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HashCash – Proof of Work CAPTCHA

Invisible Proof-of-Work CAPTCHA. No checkboxes, no image grids, no Google, no tracking. Just spam protection that gets out of the way.

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Are you tired of anti-spam plugins that bloat your site, inject third-party tracking, or constantly upsell you on premium SaaS subscriptions? Are you looking for a straightforward, lightweight solution that just protects your comments without the extra baggage?
Then Cardea is the right tool for you.
Cardea is a radically simple, zero-dependency Proof-of-Work (PoW) comment spam protector. Developed transparently on GitHub as a purely open-source project, it does one thing and does it perfectly: it stops automated bot spam on native WordPress comments.
<h3>Why Choose Cardea?</h3>
Cardea offers distinct advantages for site owners who value simplicity, privacy, and performance:
<ol>
<li>
Hyper-Focused & Zero Bloat – Cardea is strictly dedicated to the native WordPress comment system. Unlike multi-purpose anti-spam plugins that inject heavy compatibility layers for various form builders and e-commerce platforms, Cardea remains extremely lightweight and performant—protecting only what you need protected.
</li>
<li>
100% Standalone & Sovereign – No external API keys. No commercial SaaS tiers. No phone-home telemetry. Cardea is entirely self-hosted and self-contained. Your comment protection never depends on a third-party service staying alive.
</li>
<li>
Strict Privacy (GDPR Compliant) – Because the Proof-of-Work computation happens locally in each visitor’s browser, there are no tracking cookies, no user profiles, and no third-party data transfers. Unlike cloud-based CAPTCHA solutions, Cardea transmits nothing to external servers—making it inherently GDPR-friendly.
</li>
<li>
Reduced Attack Surface – By doing one thing perfectly (protecting native comments), Cardea avoids the security vulnerabilities inherent in massive, multi-ecosystem integrations. A focused codebase means fewer CVEs and tighter security.
</li>
<li>
Plug-and-Play Simplicity – No complex routing rules. No integration toggles. No configuration mazes. Users simply activate Cardea and their discussion threads are protected immediately.
</li>
</ol>
To view the source code, contribute, or report issues, visit the <a href="https://github.com/bornmw/cardea" rel="nofollow ugc">Cardea GitHub Repository</a>.
<h3>How It Works</h3>
<ol>
<li>Challenge Generation: When a page with a comment form loads, the server generates a cryptographically signed challenge using HMAC-SHA256. No database write occurs at this stage.</li>
<li>Client-Side Mining: When a user focuses on the comment textarea, a JavaScript Web Worker begins mining in the background.</li>
<li>Solution Discovery: The worker repeatedly hashes the challenge string (nonce + timestamp + salt) with incrementing counter values until it finds a hash with the required number of leading zeros.</li>
<li>Server Verification: On submission, the server first verifies the HMAC signature (ensuring the challenge wasn’t tampered with), then validates the PoW solution, and finally stores a transient to prevent replay attacks.</li>
</ol>
<h3>Features</h3>
<ul>
<li>Zero Database Bloat on Load – Challenges are generated using stateless HMAC signatures, meaning the plugin requires exactly zero database writes when a visitor loads a page.</li>
<li>Zero Dependencies – No external APIs or services required.</li>
<li>Client-Side Mining – Heavy computation happens in the user’s browser using Web Workers.</li>
<li>Deferred Execution – The cryptographic mining engine only spins up when a user interacts with the comment field, ensuring casual readers incur zero performance penalty.</li>
<li>Self-Cleaning Replay Protection – Server-side state is only stored upon a successful comment submission to prevent bot replay attacks, and expired tokens are automatically swept by WordPress cron.</li>
<li>Server-Side Verification – Server verifies HMAC signature first, then performs SHA-256 PoW validation.</li>
<li>Configurable Difficulty – Adjust the number of leading zeros required (1-8).</li>
<li>Configurable Time Window – Set how long challenges remain valid (5-120 minutes).</li>
<li>Non-Intrusive – Works transparently for legitimate users; spammers must complete the PoW challenge.</li>
<li>WordPress Standards – Follows WordPress coding standards and best practices.</li>
<li>Privacy First (GDPR Friendly) – No cookies, no user tracking, no CAPTCHA popups, and absolutely zero data sent to third-party cloud APIs.</li>
<li>Smart Pathway Protection – Flawlessly protects frontend forms and blocks XML-RPC botnets, while seamlessly allowing native Trackbacks and authenticated REST API requests.</li>
<li>Page Caching Compatible – Uses dynamic REST API endpoint to fetch fresh challenges, ensuring compatibility with edge caching (Cloudflare, Varnish) and full-page caching plugins.</li>
<li>Logged-In User Bypass – Skips PoW challenge for authenticated users, eliminating unnecessary CPU usage on the frontend.</li>
</ul>
<h3>Architecture & Testing</h3>
Cardea is built with an enterprise-grade engineering stack focused on reliability and performance:
Frontend Architecture: 
* Zero-dependency JavaScript using native Web Crypto APIs (crypto.subtle) 
* Web Workers for background cryptographic mining (non-blocking UI) 
* Dynamic challenge fetching via REST API (compatible with page caching) 
* Skip PoW for logged-in users (zero CPU overhead for authenticated commenters)
Backend Architecture: 
* Localized replay protection using WordPress transients 
* Auto-cleaning expired tokens via WordPress cron 
* Single verification pass: signature check + PoW validation
Testing Stack: 
* PHPUnit – Backend logic verification (HMAC generation, challenge validation, replay prevention) 
* Jest – Cryptographic worker validation (difficulty checking, solution finding, message interface) 
* Playwright – End-to-End browser testing integrated with WordPress Playground (full WordPress environment)
This comprehensive testing approach ensures the plugin handles legitimate users seamlessly while actively blocking sophisticated bot attacks.
<h3>Developer Rigor</h3>
Cardea is built with an enterprise-grade engineering stack focused on reliability and performance:
Architecture: 
* Zero Database Bloat on Load – Stateless HMAC signatures ensure zero database writes on page load 
* Self-Cleaning Replay Protection – Uses WordPress transients that auto-expire via cron 
* Deferred Execution – Mining only starts when user interacts with comment field
Testing Stack: 
* PHPUnit – Backend logic verification (HMAC generation, challenge validation, replay prevention) 
* Jest – Cryptographic worker validation (difficulty checking, solution finding, message interface) 
* Playwright – End-to-End browser testing integrated with WordPress Playground (full WordPress environment)
Cross-Theme Compatibility: 
* Uses HTMLFormElement.prototype.submit.call() to bypass DOM clobbering issues 
* Graceful fallback for browsers without Web Worker support

Cardea – Proof-of-Work Comment Spam Protection

Lightweight, zero-dependency Proof-of-Work anti-spam protection for WordPress comments.

HashCash – Proof of Work CAPTCHA vs Cardea – Proof-of-Work Comment Spam Protection