1. Create a sitekey
Sign in to your account, open "Sitekeys"
and create one for your website.
The sitekey is public.
Optionally restrict it to your origins, choose a difficulty and the widget mode.
2. Install the widget
Add the stylesheet and the script to your page.
Both are served by your CDPCaptcha installation;
there is no CDN.
You may also copy the two files to your own server.
<link rel="stylesheet" href="https://cdpcaptcha.com/core/assets/css/cdp-captcha.css">
<script src="https://cdpcaptcha.com/core/assets/js/cdp-captcha.js" defer></script>Then put one element with the class cdp-captcha inside the form you want to protect:
<form method="post" action="/contact">
<input name="email" type="email" required>
<div class="cdp-captcha" data-sitekey="YOUR_SITEKEY"></div>
<button type="submit">Send</button>
</form>The script mounts a widget on every such element.
When the check completes, a hidden field named cdp-captcha-solution is added to the form.
If the visitor submits before the widget is done, the widget finishes first and then submits the form.
3. Verify the response on your server
Create an API key under "API keys".
Then, when the form arrives, POST the field value to the siteverify endpoint.
Without this step there is no protection.
curl -X POST 'https://cdpcaptcha.com/api/v1/siteverify' \
-H 'X-API-Key: ck_YOUR_API_KEY' \
-H 'Content-Type: application/json' \
-d '{"response": "VALUE_OF_cdp-captcha-solution", "sitekey": "YOUR_SITEKEY"}'A valid response answers with success: true.
Any other answer means the form must be rejected:
{
"success": true,
"data": {
"verification_id": "cv_3f9a…",
"challenge": { "solved_at": "2026-10-13T10:12:04Z", "issued_at": "2026-10-13T10:11:58Z", "sitekey": "YOUR_SITEKEY", "difficulty_bits": 14 }
}
}Ready-made client code for PHP, Node, Python and others is on the examples page.