reCAPTCHA 2 for MVC 6

Implementation of Google's reCAPTCHA for ASP.Net 5/MVC 6 (dnxcore and dnx451)

Getting started

Once you have addded the project to your solution, you will need to add a dependency reference to it from your ASP.Net 5 project. You can do this via the package manager console:

Install-Package Recaptcha2-MVC6 -IncludePrerelease

You will now need to configure the reCAPTCHA. If you haven't done so already, visit https://www.google.com/recaptcha/intro/index.html and sign up for a reCAPTCHA account. Once you have, you will be provided with a site key and secret. Add those to an appsettings.json file off the root of your ASP.Net 5 application. Your file should look like this (the following keys are invalid):

  {
    "RecaptchaOptions": {
      "SiteKey": "6LcQfA8TAAAAAP2eIEy5wRsAdsTa99rDSI3Qwf_o",
      "Secret": "6LcQfA8TAAAAALj6jHM6z60M9DgMWoJYHIatXXXW"
    },
    "Data": {
      "DefaultConnection": {
        "ConnectionString": "Server=(localdb)\\mssqllocaldb;Database=aspnet5-WebApplication3-XXXXX;Trusted_Connection=True;MultipleActiveResultSets=true"
      }
    },

The reCAPTCHA relies on a Javascript file, so somewhere in your View or Layout, reference the following:

      <script src='https://www.google.com/recaptcha/api.js'></script>

You will need to notify your application that you have added a custom tag helper from this project. In /Views/_ViewImports.cshtml, add this line:

@addTagHelper "*, Recaptcha2-MVC6"

To use the reCAPTCHA, identify a controller action to which it is being applied and add this action filter:

    [HttpPost, ValidateRecaptcha]
    public async Task<IActionResult> AddComment(AddCommentViewModel viewmodel)

Note this will emit a hard-coded "We could not verify that you are a human." ModelState error message if verification fails. If you want to change that message or add localization, add the filter like this:

    [HttpPost, ValidateRecaptcha(ErrorMessage = Resource.RecaptchaError)]

Finally, in any form where you would like to add the Recaptcha, add this tag helper:

      <form asp-action="AddComment" asp-controller="Home">
        <recaptcha multi-site="false"></recaptcha>
         ...
      </form>

Supporting multiple sites with the same tokens

As described at https://developers.google.com/recaptcha/docs/secure_token, you can use the same key and secret tokens across multiple domains. To enable this behaviour, simply add the multi-site attribute to the recaptcha tag helper and set it to true:

    <form asp-action="AddComment" asp-controller="Home">
      <recaptcha multi-site="true"></recaptcha>
      ...
    </form>