altcha-org / altcha

GDPR compliant, self-hosted CAPTCHA alternative with PoW mechanism and advanced anti-spam filter.

ALTCHA

ALTCHA uses a proof-of-work mechanism to protect your website, APIs, and online services from spam and abuse. Unlike other solutions, ALTCHA is self-hosted, does not use cookies nor fingerprinting, does not track users, and is fully compliant with GDPR.

Visit ALTCHA for more information.

Features

• Frictionless CAPTCHA Alternative - Employs proof-of-work (PoW) instead of visual puzzles.
• Data Obfuscation - Safeguards your email address from scraping.
• Cookie-less - Designed to be GDPR compliant by default.
• Self-hosted - Operates independently without relying on external providers.
• SaaS Available - Get started with the SaaS API at altcha.org/docs/api.

Examples

• React
• Vue
• Svelte
• Solid
• Lit
• Angular

Server Integrations

• TypeScript
• PHP
• Go
• Python
• Java
• Ruby
• Elixir

CMS

• WordPress plugin
• Other libraries and plugins

More anti-spam solutions

• Spam Filter - stop sophisticated attacks and human-generated spam by classifying data.

Usage

ALTCHA widget is distributed as a "Web Component" and supports all modern browsers.

1. Install ALTCHA

npm install altcha

import altcha in your main file:

import 'altcha';

or insert <script> tag to your website:

<script async defer src="/altcha.js" type="module"></script>

CDN: https://cdn.jsdelivr.net/gh/altcha-org/altcha@main/dist/altcha.min.js

2. Use <altcha-widget> tag in your forms

<form>
  <altcha-widget
    challengeurl="https://..."
  ></altcha-widget>  
</form>

See the configuration below or visit the website integration documentation.

3. Integrate ALTCHA with your server

See server documentation for more details.

Bundle Size

ALTCHA's default bundle is lightweight, combining all assets, including CSS and the JavaScript Web Worker, into a single file. When GZIPped, it totals only 17 kB, making ALTCHA’s widget 94% smaller than reCAPTCHA.

Distribution	Size (GZIPped)
ALTCHA (v1.x)	17 kB
hCaptcha	48+ kB
reCAPTCHA	270+ kB

Content Security Policy (CSP)

The default distribution bundle of the WebComponent includes styles and the worker in a single file. This might cause issues with strict CSP rules. If you require strict CSP compliance, consider using the scripts located in the /dist_external directory. For more details, please refer to the documentation.

Configuration

Required options (at least one is required):

• challengeurl: URL of your server to fetch the challenge from. Refer to server integration.
• challengejson: JSON-encoded challenge data. If avoiding an HTTP request to challengeurl, provide the data here.

Additional options:

• auto: Automatically verify without user interaction (possible values: off, onfocus, onload, onsubmit).
• delay: Artificial delay in milliseconds before verification (defaults to 0).
• expire: Challenge expiration duration in milliseconds.
• floating: Enable floating UI (possible values: auto, top, bottom).
• floatinganchor: CSS selector of the "anchor" to which the floating UI will be attached (defaults to the button[type="submit"] in the related form).
• floatingoffset: Y offset from the anchor element for the floating UI in pixels (defaults to 12).
• hidefooter: Hide the footer (ALTCHA link).
• hidelogo: Hide the ALTCHA logo.
• maxnumber: Max number to iterate to (defaults to 1,000,000).
• name: Name of the hidden field containing the payload (defaults to "altcha").
• strings: JSON-encoded translation strings. Refer to customization.
• refetchonexpire: Automatically re-fetch and re-validate when the challenge expires (defaults to true).
• workers: Number of workers to utilize for PoW (defaults to navigator.hardwareConcurrency || 8, max value 16).
• workerurl: URL of the Worker script (defaults to ./worker.js, only works with external build).

Spam Filter-related options:

• blockspam: Only used with the spamfilter option. If enabled, it will block form submission and fail verification if the Spam Filter returns a negative classification. This prevents form submission.
• spamfilter: Enable Spam Filter.
• verifyurl: URL for server-side verification requests. This option is automatically configured when the spamfilter option is used. Override this setting only if using a custom server implementation.

Data Obfuscation options:

• obfuscated: The obfuscated data provided as a base64-encoded string (requires altcha/obfuscation plugin). Use only without challengeurl/challengejson.

Development / Testing options:

• debug: Print log messages in the console.
• mockerror: Causes verification to always fail with a "mock" error.
• test: Generates a "mock" challenge within the widget, bypassing the request to challengeurl.

Plugins

Version 0.9.x introduced plugins that can be enabled by importing individual plugin scripts:

import 'altcha/obfuscation';
import 'altcha';

It is recommended to import plugins before the main altcha package to ensure proper registration before any widget instance is created.

Available plugins built-in to the altcha package:

• altcha/analytics: Enable analytics with ALTCHA Forms. See HTML submissions documentation.
• altcha/obfuscation: Enable obfuscation for sensitive data such as email addresses or phone numbers.
• altcha/upload: Enable file upload from type=file fields to ALTCHA Forms. See HTML submissions documentation.

To enable specific plugins for a particular instance of the widget, use the plugins attribute in the widget tag. List the names of the plugins you want to enable, separated by commas, such as plugins="analytics,obfuscation". Plugins still need to be imported as described above. The plugins attribute only specifies which plugins should be active for that instance, even if other plugins are already imported.

Programmatic Configuration

To configure the widget programmatically, use the configure() method:

document.querySelector('#altcha').configure({
  challenge: {
    algorithm: 'SHA-256',
    challenge: '...',
    salt: '...',
    signature: '...',
  },
  strings: {
    label: 'Verify',
  },
});

Available configuration options:

export interface Configure {
  auto?: 'off' | 'onfocus' | 'onload' | 'onsubmit';
  challenge?: {
    algorithm: string;
    challenge: string;
    maxnumber?: number;
    salt: string;
    signature: string;
  };
  challengeurl?: string;
  debug?: boolean;
  delay?: number;
  expire?: number;
  floating?: 'auto' | 'top' | 'bottom';
  floatinganchor?: string;
  floatingoffset?: number;
  autorenew?: boolean;
  hidefooter?: boolean;
  hidelogo?: boolean;
  maxnumber?: number;
  mockerror?: boolean;
  name?: string;
  obfuscated?: string;
  refetchonexpire?: boolean;
  spamfilter?: boolean | 'ipAddress' | SpamFilter;
  strings?: {
    error: string;
    expired: string;
    footer: string;
    label: string;
    verified: string;
    verifying: string;
    waitAlert: string;
  }
  test?: boolean | number | 'delay';
  verifyurl?: string;
  workers?: number;
  workerurl?: string;
}

Events

• load - Triggers when the widget loads. The exported methods become available after this event.
• serververification - Triggers upon a server verification (only in conjunction with spamfilter).
• statechange - Triggers whenever an internal state changes.
• verified - Triggers when the challenge is verified.

enum State {
  ERROR = 'error',
  VERIFIED = 'verified',
  VERIFYING = 'verifying',
  UNVERIFIED = 'unverified',
  EXPIRED = 'expired',
};

Using events:

document.querySelector('#altcha').addEventListener('statechange', (ev) => {
  // See enum State above
  console.log('state:', ev.detail.state);
});

Important

Both programmatic configuration and event listeners have to called/attached after the ALTCHA script loads, such as within window.addEventListener('load', ...).

Spam Filter

The widget integrates with ALTCHA's Anti-Spam solution to allow checking submitted form data for potential spam.

The Spam Filter API analyzes various signals in the submitted data to determine if it exhibits characteristics of spam. This non-invasive filtering helps reduce spam submissions without frustrating legitimate users.

Spam Filter Configuration

The Spam Filter can be enabled with default configuration by setting the spamfilter option to true, or ipAddress to verify only the IP address and the time zone, or it can be customized using the following configuration schema:

interface SpamFilter {
  blockedCountries?: string[];
  classifier?: string;
  disableRules?: string[];
  email?: string | false;
  expectedCountries?: string[];
  expectedLanguages?: string[];
  fields?: string[] | false;
  ipAddress?: string | false;
  text?: string | string[];
  timeZone?: string | false;
}

SpamFilter configuration options:

• blockedCountries - An array of country codes (ISO 3166 alpha-2) that you want to block.
• classifier - Enforce a specific classifier.
• disableRules - An array of rules to disable.
• email - The name of the input field for the user's email. Disable email checking with false.
• expectedCountries - An array of expected countries as 2-letter codes (ISO 3166-1 alpha-2).
• expectedLanguages - An array of expected languages as 2-letter codes (ISO 639 alpha-2).
• fields - An array of input names to send to the spam filter.
• ipAddress - The user's IP is detected automatically but can be overridden or disabled with false.
• text - The text to classify. An array of strings can also be submitted.
• timeZone - The user's timezone is detected automatically but can be overridden or disabled with false.

To include the email field into fields (for easier server-side verification), configure the list of input names using the spamfilter.fields: string[] option.

Exclude Inputs from Spam Checking

By default, all text inputs and textareas within the parent form are spam-checked. To exclude a specific input, add the data-no-spamfilter attribute. Alternatively, explicitly list the checked fields using the fields config option.

Contributing

See Contributing Guide and please follow our Code of Conduct.

License

MIT