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Mautic ALTCHA Plugin

Adds a self-hosted, privacy-friendly ALTCHA (proof-of-work) CAPTCHA field to Mautic forms, built the same way as FireMultimedia's Multi-CAPTCHA bundle (hCaptcha / reCAPTCHA / Turnstile) so it slots into Mautic exactly like those providers - but without ever calling a third-party API.

Why ALTCHA is different from hCaptcha/reCAPTCHA/Turnstile

The other CAPTCHA plugins all POST the visitor's token to a third party (https://api.hcaptcha.com/siteverify, Google, Cloudflare) to find out if the submission was human. Plain, self-hosted ALTCHA never does this. Instead:

1. Mautic generates a small proof-of-work puzzle (a random salt + max number) and signs it with an HMAC secret that only your server knows.
2. The visitor's browser (the <altcha-widget> JS component) brute-forces the answer - a cheap operation for a real browser, a deliberately wasteful one to run at scale for a spam bot.
3. On submit, Mautic re-checks the HMAC signature and recomputes the hash locally. No network call, no cookies, no third-party tracking, GDPR friendly by design.

That's also why the ALTCHA field has no "explicit consent" toggle like the other providers: there's no third-party script or cookie to consent to.

This plugin also supports ALTCHA Sentinel as an optional upgrade - see below.

Self-hosted vs. Sentinel, at a glance

If you fill in both, this plugin prefers Sentinel.

Installation

1. Copy this MauticAltchaBundle folder into plugins/ in your Mautic root (so the path is plugins/MauticAltchaBundle/...), or publish it as a Composer package and run:

2. Make sure altcha-org/altcha (^2.0) is installed - it's declared as a dependency in composer.json, so a plain composer install inside your Mautic root will pull it in.

3. Clear the cache:

4. Go to Settings → Plugins, click Install/Upgrade Plugins. You should see a new "ALTCHA" plugin appear.

Configuration

Go to Settings → Plugins → ALTCHA. You'll see four fields; fill in either the first one, or the last three - not necessarily all four.

Option A: Self-hosted (just an HMAC secret)

Fill in HMAC Secret. This is not an API key - it never leaves your server. Generate a long, random string, for example:

Treat it like any other application secret (don't commit it, don't reuse it across environments). If it ever leaks, anyone who has it could forge valid "human" submissions - though even then they'd only bypass the CAPTCHA, not gain any other access.

Leave the three Sentinel fields blank.

Option B: ALTCHA Sentinel

If you run (or subscribe to) ALTCHA Sentinel, fill in these three fields instead and leave HMAC Secret blank:

1. Sentinel Domain / Base URL - your Sentinel instance's base URL, e.g. https://sentinel.example.com for a self-hosted install, or your ALTCHA Cloud region's URL. No trailing slash, no path - Mautic appends /v1/challenge itself when it builds the widget URL.
2. Sentinel API Key - from Sentinel's API Keys admin section (https://<your-sentinel-domain>/admin → Configuration → API Keys). It looks like key_.... Create a Security Group first if you haven't (that's where you configure Sentinel's actual anti-spam rules), then create an API Key bound to it.
3. Sentinel API Key Secret - shown alongside the API Key when you create it. Unlike the API Key itself, this value never leaves your server.

With Sentinel configured, the widget fetches its challenge directly from your Sentinel instance instead of from Mautic, and gains Sentinel's extra features (Adaptive Captcha, Threat Intelligence, Classifier, rate limiting, etc., depending on how your Security Group is configured). The Complexity and Challenge expiry field settings (see below) are ignored in this mode, since Sentinel controls its own difficulty.

Add the field to a form

Open any Mautic form in the builder and drag in the ALTCHA field. Under its Properties tab you can set:

The field will only appear in the builder once step 1 (HMAC secret) is done - this mirrors how the hCaptcha/reCAPTCHA/Turnstile fields hide themselves until their keys are configured.

How it works internally (for developers)

• Controller/ChallengeController.php - a public, unauthenticated, invokable endpoint (/altcha/challenge) that issues a fresh challenge on every single request. This exists because Mautic caches an entire form's rendered HTML (FormModel::generateHtml()/getContent()) and only regenerates it when the form is saved - not on every page view. A challenge embedded directly into that HTML would be reused by every visitor until the form's next save. Pointing the widget at this URL instead sidesteps that caching entirely, the same way Sentinel mode already works.
• Integration/AltchaIntegration.php - registers the "ALTCHA" integration with getAuthenticationType() === 'none'. Deliberately returns an empty getRequiredKeyFields() - that method's name is literal, and Mautic renders every field it lists as mandatory. Since no single field here is always needed (only "the self-hosted secret, or the full Sentinel set"), all four credential fields are instead added as plain, optional fields via appendToForm() for the "keys" tab, and isConfigured() is overridden with the actual "one set or the other" logic.
• Service/AltchaClient.php - the only class that talks to the altcha-org/altcha library. Self-hosted mode uses its V1 API (AltchaOrg\Altcha\V1\*, the classic flat {algorithm, challenge, salt, signature, maxNumber} challenge shape); Sentinel mode uses AltchaOrg\Altcha\ServerSignature::verifyServerSignature() to check the signature Sentinel embeds in the submitted payload, locally, using the API Key Secret. AltchaClient::buildWidgetChallenge() is the single method the Twig layer calls, and it always returns a URL - our own challenge endpoint in self-hosted mode, or Sentinel's in Sentinel mode.
• Resources/views/Integration/altcha.html.twig - renders <altcha-widget challengeurl="...">. Specifically challengeurl, not challenge - the widget treats those as two different attributes (an inline challenge object vs. a URL to fetch one from), and since this plugin always produces a URL, challengeurl is the correct one unconditionally.

Security notes

• Keep the altcha-org/altcha composer package up to date. A signature- bypass advisory was published for altcha-lib-php (GHSA-82w8-65qw-gch6). This plugin pins ^2.0, which includes the fix; run composer update altcha-org/altcha periodically regardless.
• The HMAC secret and the Sentinel API Key Secret are stored encrypted at rest via Mautic's own mautic.helper.encryption, and masked in the settings UI. The Sentinel API Key itself is not masked, since it's designed to be sent to the visitor's browser as part of the widget's challenge URL and isn't sensitive on its own - only its Secret is.

Compatibility

Mautic 5, 6, and 7 (same AppVersion-based branch selection as the reference Multi-CAPTCHA bundle). PHP 8.1+.

License

GPL-3.0, consistent with the reference plugin this was modeled on.