ReCaptcha Secure Token

This library provides a PHP implementation of the ReCaptcha v2 Secure Token algorithm.

Usage

You should add this library to your composer require section:

"require": {
  "slushie/recaptcha-secure-token": "~1.0",
  /* ... */
}

From within your PHP code, you can create an instance of the ReCaptchaToken class and pass in your site_key and site_secret values:

$config = ['site_key' => 'YOUR_SITE_KEY', 'site_secret' => 'YOUR_SITE_SECRET'];
$recaptchaToken = new \ReCaptchaSecureToken\ReCaptchaToken($config);

To generate a secure token you must provide a unique session_id:

$sessionId = uniqid('recaptcha');
$secureToken = $recaptchaToken->secureToken($sessionId);

Finally, use this token value in your HTML output. For example:

<div class="g-recaptcha" 
     data-sitekey="YOUR_SITE_KEY"
     data-stoken="<?php echo $secureToken ?>"></div>

Timestamp

Being a time-based protocol, the timestamp must be accurate. If your system clock is not accurate (try ntpdate), you must pass an accurate timestamp (in ms) to secureToken. You can obtain one from an NTP server, e.g.:

$socket = new Bt51\NPM\Socket('0.pool.ntp.org', 123);
$ntp_client = new Bt51\NPM\Client($socket);
$timestamp = $ntp_client->getTime()->getTimestamp() * 1000;

$sessionId = uniqid('recaptcha');
$secureToken = $recaptchaToken->secureToken($sessionId, $timestamp);

Algorithm Implementation

The original ReCaptcha algorithm is undocumented, although example source code is provided in Java.

This implementation is based on the original Java implementation, as well as some resources from around the web. Of important note are the follow:

For more implementation details, please see the source code.

Security Considerations

There are multiple security flaws in the original implementation. Of particular note is the use of the ECB block mode, which is known to be insecure. A simple example of this insecurity is available on the Wikipedia article on Block cipher modes.

This could theoretically lead to spammers acquiring your site_secret. No workaround is provided, the secure token algorithm is inherently insecure.

License and Copyright

This code is provided under the Apache 2.0 license.