We should use UserID object to identify users instead of username.

chromeos/cryptohome/cryptohome_parameters.h

 // Copyright 2014 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.
 
 #ifndef CHROMEOS_CRYPTOHOME_CRYPTOHOME_PARAMETERS_H_
 #define CHROMEOS_CRYPTOHOME_CRYPTOHOME_PARAMETERS_H_
 
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "chromeos/chromeos_export.h"
+#include "components/user_manager/user_id.h"
 
 namespace cryptohome {
 
 enum AuthKeyPrivileges {
   PRIV_MOUNT = 1 << 0,              // Can mount with this key.
   PRIV_ADD = 1 << 1,                // Can add new keys.
   PRIV_REMOVE = 1 << 2,             // Can remove other keys.
   PRIV_MIGRATE = 1 << 3,            // Destroy all keys and replace with new.
   PRIV_AUTHORIZED_UPDATE = 1 << 4,  // Key can be updated in place.
   PRIV_DEFAULT = PRIV_MOUNT | PRIV_ADD | PRIV_REMOVE | PRIV_MIGRATE
 };
 
 // Identification of the user calling cryptohome method.
 struct CHROMEOS_EXPORT Identification {
-  explicit Identification(const std::string& user_id);
+  explicit Identification(const user_manager::UserID& user_id);
 
   bool operator==(const Identification& other) const;
 
-  std::string user_id;
+  user_manager::UserID user_id;
 };
 
 // Definition of the key (e.g. password) for the cryptohome.
 // It contains authorization data along with extra parameters like permissions
 // associated with this key.
 struct CHROMEOS_EXPORT KeyDefinition {
   enum Type {
     TYPE_PASSWORD = 0
   };
 
@@ skipping 97 matching lines @@
   // used as the backend.
   bool ephemeral;
 
   // If not empty, home dir will be created with these keys if it exist.
   std::vector<KeyDefinition> create_keys;
 };
 
 }  // namespace cryptohome
 
 #endif  // CHROMEOS_CRYPTOHOME_CRYPTOHOME_PARAMETERS_H_