AU: Sign delta payloads

update_metadata.proto

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Update file format: A delta update file contains all the deltas needed
 // to update a system from one specific version to another specific
 // version. The update format is represented by this struct pseudocode:
 // struct delta_update_file {
 //   char magic[4] = "CrAU";
 //   uint64 file_format_version = 1;
@@ skipping 102 matching lines @@
     // that bsdiff writes with '\0' bytes.
     optional uint64 dst_length = 7;
   }
   repeated InstallOperation install_operations = 1;
   repeated InstallOperation kernel_install_operations = 2;
 
   // (At time of writing) usually 4096
   optional uint32 block_size = 3 [default = 4096];
 
   // If signatures are present, the offset into the blobs, generally
-  // tacked onto the end of the file. We use an offset rather than
-  // a bool to allow for more flexibility in future file formats.
-  // If this is absent, it means signatures aren't supported in this
+  // tacked onto the end of the file, and the length. We use an offset
+  // rather than a bool to allow for more flexibility in future file formats.
+  // If either is absent, it means signatures aren't supported in this
   // file.
   optional uint64 signatures_offset = 4;
+  optional uint64 signatures_size = 5;
 }