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

chromeos/cryptohome/cryptohome_parameters.h

Index: chromeos/cryptohome/cryptohome_parameters.h
diff --git a/chromeos/cryptohome/cryptohome_parameters.h b/chromeos/cryptohome/cryptohome_parameters.h
index 0b744a42416b2c49a98e05c5e3c08f177eb86f22..13a2ed56cd0c4adcc861231b2346802fb800cb0f 100644
--- a/chromeos/cryptohome/cryptohome_parameters.h
+++ b/chromeos/cryptohome/cryptohome_parameters.h
@@ -11,6 +11,7 @@
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "chromeos/chromeos_export.h"
+#include "components/user_manager/user_id.h"
 
 namespace cryptohome {
 
@@ -25,11 +26,11 @@ enum AuthKeyPrivileges {
 
 // 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.