diff --git a/src/api2/admin/datastore.rs b/src/api2/admin/datastore.rs
index 81ca02eb..fe72ea32 100644
--- a/src/api2/admin/datastore.rs
+++ b/src/api2/admin/datastore.rs
@@ -67,7 +67,10 @@ fn read_backup_index(
 
     result.push(BackupContent {
         filename: MANIFEST_BLOB_NAME.to_string(),
-        crypt_mode: Some(CryptMode::None),
+        crypt_mode: match manifest.signature {
+            Some(_) => Some(CryptMode::SignOnly),
+            None => Some(CryptMode::None),
+        },
         size: Some(index_size),
     });
 
diff --git a/src/backup/manifest.rs b/src/backup/manifest.rs
index 6af110d1..56f6cf42 100644
--- a/src/backup/manifest.rs
+++ b/src/backup/manifest.rs
@@ -72,6 +72,7 @@ pub struct BackupManifest {
     files: Vec<FileInfo>,
     #[serde(default="empty_value")] // to be compatible with < 0.8.0 backups
     pub unprotected: Value,
+    pub signature: Option<String>,
 }
 
 #[derive(PartialEq)]
@@ -105,6 +106,7 @@ impl BackupManifest {
             backup_time: snapshot.backup_time().timestamp(),
             files: Vec::new(),
             unprotected: json!({}),
+            signature: None,
         }
     }