Saturday, November 26, 2011

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 KASME
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

 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



  1. Thanks for this. Now I understood the user authentication side but what about the Network side.
    How is the network authenticated?

  2. Need help to understand use of Op and OPc key use in AuC. Network authenticate USIM by comparing card's RES with its XRES. XRES = RAND(f2)Ki. What is OPc/OP used for by the AuC? I know the AuC generates XMAC-A which will compa