EAP-SIM is an Extensible Authentication Protocol (EAP) [RFC3748] mechanism for authentication and session key distribution using the Global System for Mobile communications (GSM) Subscriber Identity Module (SIM). [Hence the name !] .
As you all know EAP is an authentication framework which supports multiple authentication methods.
EAP typically runs directly over data link layers such as Point-to-Point Protocol (PPP) or IEEE 802 .
GSM cellular networks use a subscriber identity module (SIM) card to carry out user authentication..
EAP-SIM uses a SIM authentication algorithm between the client and an Authentication, Authorization and Accounting (AAA) server providing mutual authentication between the client and the network.
In EAP-SIM the communication between the SIM card and the Authentication Centre (AuC) replaces the need for a pre-established password between the client and the AAA server.
Note - GSM is a second generation mobile network standard.
Second generation mobile networks and third generation mobile networks use different authentication and key agreement mechanisms.
EAP-AKA specifies an EAP method that is based on the Authentication and Key Agreement (AKA) mechanism used in 3rd generation mobile networks Universal Mobile Telecommunications System (UMTS) and CDMA2000.
Advantages of EAP - AKA
- The use of the AKA also as a secure PPP authentication method in devices that already contain an identity module.
- The use of the 3rd generation mobile network authentication infrastructure in the context of wireless LANs
- Relying on AKA and the existing infrastructure in a seamless way with any other technology that can use EAP.
- To run an EAP-SIM/AKA authentication, a client as wpa_supplicant, which has access to a (U)SIM, is needed.
- The WLAN access point on the network has to support EAP (which is often transcribed as WPA(2)-RADIUS or WPA(2)-ENTERPRISE).
- The WLAN AP/Controller has to have access to a RADIUS server, which handles the authentication and session key generation.
- The RADIUS server in a productive environment needs for EAP-SIM/AKA access to the home location register (HLR) of the MNO where the (U)SIMs are registered .
This is explained in the diagram below
- EAP SIM is based on the authentication and encryption algorithms stored on the Global System for Mobile Communications (GSM) SIM.
- Its based on a challenge-response mechanism and employs a shared secret key, Ki, which is stored on the SIM and otherwise known only to the GSM operator's Authentication Center (AuC).
- When a GSM SIM is given a 128-bit random number (RAND) as a challenge, it calculates a 32-bit response (SRES) and a 64-bit encryption key (Kc)
- EAP SIM also enhances the basic GSM authentication mechanism by providing for mutual authentication between the client and the AAA server
- Secure keyed hashing algorithm, HMAC-SHA1 (one way hashing) .
Sample call flow - EAP SIM
Call flows are exactly same as EAP-SIM with the type AKA. The USIM security algorithms are used in this case.
- GSM - Global System for Mobile communications.
- IMS - International Mobile Subscriber Identifier, used in GSM to identify subscribers.
- MAC - Message Authentication Code .
- HLR - Home Location Register .
- NAI - Network Access Identifier .
- RAND is a 128-bit random challenge issued from the base station to the mobile.
- SRES - The authentication result parameter in GSM, corresponds to the RES parameter in 3G AKA, 32 bits.
- USIM - UMTS Subscriber Identity Module. USIM is an application that is resident on devices such as smart cards distributed by UMTS operators.