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Adaptive Login Form: Adjusting compromise between Comfort and Paranoia.
Conception: 
If my current IP address is not marked as Dangerous since my last successful login, then there is no need to distrust me and force me to go through Quests to solve different types of Captchas. 
In this case, the standard “Password” field is sufficient for one attempt.
But if the Attempt is unsuccessful, then we mark the IP address as Dangerous, and then it is possible and necessary to trick me (or the one who is trying to be me) with a more thorough login procedure.
There may be multilevel options. It doesn’t matter (this will be gradually added to the functionality). We are now talking about the General Principle.
Separate statistics are generated for each IP address and the ratio “Successful number of entries” / “Total number of entries” is determined. Depending on how close this parameter is to 100%, we can talk about the need for the Toughness of the Mistrust process.
This mechanism starts before the User enters his Login.
The more Unsuccessful Login attempts occur from a given IP Address, the more thoroughly it is checked. 
Conversely, the Login procedure can be simplified as much as possible if there is no obvious reason.
<h4>Futured</h4>
<ul>
<li>Regardless of what kind of Authentication Error occurred, be it:</li>
<li>Invalid Username;</li>
<li>Invalid User Password;</li>
<li>Incorrectly specified additional security elements: “Secret Key” / Captcha / etc. 
This will not be indicated in the error message. There will always be only one message: “Authentication Failed”. 
Thus, we do not explicitly indicate to the potential Villain / Bot the reason for the denial of access. And the more such Reasons there are, the more complicated the Entry procedure becomes.</li>
<li><a href="https://wordpress.org/plugins/new-users-monitor/" rel="ugc">Integration with “New Users Monitor”</a></li>
</ul>

Adaptive Login Action

Adaptive Login Form: Adjusting compromise between Comfort and Paranoia.

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.

Adaptive Login Action vs Really Simple CAPTCHA