3.9.6.2.3 User Identity Confidentiality The identity of the user is to be protected to prevent unlawful use of privileges. Temporary identities are assigned and used wherever possible to avoid unnecessary exchange of permanent identities between entities. Some temporary identities are M-temporary mobile subscriber identity (M-TMSI), which is used to identify the UE within the MME, the S-TMSI, which is constructed from the MME code and the M-TMSI, used for paging the UE, and the globally unique temporary UE identity (GUTI), allocated by the MME with two components, one uniquely identifying the MME that allocated the GUTI and the 21 Forward security has been proposed as a method to minimize the consequences of key exposure. Next Generation Mobile Communications Ecosystem other uniquely identifying the UE within that MME. Apart from the temporary identities, the permanent identities (IMSI and IMEI) are stored securely. There is one allowed security breach: if the MME queries for the UEs IMSI in the Identity Request message, the UE should send it, even if security is not configured. 3.9.6.2.4 Data Protection Once the UE and network have completed their mutual authentication, they can start communicating in a secure manner, using ciphering and integrity protection. Ciphering is applied on signaling messages (in both NAS and AS) and user plane data (at AS). Integrity protection is applied to all signaling messages at both the NAS and AS levels. Integrity protection is not applied to user plane data because it would become too much of an overhead at the packet level, impacting the data rates. While the Ck and Ik are assigned in the AKA procedure, the ciphering and integrity algorithms and other inputs to these algorithms are communicated in the security mode command (SMC) procedure. There are two SMC procedures defined, one at the NAS level and the other one as the AS level. The NAS SMC procedure is triggered by the MME immediately after the AKA authentication. The AS SMC procedure is triggered by the eNode-B by sending