receiving, by a terminal, an authentication request message from an authentication function; and

in a case where authentication on the authentication request message fails, feeding back, by the terminal, an authentication failure message to the authentication function, wherein in a case where a cause of authentication failure is a Message Authentication Code (MAC) failure and in cases where a cause of authentication failure is a Synchronization (Sync) failure, the terminal feeds back authentication failure messages of the same type to the authentication function;

wherein in the case where the cause of the authentication failure is the MAC failure and in the case where the cause of the authentication failure is the Sync failure, the terminal feeds back the authentication failure messages of the same type to the authentication function, comprising:

in the case where the cause of the authentication failure is the MAC failure and in the case where the cause of the authentication failure is the Sync failure, the authentication failure messages do not carrying a failure cause parameter, or all carry the same failure cause parameter.