- Patch #28420 by Jeremy: provide a more generic interface that can be used

  to validate other form submissions, not just comments. Two new functions
  are introduced, form_token() and form_validate(). The first function uses
  a private key and a public key to set a token in a hidden field. The second
  function validates the token. The comment and contect module are updated to
  use these functions.

includes/common.inc

@@ -1039,6 +1039,53 @@ function form($form, $method = 'post', $action = NULL, $attributes = NULL) {
   return '<form action="'. check_url($action) .'" method="'. $method .'"'. drupal_attributes($attributes) .">\n<div>". $form ."\n</div></form>\n";
 }
 
+/**
+ * Set a hidden 'form_token' field to be included in a form, used to validate
+ * that the resulting submission was actually generated by a local form.
+ *
+ * @param $key
+ *   A unique key to identify the form that is currently being displayed.
+ *   This identical key is later used to validate that the resulting submission
+ *   actually originated with this form.
+ * @result
+ *   A themed HTML string representing the hidden token field.
+ */
+function form_token($key) {
+  // this private key should always be kept secret
+  if (!variable_get('drupal_private_key', '')) {
+    variable_set('drupal_private_key', mt_rand());
+  }
+
+  // the verification token is an md5 hash of the form key and our private key
+  return form_hidden('form_token', md5($key . variable_get('drupal_private_key', '')));
+}
+
+/**
+ * Verify that the hidden 'form_token' field was actually generated with our
+ * private key.
+ *
+ * @param $edit
+ *  An array containing the form that needs to be validated.
+ * @param $key
+ *  The same key that was used to generate the 'form_token'.
+ * @param $error_message
+ *  An optional error message to display if the form does not validate.
+ * @result
+ *  There is nothing returned from this function, but if the 'form_token' does
+ *  not validate an error is generated, preventing the submission.
+ */
+function form_validate($edit, $key, $error_message = NULL) {
+  if ($error_message == NULL) {
+    // set a generic default error message
+    $error = t('Validation error, please try again.  If this error persists, please contact the site administrator.');
+  }
+
+  if ($edit['form_token'] != md5($key . variable_get('drupal_private_key', ''))) {
+    // setting this error will cause the form to fail validation
+    form_set_error('form_token', $error);
+  }
+}
+
 /**
  * File an error against the form element with the specified name.
  */