Add the sample code below to your child theme's functions.php file or in a site specific plugin.
<?php
//* Do NOT include the opening php tag
//* Add secure processing message to checkout for Easy Digital Downloads
function edd_checkout_secure_messaging() {
?>
<p style="font-size:small;text-align:center;padding-top:15px;"><span class="dashicons dashicons-lock"></span> Your transaction will be securely processed.</p>
<?php
}
add_action( 'edd_purchase_form_after_submit', 'edd_checkout_secure_messaging' );
Preview: http://cl.ly/2c461Y260C3I