Update CastSocket connection flow to check for receiver credentials.

chrome/browser/extensions/api/cast_channel/cast_channel.proto

 // Copyright 2013 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package extensions.api.cast_channel;
 
@@ skipping 35 matching lines @@
     BINARY = 1;
   }
   required PayloadType payload_type = 5;
 
   // Depending on payload_type, exactly one of the following optional fields
   // will always be set.
   optional string payload_utf8 = 6;
   optional bytes payload_binary = 7;
 }
 
+// Messages for authentication protocol between a sender and a receiver.
+message AuthChallenge {
+}
+
+message AuthResponse {
+  required bytes signature = 1;

[akalin, 2013/10/26 23:33:37]

another problem -- required fields in protobufs are almost always a bad idea

+  required bytes client_auth_certificate = 2;
+}
+
+message AuthError {
+  enum ErrorType {
+    INTERNAL_ERROR = 0;
+    NO_TLS = 1;  // The underlying connection is not TLS
+  }
+  required ErrorType error_type = 1;
+}
+
+message DeviceAuthMessage {
+  // Request fields
+  optional AuthChallenge challenge = 1;
+  // Response fields
+  optional AuthResponse response = 2;
+  optional AuthError error = 3;
+}