Improve message about insecure S3 settings (#116956)

* Improve message about insecure S3 settings

Clarifies that insecure settings are stored in plaintext and must not be
used. Also removes the mention of the (wrong) system property from the
error message if insecure settings are not permitted.

Backport of #116915 to `7.17`

* CI poke

docs/changelog/116915.yaml

@@ -0,0 +1,5 @@
+pr: 116915
+summary: Improve message about insecure S3 settings
+area: Snapshot/Restore
+type: enhancement
+issues: []

plugins/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3Repository.java

@@ -267,8 +267,7 @@ class S3Repository extends MeteredBlobStoreRepository {
             deprecationLogger.critical(
                 DeprecationCategory.SECURITY,
                 "s3_repository_secret_settings",
-                "Using s3 access/secret key from repository settings. Instead "
-                    + "store these in named clients and the elasticsearch keystore for secure settings."
+                INSECURE_CREDENTIALS_DEPRECATION_WARNING
             );
         }
 
@@ -285,6 +284,10 @@ class S3Repository extends MeteredBlobStoreRepository {
         );
     }
 
+    static final String INSECURE_CREDENTIALS_DEPRECATION_WARNING =
+        "This repository's settings include a S3 access key and secret key, but repository settings are stored in plaintext and must not "
+            + "be used for security-sensitive information. Instead, store all secure settings in the keystore.";
+
     private static Map<String, String> buildLocation(RepositoryMetadata metadata) {
         return org.elasticsearch.core.Map.of(
             "base_path",

plugins/repository-s3/src/test/java/org/elasticsearch/repositories/s3/RepositoryCredentialsTests.java

@@ -108,12 +108,11 @@ public void testRepositoryCredentialsOverrideSecureCredentials() {
         assertThat(credentials.getAWSSecretKey(), is("insecure_aws_secret"));
 
         assertWarnings(
+            "[access_key] setting was deprecated in Elasticsearch and will be removed in a future release!"
+                + " See the breaking changes documentation for the next major version.",
             "[secret_key] setting was deprecated in Elasticsearch and will be removed in a future release!"
                 + " See the breaking changes documentation for the next major version.",
-            "Using s3 access/secret key from repository settings. Instead store these in named clients and"
-                + " the elasticsearch keystore for secure settings.",
-            "[access_key] setting was deprecated in Elasticsearch and will be removed in a future release!"
-                + " See the breaking changes documentation for the next major version."
+            S3Repository.INSECURE_CREDENTIALS_DEPRECATION_WARNING
         );
     }
 
@@ -194,12 +193,11 @@ public void testReinitSecureCredentials() {
 
         if (hasInsecureSettings) {
             assertWarnings(
+                "[access_key] setting was deprecated in Elasticsearch and will be removed in a future release!"
+                    + " See the breaking changes documentation for the next major version.",
                 "[secret_key] setting was deprecated in Elasticsearch and will be removed in a future release!"
                     + " See the breaking changes documentation for the next major version.",
-                "Using s3 access/secret key from repository settings. Instead store these in named clients and"
-                    + " the elasticsearch keystore for secure settings.",
-                "[access_key] setting was deprecated in Elasticsearch and will be removed in a future release!"
-                    + " See the breaking changes documentation for the next major version."
+                S3Repository.INSECURE_CREDENTIALS_DEPRECATION_WARNING
             );
         }
     }
@@ -239,10 +237,7 @@ public void sendResponse(RestResponse response) {
             throw error.get();
         }
 
-        assertWarnings(
-            "Using s3 access/secret key from repository settings. Instead store these in named clients and"
-                + " the elasticsearch keystore for secure settings."
-        );
+        assertWarnings(S3Repository.INSECURE_CREDENTIALS_DEPRECATION_WARNING);
     }
 
     private void createRepository(final String name, final Settings repositorySettings) {

server/src/main/java/org/elasticsearch/common/settings/SecureSetting.java

@@ -182,9 +182,7 @@ private InsecureStringSetting(String name) {
         @Override
         public SecureString get(Settings settings) {
             if (ALLOW_INSECURE_SETTINGS == false && exists(settings)) {
-                throw new IllegalArgumentException(
-                    "Setting [" + name + "] is insecure, " + "but property [allow_insecure_settings] is not set"
-                );
+                throw new IllegalArgumentException("Setting [" + name + "] is insecure, use the elasticsearch keystore instead");
             }
             return super.get(settings);
         }