Make crypto store settings immutable

CHANGELOG.md

@@ -35,6 +35,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Support pull-based ingestion message mappers and raw payload support ([#19765](https://github.com/opensearch-project/OpenSearch/pull/19765))
 - Add search API tracker ([#18601](https://github.com/opensearch-project/OpenSearch/pull/18601))
 - Support dynamic consumer configuration update in pull-based ingestion ([#19963](https://github.com/opensearch-project/OpenSearch/pull/19963))
+- Add validation to make crypto store settings immutable ([#20123](https://github.com/opensearch-project/OpenSearch/pull/20123))
 - Cache the `StoredFieldsReader` for scroll query optimization ([#20112](https://github.com/opensearch-project/OpenSearch/pull/20112))
 
 ### Changed

server/src/main/java/org/opensearch/cluster/metadata/MetadataUpdateSettingsService.java

@@ -142,6 +142,7 @@ public void updateSettings(
         validateRefreshIntervalSettings(normalizedSettings, clusterService.getClusterSettings());
         validateTranslogDurabilitySettings(normalizedSettings, clusterService.getClusterSettings(), clusterService.getSettings());
         validateIndexTotalPrimaryShardsPerNodeSetting(normalizedSettings, clusterService);
+        validateCryptoStoreSettings(normalizedSettings, request.indices(), clusterService.state());
         final int defaultReplicaCount = clusterService.getClusterSettings().get(Metadata.DEFAULT_REPLICA_COUNT_SETTING);
 
         Settings.Builder settingsForClosedIndices = Settings.builder();
@@ -589,4 +590,33 @@ public static void validateIndexTotalPrimaryShardsPerNodeSetting(Settings indexS
             );
         }
     }
+
+    /**
+     * Validates crypto store settings are immutable after index creation.
+     */
+    public static void validateCryptoStoreSettings(Settings indexSettings, Index[] indices, ClusterState clusterState) {
+        final String storeTypeKey = "index.store.type";
+
+        // Only validate if store.type is being explicitly modified
+        if (!indexSettings.keySet().contains(storeTypeKey)) {
+            return;
+        }
+
+        for (Index index : indices) {
+            String currentStoreType = clusterState.metadata().getIndexSafe(index).getSettings().get(storeTypeKey, "");
+            String newStoreType = indexSettings.get(storeTypeKey);
+
+            // Prevent changing FROM cryptofs to anything else (including null)
+            if ("cryptofs".equals(currentStoreType) && !"cryptofs".equals(newStoreType)) {
+                throw new IllegalArgumentException(
+                    "Cannot change store type from 'cryptofs' for index [" + index.getName() + "] - cryptofs store type is immutable"
+                );
+            }
+
+            // Prevent changing TO cryptofs from any other type
+            if (!"cryptofs".equals(currentStoreType) && "cryptofs".equals(newStoreType)) {
+                throw new IllegalArgumentException("Cannot change store type to 'cryptofs' for index [" + index.getName() + "]");
+            }
+        }
+    }
 }