arnesonium

This plugin uses <a href="http://openpgpjs.org/" rel="nofollow ugc">OpenPGP.js</a> to provide public key encryption for a 
textarea. It is most useful for any kind of text area that will be 
submitted via email or over an unsecured network connection.
The GitHub repository for this plugin is located at https://github.com/pymander/wordpress-openpgp
<h4>Usage</h4>
This plugin provides a simple shortcode which you can add to your 
forms. To use the shortcode, you must first upload your ASCII-armored 
public key to your blog’s media section. Note that the public key must 
reside on the same server as your blog.
The <code>cryptbutton</code> shortcode takes the following arguments.
keyid 
The media ID of your ASCII-armored public key. Either this or <code>keyurl</code> 
are required.
keyurl 
The URL for your ASCII-armored public key. Either this argument or 
 keyid are required.
textarea 
Optional. The HTML ID for the textarea element to be encrypted. When 
this is omitted, the plugin will try to find the correct textarea 
automatically.
class 
Optional. Specify additional CSS classes for the button element.
text 
Optional. Specify the button text. This defaults to “Encrypt”. You can 
also use the shortcode as an open/close tag, and the contents will be 
used as the button text.
<h4>Example</h4>
This example uses the <a href="http://jetpack.me/" rel="nofollow ugc">Jetpack for WordPress</a> contact form. You can 
see an example of the output on my <a href="http://arnesonium.com/contact/" rel="nofollow ugc">Contact page</a>. The WordPress code 
looks something like this:
<pre><code>[contact-form subject='ARNESONIUM CONTACT']
[contact-field label='Name' type='name' required='1'/]
[contact-field label='Email' type='email' required='1'/]
[contact-field label='Phone' type='text'/]
[contact-field label='Comment' type='textarea' required='1'/]
[cryptbutton keyid=42]Encrypt[/cryptbutton]
[/contact-form]
</code></pre>
Line 6 displays the cryptbutton usage. Note that I changed some 
elements of this example to make things clearer. You will need to play 
with layout and CSS to get things looking nice.
<h4>Contact Form 7</h4>
This plugin also adds a <code>cryptbutton</code> shortcode to 
<a href="http://contactform7.com/" rel="nofollow ugc">Contact Form 7</a>. Shortcodes use a slightly 
different syntax with CF7. All of the options are still available, but 
the example above would be used in a form like this:
<pre><code>
[cryptbutton keyid:42 "Encrypt"]
[submit "Send"]

</code></pre>

OpenPGP Form Encryption for WordPress

OpenPGP public key encryption for any textarea with a shortcode button.

wp2pgpmail

With PGP Key Generator, your visitors can generate their own private and public PGP keys. It is also possible to use the plugin to encrypt and unencrypt a PGP message.
Check <a href="https://wp2pgpmail.com" rel="nofollow ugc">https://wp2pgpmail.com</a> for more info.
Is it secure ?
<ul>
<li>All code is implememented in readable Javascript.</li>
<li>You can verify the source code.</li>
<li>No binaries are loaded from a server or used embedded.</li>
<li>No hidden transfer of plain text.</li>
</ul>
This plugin is based on the tool developed by <a href="http://ianpurton.com/" rel="nofollow ugc">Ian Purton</a>.

PGP Key Generator

A plugin to generate private and public PGP keys. No need to install any software to encrypt and decrypt PGP messages.

OpenPGP Form Encryption for WordPress vs PGP Key Generator