LTE: Authentication Request

Ø    The purpose of the EPS authentication and key agreement (AKA) procedure is to provide mutual authentication between the user and the network and to agree on a key K_ASME

Ø    The EPS AKA procedure is always initiated and controlled by the network. However, the UE can reject the EPS authentication challenge sent by the network. The UE shall proceed with an EPS authentication challenge only if an USIM is present

Ø    When a NAS signalling connection exists, the network can initiate an authentication procedure at any time. The network initiates the authentication procedure by sending an AUTHENTICATION REQUEST message to the UE

Ø    The MME sends unciphered AUTHENTICATION REQUEST message to the UE which includes a random number RAND and an authentication parameter AUTN. Now the UE’s job is to compute the authentication response parameter RES and send it back to the MME in AUTHENTICATION RESPONSE message

Ø    The value of RES is computed by the USIM using RAND, AUTN and the secret key ‘K’ which is stored in the USIM.

Ø    The IE Authentication parameter RAND (EPS challenge) will carry the RAND of length 128-bits. It provides the MS with a non-predictable number to be used to calculate the authentication response parameter RES

Ø    The IE Authentication parameter AUTN (EPS challenge) will carry the AUTN of length 128-bits. It provides the MS with a means of authenticating the network. The AUTN consists of (SQN xor AK)||AMF||MAC  = 48 + 16 + 64  = 128-bits. In the AUTHENTICATION REQUEST example below, AUTN value = 6e323b36c46c5555a3df0e6e323b6391 which means that, 

            SQN xor AK = 6e323b36c46c

                        AMF: 5555 
                        MAC: a3df0e6e323b6391

Abbreviations:

 AMF – Authentication Management Field

 AK – Anonymity Key

 ASME – Access Security Management Entity

 AUTN – Authentication Token

 MAC – Message Authentication Code

 RAND – RANDom number

 SQN – Sequence Number

Reference: 3GPP TS 24.301

Example: AUTHENTICATION REQUEST