Require basic authentication for all pages except home page

functions.php

<?php

/**
 * Require basic authentication for all pages except home page
 * 
 * @return void
 */
function motivast_require_auth() {

	define('AUTH_USER', 'admin');
	define('AUTH_PASS', 'admin');

	header('Cache-Control: no-cache, must-revalidate, max-age=0');

	$is_authenticated = (
		!empty( $_SERVER['PHP_AUTH_USER'] ) && !empty( $_SERVER['PHP_AUTH_PW'] ) &&
		$_SERVER['PHP_AUTH_USER'] == AUTH_USER &&
		$_SERVER['PHP_AUTH_PW']   == AUTH_PASS
	);

	if ( !is_front_page() && !$is_authenticated ) {

		header('HTTP/1.1 401 Authorization Required');
		header('WWW-Authenticate: Basic realm="Access denied"');
		exit;
	}
}

add_action('wp', 'motivast_require_auth');