diff --git a/core/lib/Drupal/Core/Config/ConfigObject.php b/core/lib/Drupal/Core/Config/ConfigObject.php
index 0884e9a..583af92 100644
--- a/core/lib/Drupal/Core/Config/ConfigObject.php
+++ b/core/lib/Drupal/Core/Config/ConfigObject.php
@@ -214,11 +214,32 @@ class ConfigObject {
    * Saves the configuration object.
    */
   public function save() {
+    $this->sortByKey($this->data);
     $this->storageManager->selectStorage('write', $this->name)->write($this->name, $this->data);
     return $this;
   }
 
   /**
+   * Sorts all keys in configuration data.
+   *
+   * Ensures that re-inserted keys appear in the same location as before, in
+   * order to ensure an identical order regardless of storage controller.
+   * A consistent order is important for any storage that allows any kind of
+   * diff operation.
+   *
+   * @param array $data
+   *   An associative array to sort recursively by key name.
+   */
+  public function sortByKey(array &$data) {
+    ksort($data);
+    foreach ($data as &$value) {
+      if (is_array($value)) {
+        $this->sortByKey($value);
+      }
+    }
+  }
+
+  /**
    * Deletes the configuration object.
    */
   public function delete() {
diff --git a/core/modules/config/lib/Drupal/config/Tests/ConfigCRUDTest.php b/core/modules/config/lib/Drupal/config/Tests/ConfigCRUDTest.php
index 5e5b926..b8848b1 100644
--- a/core/modules/config/lib/Drupal/config/Tests/ConfigCRUDTest.php
+++ b/core/modules/config/lib/Drupal/config/Tests/ConfigCRUDTest.php
@@ -85,4 +85,31 @@ class ConfigCRUDTest extends WebTestBase {
     // Verify config() returned the existing config instance.
     $this->assertIdentical($new_config, $config);
   }
+
+  /**
+   * Tests Drupal\Core\Config\ConfigObject::sortByKey().
+   */
+  function testDataKeySort() {
+    $config = config('config_test.keysort');
+    $config->set('new', 'Value to be replaced');
+    $config->set('static', 'static');
+    $config->save();
+    // Clone this ConfigObject, so this test does not rely on any particular
+    // architecture.
+    $config = clone $config;
+
+    // Load the configuration data into a new object.
+    $new_config = config('config_test.keysort');
+    // Clear the 'new' key that came first.
+    $new_config->clear('new');
+    // Add a new 'new' key and save.
+    $new_config->set('new', 'Value to be replaced');
+    $new_config->save();
+
+    // Verify that the data of both objects is in the identical order.
+    // assertIdentical() is the required essence of this test; it performs a
+    // strict comparison, which means that keys and values must be identical and
+    // their order must be identical.
+    $this->assertIdentical($new_config->get(), $config->get());
+  }
 }