Table of Contents   1. Introduction 2. LTE Security Concept  3. LTE Authentication Procedure 4. Closing and References

 

 

2. LTE Security Concept

 

2.1 Scope and Concept of LTE Security 

 

Figure 1 below shows the scope and overall concept of the LTE Security documents. The scope of these documents will include the following three areas:

 

  LTE Authentication: performs mutual authentication between a UE and a network.

  NAS Security: performs integrity protection/verification and ciphering (encryption/decryption) of NAS signaling between a UE and an MME.

  AS Security

•  performs integrity protection/verification and ciphering of RRC signaling between a UE and an eNB.

•  performs ciphering of user traffic between a UE and an eNB.

 

 

Figure 1. Scope and concept of LTE security

 

LTE Authentication 

 

In mobile communication networks, authentication refers to the process of determining whether a user is an authorized subscriber to the network that he/she is trying to access. Among various authentication procedures available in such networks, EPS AKA (Authentication and Key Agreement) procedure is used in LTE networks for mutual authentication between users and networks.

 

The EPS AKA procedure consists of two steps. First, an HSS (Home Subscriber Server) generates EPS authentication vector(s) (RAND, AUTN, XRES, K_ASME) and delivers them to an MME. Then in the second step, the MME selects one of the authentication vectors and uses it for mutual authentication with a UE and shares the same authentication key (K_ASME) each other. Mutual authentication is the process in which a network and a user authenticate each other. In LTE networks, since the ID of the user's serving network is required when generating authentication vectors, authentication of the network by the user is performed in addition to authentication of the user by the network.

 

ASME (Access Security Management Entity) is an entity that receives top-level key(s), from an HSS, to be used in an access network. In EPS, an MME serves as ASME and K_ASME is used as the top-level key to be used in the access network. The MME, on behalf of an HSS, conducts mutual authentication with a UE using K_ASME. Once mutually authenticated, the UE and MME get to share the same K_ASME as an authentication key.

 

To avoid any possible eavesdropping or manipulation of data across radio links, K_ASME is not delivered to the UE via E-UTRAN. Instead, the MME delivers part of authentication vector to the UE, which uses it to authenticate the network and generates K_ASME as the HSS does.

 

NAS Security

 

NAS security, designed to securely deliver signaling messages between UEs and MMEs over radio links, performs integrity check (i.e., integrity protection/verification) and ciphering of NAS signaling messages. Different keys are used for integrity check and for ciphering. While integrity check is a mandatory function, ciphering is an optional function. NAS security keys, such as integrity key (K_NASint) and ciphering key (K_NASenc), are derived by UEs and MMEs from K_ASME.

 

AS Security

 

AS security is purposed to ensure secure delivery of data between a UE and an eNB over radio links. It conducts both integrity check and ciphering of RRC signaling messages in control plane, and only ciphering of IP packets in user plane. Different keys are used for integrity check/ciphering of RRC signaling messages and ciphering of IP packets. Integrity check is mandatory, but ciphering is optional.

 

AS security keys, such as K_RRCint, K_RRCenc and K_UPenc, are derived from K_eNB by a UE and an eNB. K_RRCint and K_RRCenc are used for integrity check and ciphering of control plane data (i.e., RRC signaling messages), and K_UPenc is used for ciphering of user plane data (i.e., IP packets). Integrity check and ciphering are performed at the PDCP (Packet Data Convergence Protocol) layer.

 

A UE can derive K_eNB from K_ASME. However, since K_ASME is not transferred to an eNB, an MME instead generates K_eNB from K_ASME and forwards it to the eNB.

 

2.2 Overview of LTE Security Procedure

 

Figure 2 shows the overview of LTE security procedure.  displays LTE authentication procedure while  and  demonstrate security setup procedures for NAS and AS respectively. A brief description of each procedure will be given below first. Then, a detailed explanation on the LTE authentication procedures and NAS and AS security setup procedures will be given in Chapter III hereof and again in Part II, LTE Security II, that follows.

 

 

Figure 2. Overview of LTE security procedure

 

 LTE Authentication

 

When a user requests for access to a LTE network, mutual authentication between the user and the network is conducted using EPS AKA procedure. An MME, upon receipt of such request, identifies the user using his/her IMSI and requests authentication vector(s) (AVs) from an HSS¹. The HSS then generates AV(s) using EPS AKA algorithm, AV={RAND, XRES, AUTN_HSS, K_ASME}, and forwards them to the MME.

 

After storing the AVs, the MME selects one of them and uses it to perform mutual authentication with the UE². The MME forwards RAND and AUTN_HSS to the UE, which then computes RES, AUTN_UE and K_ASME using EPS AKA algorithm. The UE now compares its own AUTN_UE and AUTN_HSS received from the MME for network authentication. Once authenticated, RES is forwarded to the MME, which then compares the XRES received from the HSS and the RES received from the UE for user authentication. If the UE and network have authenticated each other, they share the same key K_ASME (K_ASME is not transferred between UE and MME, though).

 

 NAS Security

 

Once the UE and MME have authenticated each other and the same key K_ASME is shared, NAS security setup procedure begins. In this procedure, NAS security keys to be used when delivering NAS signaling messages are derived from K_ASME for secure delivery of these messages. This procedure consists of a round trip of NAS signaling messages (Security Mode Command and Security Mode Complete message), and begins when the MME delivers a Security Mode Command message to the UE.

 

First, the MME selects NAS security algorithms (Alg-ID: Algorithm ID) and uses them to create an integrity key (K_NASint ) and a ciphering key (K_NASenc) from K_ASME. Then, it applies K_NASint  to the Security Mode Command message to generate an NAS message authentication code (NAS-MAC, Message Authentication Code for NAS for Integrity). The MME then delivers the Security Mode Command message including the selected NAS security algorithms and the NAS-MAC to the UE. As the UE does not know the selected encryption algorithm yet, this message is integrity protected only but not ciphered.

 

Upon receiving the Security Mode Command message, the UE verifies the integrity thereof by using the NAS integrity algorithm selected by the MME and uses NAS integrity/ciphering algorithm to generate NAS security keys (K_NASint and K_NASenc) from K_ASME. Then it ciphers the Security Command Complete message with K_NASenc and generates a message authentication code, NAS-MAC with K_NASint to the ciphered message. Now it forwards the ciphered and integrity protected message to the MME with the NAS-MAC included.

 

Once the MME successfully verifies the integrity of the received Security Mode Complete message and has them decrypted using the NAS security keys (K_NASint and K_NASenc), the NAS security setup procedure is completed.

 

Once the NAS security is set up, NAS signaling messages between the UE and the MME are ciphered and integrity protected by the NAS security keys and then securely delivered over radio links. 

 

 AS Security

 

After NAS security setup is finished, AS security setup procedure between a UE and an eNB begins. In this procedure, AS security keys to be used when delivering RRC signaling messages and IP packets are derived from K_eNB for secure delivery of these data. This procedure consists of a round trip of RRC signaling messages (Security Mode Command and Security Mode Complete message), and begins when an eNB delivers Security Mode Command message to the UE.

 

First, the MME calculates K_eNB from K_ASME and delivers it to the eNB, which uses it to perform the AS security setup procedure. The eNB selects AS security algorithms (Alg-ID: Algorithm ID) and uses them to create an integrity key (K_RRCint) and a ciphering key (K_RRCenc), from K_eNB. to be used for RRC signaling messages, and a ciphering key (K_UPenc) to be used in the user plane. Then, it applies K_RRCint to the Security Mode Command message to generate a message authentication code (MAC-I, Message Authentication Code for Integrity). The eNB now delivers the Security Mode Command message including the selected AS security algorithms and the MAC-I to the UE.

 

Upon receiving the Security Mode Command message from the eNB, the UE verifies the integrity thereof by using the AS integrity algorithm selected by the eNB and uses AS integrity/ciphering algorithm to generate AS security keys (K_RRCint, K_RRCenc and K_UPenc). Then it generates a message authentication code, MAC-I, with the RRC integrity key to the Security Command Complete message, and then forwards the message including the MAC-I to the eNB.

 

When the eNB successfully verifies the integrity of the received Security Mode Complete message by using the AS integrity key, the AS security setup procedure is completed.

 

After the AS security is set up, RRC signaling messages between the UE and the eNB are ciphered and integrity protected by the AS security keys, and user IP packets are encrypted and then securely delivered over radio links.