jojaba

This plugin makes it possible to restrict the access of posts contained in categories by setting a password and giving the impacted categories. The content and the excerpt of these posts are replaced by a password form that the user can fill out to get access. The WordPress generated feeds are modified (the description is replaced by a sentence that you can define).
Here’s the list of the settings (see screenshots for further infos):
<ul>
<li>Choose between session or cookie authentication</li>
<li>Set the password</li>
<li>Check the categories that has to be protected</li>
<li>Check the users granted to access the categories without password</li>
<li>Tell if the excerpt should be displayed or not in resources listing</li>
<li>Display a custom excerpt before password field</li>
<li>Set the info message that display before the password form</li>
<li>Set the placeholder in password field</li>
<li>Set the error message when typing the wrong password</li>
<li>Set the valdation button text</li>
<li>Set the text replacing the feed item description of protected posts</li>
<li>You can style the form using the available classes</li>
</ul>
Availabe languages : English (1.5), French (1.5), Simplified Chinese (thanks to Changmeng Hu, 1.4), German (1.4).
This plugin uses php Sessions (more secure) or a Cookie (less secure) to keep in mind the authenticated users. The password is crypted before it is stored. The regular feed content is filtered to avoid content display of restricted categories posts.

Access Category Password

protection type COOKIE is insecure.

If User set browser COOKIE, he can access protected posts.
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protection type がCOOKIEの場合は安全面に問題があります。

ある名称のCOOKIEをブラウザに埋め込めば保護された投稿にアクセス可能で、それはとても簡単にできます。

The protection type "COOKIE" is deprecated as insecure.

Protects posts in categories by setting a unique Password for all restricted categories.

marcucci

This is a very simple plugin that will check to see if a visitor is logged in before allowed your WordPress site to be displayed. 
If they are not it will display the login page of your site. It will also add text on the login page to explain that the site 
required all visitors to login.

Password Protect Plugin for WordPress

Force all visitors to login before accessing your wordpress site and add a message on the login screen explaining that login is required.

Access Category Password vs Password Protect Plugin for WordPress