EAP-SIM and EAP-AKA
acronymnetworkingISP & telecomsecurity
Stands for: EAP methods for GSM SIM (RFC 4186, Type 18) and UMTS Authentication and Key Agreement (RFC 4187, Type 23)
The EAP methods that let a mobile operator's SIM authenticate a device to a network: EAP-SIM (RFC 4186, Type 18) using GSM credentials and EAP-AKA (RFC 4187, Type 23) using the UMTS Authentication and Key Agreement of the USIM.
RFC 4187 (Arkko of Ericsson and Haverinen of Nokia, January 2006, Informational) specifies EAP-AKA, EAP Type 23. The authentication runs the same AKA exchange the mobile network uses: the operator's authentication centre produces a random challenge (RAND) and an authentication token (AUTN), the USIM in the device checks the token and returns a result (RES), and the two sides derive keys; the network is authenticated to the SIM as well as the SIM to the network. The RFC positions it against EAP-SIM of RFC 4186, EAP Type 18, which uses the GSM algorithms, with "substantially longer key lengths and mutual authentication" as the differences. Both offer optional identity privacy through pseudonyms, so the permanent identity is not sent on every connection, and fast re-authentication, so a device that recently authenticated can reconnect without a round trip to the home operator. These are the methods under operator Wi-Fi offload and roaming: the credential is the SIM the subscriber already has, the authentication server is the operator's, and the visited network is a pass-through authenticator like any other 802.1X deployment.
A mobile phone carries a credential that was designed from the start for remote authentication, the SIM, and these two EAP methods let a Wi-Fi network use it. RFC 4187, published in January 2006 by Arkko of Ericsson and Haverinen of Nokia as an Informational document, specifies EAP-AKA at EAP Type 23, the EAP method for the UMTS Authentication and Key Agreement; its companion RFC 4186 specifies EAP-SIM at Type 18 for the older GSM credentials. Both wrap an exchange the mobile network already performs: the operator's authentication centre generates a random challenge, RAND, and in the AKA case an authentication token, AUTN; the (U)SIM in the device computes a response, RES, and session keys; and the server compares. In AKA the token lets the SIM verify that the challenge came from its own operator, so authentication is mutual.
RFC 4187 states the differences from EAP-SIM as "substantially longer key lengths and mutual authentication", which is a polite way of saying that the GSM algorithms had the weaknesses GSM had, and that EAP-AKA is the method operators moved to as USIMs became universal. Both methods add two features on top of the cellular exchange. Identity privacy lets the server issue pseudonyms so that the permanent subscriber identity is not transmitted on every connection, which matters on a radio medium where the first EAP Identity packet is sent in the clear. Fast re-authentication lets a device that authenticated recently reconnect using keys derived from the earlier run, without another round trip to the home operator's authentication centre, which is what makes handover between access points tolerable.
The architecture is pure 802.1X. The access point or controller is a pass-through authenticator; the RADIUS conversation travels, often through proxies and across operator boundaries, to a server that can reach the subscriber's home authentication centre; the visited network learns only that the answer was yes and whatever authorisation attributes the home network chose to send. Everything that makes roaming work at the cellular layer, the home and visited operators and the agreements between them, reappears here at the Wi-Fi layer, and the AAA proxies in between are where the engineering lives.
For an enterprise network designer the methods are mostly something to recognise rather than to deploy: they appear when an operator's offload or roaming service is hosted on an enterprise or venue network, when a device's supplicant offers them in the method list, and in the server logs of any RADIUS proxy that participates in such a service. Where they are deployed, the operational questions are the ones RFC 4187 already raises: whether pseudonyms are issued so identities are not leaked, how fast re-authentication is bounded, and how the path to the home operator is protected, which is where RadSec and the roaming federations enter.
Also known as: eap-sim, eap-aka, sim authentication, usim