Use the reCAPTCHA Enterprise API.

publish/login.php

@@ -16,7 +16,7 @@ if (isset($_SESSION['editor'])) {
 } else if (isset($_POST['login']) && ($wait = login_blocked_for($login_name)) > 0) {
   $denied = TRUE;
   $wait_minutes = (int)ceil($wait / 60);
-} else if (isset($_POST['login']) && !captcha_verify_response()) {
+} else if (isset($_POST['login']) && !captcha_verify_response('login')) {
   $denied = TRUE;
   $captcha_failed = TRUE;
 } else if (isset($_POST['login'])) {
@@ -113,7 +113,7 @@ else if ($denied)
     <input type="checkbox" id="remember" name="remember" checked />
     </div>
 
-    <?= captcha_widget() ?>
+    <?= captcha_widget('login') ?>
 
     <div class="leftcolumn"></div>
     <div class="submission">