Password protect posts in WordPress programmatically.

password-protect-posts.php

<?php

add_action( 'the_post', 'myprefix_password_protect_post_type' );
/**
 * Add master password to post type.
 *
 * This function password protects all posts in
 * a post type with the same master password.
 *
 * @since 1.0.0
 *
 * @param WP_Post $post The Post object (passed by reference).
 */
function myprefix_password_protect_post_type( $post ) {

    // Bail if current post is not from specified post type.
    if ( 'page' !== $post->post_type ) {
        return;
    }

    // Assign master password string.
    $post->post_password = 'my-password';

}

add_action( 'the_post', 'myprefix_password_protect_category' );
/**
 * Add master password to a post category.
 *
 * This function password protects all posts in a
 * category with the same master password.
 *
 * @since 1.0.0
 *
 * @param WP_Post $post The Post object (passed by reference).
 */
function myprefix_password_protect_category( $post ) {

    // Bail if current post is not from specified category.
    if ( ! in_category( array( 'uncategorized' ) ) ) {
        return;
    }

    // Assign master password string.
    $post->post_password = 'my-password';

}