Ixnas.AltchaNet 0.2.1

Altcha.NET

Server-side implementation of the ALTCHA challenge in C#.

Features

• Compatible with the ALTCHA client-side widget
• Independent from ASP.NET (Core)
• Supports self-hosted challenges
• Supports ALTCHA's public API and their spam filter
• Replay attack prevention by denying previously verified challenges
• Expiring challenges

Contents

• Installation
• Using self-hosted challenges
  • Set up
    • Key
    • Store
  • Usage
    • Generating a challenge
    • Validating a response
• Verifying challenges from ALTCHA's API
  • Set up
  • Usage
    • Validating a regular response
    • Validating a spam filtered form
• Example
• License

Installation

This library is available on NuGet, so you can add it to your project as follows:

dotnet add package Ixnas.AltchaNet

Using self-hosted challenges

Set up

First make sure you've set up the front-end widget to use your challenge endpoint.

The entrypoint of this library contains a service builder for self-hosted configurations. This builder configures the service that is used to create ALTCHA challenges and validate their responses. The most basic configuration looks like this:

var altchaService = Altcha.CreateServiceBuilder()
                          .UseSha256(key)
                          .UseStore(store)
                          .Build();

Here is a description of the different configuration options.

Key

The library requires a key to sign and verify ALTCHA challenges. You can use a random number generator from .NET to create one for you:

var key = new byte[64];
using (var rng = RandomNumberGenerator.Create())
{
    rng.GetBytes(key);
}

Store

The library requires a store implementation to store previously verified challenge responses. You can use anything persistent, like a database or a file. As long as it implements the IAltchaChallengeStore interface, it will work. You can use expiryUtc to periodically remove expired challenges from your store. For example, the bundled in-memory store looks similar to this:

public class InMemoryStore : IAltchaChallengeStore
{
    private class StoredChallenge
    {
        public string Challenge { get; set; }
        public DateTimeOffset ExpiryUtc { get; set; }
    }

    private readonly List<StoredChallenge> _stored = new List<StoredChallenge>();

    public Task Store(string challenge, DateTimeOffset expiryUtc)
    {
        var challengeToStore = new StoredChallenge
        {
            Challenge = challenge,
            ExpiryUtc = expiryUtc
        };
        _stored.Add(challengeToStore);
        return Task.CompletedTask;
    }

    public Task<bool> Exists(string challenge)
    {
        _stored.RemoveAll(storedChallenge => storedChallenge.ExpiryUtc <= DateTimeOffset.UtcNow);
        var exists = _stored.Exists(storedChallenge => storedChallenge.Challenge == challenge);
        return Task.FromResult(exists);
    }
}

Usage

Generating a challenge

To generate a challenge:

var challenge = altchaService.Generate();

The challenge object can be serialized to JSON for the client to use. Read ALTCHA's documentation on how to use such a JSON object.

Validating a response

To validate a response:

var validationResult = await altchaService.Validate(altchaBase64);
if (!validationResult.IsValid)
    /* ... */

The altchaBase64 parameter represents the raw value from the altcha field in a submitted form, so you don't need to convert anything.

Verifying challenges from ALTCHA's API

Set up

First make sure you've set up the front-end widget to use the API.

The entrypoint of this library contains a different service builder for integrating with ALTCHA's API. The most basic configuration looks like this:

var altchaApiService = Altcha.CreateApiServiceBuilder()
                             .UseApiSecret(secret)
                             .UseStore(store)
                             .Build();

Here is a description of the different configuration options.

The store uses the same interface as it does for the self-hosted service. You can even use the same instance if you like.

Usage

Validating a regular response

To validate a regular response:

var validationResult = await altchaApiService.Validate(altchaBase64);
if (!validationResult.IsValid)
    /* ... */

This works the same way as self-hosted validation. Challenges generated by the self-hosted service can not be validated by the API service, or vice versa.

Validating a spam filtered form

To validate a spam filtered form, you need an object that represents the form fields as public string properties. By default, the library looks for a public string property named Altcha that should contain the raw value from the altcha field in a submitted form. A form class could look like this:

public class ExampleForm
{
    public string Altcha { get; set; }
    public string Email { get; set; }
    public string Text { get; set; }
}

To validate the form:

var validationResult = await altchaApiService.ValidateSpamFilteredForm(form);
if (!validationResult.IsValid)
    /* ... */

if (!validationResult.PassedSpamFilter)
    /* ... */

If you prefer to use a different property for the ALTCHA payload, you can use a member expression to select it:

var validationResult = await altchaApiService.ValidateSpamFilteredForm(form, x => x.AnotherProperty);

The result's IsValid property tells you whether the form data, verification data and the signature are valid. You should probably reject the form submission if this is not the case.

The result's PassedSpamFilter property tells you whether the form data successfully passed through the spam filter. You might want to keep the form submission and mark it as spam in your application for manual approval.

Example

You can examine the AspNetCoreExample project as a minimal example.

License

See LICENSE.txt

See LICENSE-ALTCHA.txt for ALTCHA's original license.