CAPTCHA for Email-Addresses and Forms, with PHP examples

BotStop is a CAPTCHA implementation protecting email addresses and forms from robots. To use it you have to:

register on BotStop.org
create a secret key
insert an iframe with the code view into your web page
validate the codes entered by your visitors

To verify that website visitors are not robots, they must enter a code which can be easily recognized by humans, but not by robots. Different measures have been taken to prevent automated robots from breaking this protection. And a rate limiter protects from brute force attacks.

Compared to other solutions, the BotStop architecture offers some advantages. It does not depend on CORS (Cross-Origin Resource Sharing), and is thus not exposed to the corresponding security risks and possible implementation issues. Moreover, no request from your server to the BotStop service is required, allowing for optimal performance.

For a future release, we plan to build a server package which can be downloaded and operated independently, without registering on our server.

Code View

The code view is a small HTML document which can be displayed in an iframe. In the code view request, the code must be encrypted with your secret key. Here is the specification:

URL: https://botstop.org/v1/code?keyId=...&code=...
The first parameter is the id of your secret key. The second parameter must contain an encrypted and URL-encoded random string. In our example source code you can see that this parameter can be generated with two lines of program code. The complete source code of the examples can be downloaded here.
Dimensions of the iframe: width:214px; height:42px;

Use Cases

This page shows some typical use cases and variants of the BotStop protection for links, email addresses and forms.

PHP Source Code

Here you can find PHP implementations with demos of the example use cases. The PHP source code of these examples is also on GitHub.

About us

This service is developed and operated by a small team of independent IT consultants in Switzerland. To contact us, you can use the link in the header of this page.