.so
• Docker

  • RUN apt-get update \

    && apt-get install -y libgdiplus

• MacOS

  • brew install mono-libgdiplus

    After installing the Mono MDK, Copy Mono MDK Files:

    • /Library/Frameworks/Mono.framework/Versions/4.6.2/lib/libgdiplus.0.dylib
    • /Library/Frameworks/Mono.framework/Versions/4.6.2/lib/libgdiplus.0.dylib.dSYM
    • /Library/Frameworks/Mono.framework/Versions/4.6.2/lib/libgdiplus.dylib
    • /Library/Frameworks/Mono.framework/Versions/4.6.2/lib/libgdiplus.la

    And paste them to: /usr/local/lib