Takayuki Miyoshi

Really Simple CAPTCHA does not work alone and is intended to work with other plugins. It is originally created for <a href="https://contactform7.com/" rel="nofollow ugc">Contact Form 7</a>, however, you can use it with your own plugin.
Note: This product is “really simple” as its name suggests, i.e., it is not strongly secure. If you need perfect security, you should try other solutions.
<h4>How does it work?</h4>
Really Simple CAPTCHA does not use PHP “Sessions” for storing states, unlike many other PHP CAPTCHA solutions, but stores them as temporary files. This allows you to embed it into WordPress without worrying about conflicts.
When you generate a CAPTCHA, Really Simple CAPTCHA creates two files for it; one is an image file of CAPTCHA, and the other is a text file which stores the correct answer to the CAPTCHA.
The two files have the same (random) prefix in their file names, for example, “a7hk3ux8p.png” and “a7hk3ux8p.txt.” In this case, for example, when the respondent answers “K5GF” as an answer to the “a7hk3ux8p.png” image, then Really Simple CAPTCHA calculates hash of “K5GF” and tests it against the hash stored in the “a7hk3ux8p.txt” file. If the two match, the answer is confirmed as correct.
<h4>How to use with your plugin</h4>
Note: Below are instructions for plugin developers.
First, create an instance of ReallySimpleCaptcha class:
<pre><code>$captcha_instance = new ReallySimpleCaptcha();
</code></pre>
You can change the instance variables as you wish.
<pre><code>// Change the background color of CAPTCHA image to black
$captcha_instance->bg = array( 0, 0, 0 );
</code></pre>
See really-simple-captcha.php if you are interested in other variables.
Generate a random word for CAPTCHA.
<pre><code>$word = $captcha_instance->generate_random_word();
</code></pre>
Generate an image file and a corresponding text file in the temporary directory.
<pre><code>$prefix = wp_rand();
$captcha_instance->generate_image( $prefix, $word );
</code></pre>
Then, show the image and get an answer from respondent.
Check the correctness of the answer.
<pre><code>$correct = $captcha_instance->check( $prefix, $the_answer_from_respondent );
</code></pre>
If the $correct is true, go ahead. Otherwise, block the respondent — as it would appear not to be human.
And last, remove the temporary image and text files, as they are no longer in use.
<pre><code>$captcha_instance->remove( $prefix );
</code></pre>
That’s all.
If you wish to see a live sample of this, you can try <a href="https://contactform7.com/captcha/" rel="nofollow ugc">Contact Form 7</a>.

Really Simple CAPTCHA

Needed this for an installation with custom forms. I used the plugin developers instructions and was able to make a fully custom implementation based on server side validation within 30 minutes. No more spam bots misusing the forms. Thank you!

Great work - easy to implement

Really Simple CAPTCHA is a CAPTCHA module intended to be called from other plugins. It is originally created for my Contact Form 7 plugin.

Tareq Hasan

weDevs

Nizam Uddin

podpirate

Integrate reCaptcha in your blog. Supports no Captcha as well as old style recaptcha. 
Provides of the box integration for signup, login, comment formsand Ninja Forms as well 
as a plugin API for your own integrations.
<h4>Features:</h4>
<ul>
<li>Secures login, signup und comments with a recaptcha.</li>
<li>Supports old as well as new reCaptcha.</li>
<li>
Works together with
<ul>
<li>WP Multisite</li>
<li>bbPress</li>
<li>BuddyPress</li>
<li>AwesomeSupport (thanks to <a href="http://julienliabeuf.com/" rel="nofollow ugc">Julien Liabeuf</a> )</li>
<li>WooCommerce (Only checkout, registration and login form. Not password reset)</li>
<li><a href="http://ninjaforms.com/" rel="nofollow ugc">Ninja Forms</a></li>
<li>cformsII</li>
</ul>
</li>
<li>
For integration in your self-coded forms see this <a href="https://github.com/tareq1988/wp-recaptcha-integration/wiki/Custom-Themes-and-Forms" rel="nofollow ugc">wiki article</a> for details.
</li>
</ul>
<h4>Localizations</h4>
<ul>
<li>Brazilian Portuguese (thanks to <a href="http://www.viniciusferraz.com" rel="nofollow ugc">Vinícius Ferraz</a>)</li>
<li>Spanish (thanks to <a href="https://github.com/yivi" rel="nofollow ugc">Ivan Yivoff</a>)</li>
<li>Italian (thanks to <a href="http://blog.salaros.com/" rel="nofollow ugc">Salaros</a>)</li>
<li>German</li>
</ul>
Latest Files on GitHub: <a href="https://github.com/tareq1988/wp-recaptcha-integration" rel="nofollow ugc">https://github.com/tareq1988/wp-recaptcha-integration</a>
<h4>Compatibility</h4>
On a WP Multisite you can either activate the plugin network wide or on a single site.
Activated on a single site everything works as usual.
With network activation entering the API key and setting up where a captcha is required 
is up to the network admin. A blog admin can override the API key e.g. when his blog is 
running under his/her own domain name.
<h4>Known Limitations</h4>
<ul>
<li>
You can’t have more than one old style reCaptcha on a page. This is a limitiation of 
reCaptcha itself. If that’s an issue for you, you should use the no Captcha Form.
</li>
<li>
A No Captcha definitely requires client side JavaScript enabled. That’s how it does its 
sophisticated bot detection magic. There is no fallback. If your visitor does not have 
JS enabled the captcha test will not let him through.
</li>
<li>
On a Contact Form 7 when the reCaptcha is disabled (e.g. for logged in users) the field 
label will be still visible. This is due to CF7 Shortcode architecture, and can’t be fixed.
To handle this there is a filter <code>recaptcha_disabled_html</code>. You can return a message for your logged-in 
users here. Check out the <a href="https://github.com/tareq1988/wp-recaptcha-integration" rel="nofollow ugc">GitHub Repo</a> for details.
</li>
<li>
As of version 4.3 CF7 comes with its own recaptcha. Both are supposed to work together. 
I you want to keep the WP ReCaptcha functionality, e.g. if you want to hide the captcha 
from known users, leave the integration in the CF7 settings unconfigured.
</li>
<li>
Old style reCaptcha does not work together with WooCommerce.
</li>
<li>
In WooCommerce the reset password form can not be protected by a captcha. Woocommerce does 
not fire any action in the lost password form, so there is no way for the plugin to hook in. 
Take a look at <a href="https://wordpress.org/support/topic/captcha-not-showing-on-lost-password-page?replies=7" rel="ugc">this thread</a> for a workaround.
</li>
<li>
Due to a lack of filters there is no (and as far as one can see, there will never be) 
support for the MailPoet subscription form.
</li>
</ul>
<h3>Plugin API</h3>
The plugin offers some filters to allow themes and other plugins to hook in.
See <a href="https://github.com/tareq1988/wp-recaptcha-integration" rel="nofollow ugc">GitHub-Repo</a> for details.

ReCaptcha Integration for WordPress

This plugin is very robust. It replaces another reCAPTCHA plugin I was using. I like this one because it works with WooCommerce, and protects login, lost password, and commenting forms.

Very Robust. Easy To Setup

This was the only plugin I could find that added a Captcha to the WooCommerce guest checkout. All the others either need to be paid for or only add a captcha to the login and registration pages. I wish this plugin was more visible so I hadn't wasted time with inferior offerings.

Nice job weDevs and thanks!

Only plugin to add WooCommerce checkout Captcha

reCaptcha for login, signup, comment forms, Ninja Forms and woocommerce.

Really Simple CAPTCHA vs ReCaptcha Integration for WordPress