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SIM Based WLAN Authentication for Open Platforms
Contributed by Intel Corporation
This paper discusses approaches to authenticating users with open platforms, such as notebook PCs, for WLAN access using SIM cards. Using SIM cards to authenticate WLAN access is desirable by wireless operators as this approach minimizes additional infrastructure investments since the existing authentication processes and equipment are re-used. This paper starts with an overview of the authentication methodology in todayâ€™s GSM networks. Applying the GSM authentication methodology in a WLAN environment is then discussed. Enabling use of open platforms in WLANs requires consideration of potential security threats to authentication data due to the presence of open pathways. These open pathways are illustrated and possible mitigation techniques are discussed. With the appropriate mitigation approaches, SIM based user authentication can be easily extended to the WLAN environment.
Worldwide, traditional cellular operators are investigating how to integrate WLANs (Wireless Local Area Network) in their service offerings and business models. Currently GSM (Global System for Mobile Communications) operators provide Subscriber Identity Module (SIM) for each subscriber on their network. Worldwide, GSM networks are the most widely deployed digital cellular mobile network standard. The role of SIM is to authenticate the user on the GSM network and to facilitate effective billing. Operators are now seeking to extend this SIM-authentication functionality to WLAN services, instead of the existing username/password or prepaid service WLAN authentication methods. This paper will provide a brief overview of the authentication process in a GSM network, and will then describe a method of WLAN authentication using SIM cards. Finally the paper will go through various SIM security issues, as well as describe an assortment of reader attachments and their advantages and disadvantages.
2. Overview of GSM SIM Based Authentication
2.1. SIM - Subscriber Identity Module
The role of SIM in GSM networks is to ensure that only authorized users can access the network. In order to properly authenticate a user, it must be able to store data, guard against unauthorized access to the stored data, and execute a cryptographic algorithm under secure conditions. The SIM and mobile device is authenticated with a background system. The data transferred between the mobile station and base station across the air interface is encrypted.
2.2. SIM Authentication on GSM Networks
There are three major components in the GSM network:
The SIM authentication procedure on GSM networks checks the validity of the subscriberâ€™s SIM card and then decides whether the mobile station is allowed on a particular network. The parties involved in the authentication process are: a) the end user or holder of the SIM card, a non-grey listed (i.e., not stolen) and certified handset and b) the network operator (GSM service provider). The authentication process is one-way since the user is being first authenticated to the phone via the PIN number and then to the operator via their SIM based AAA (Authentication, Authorization, and Accounting) mechanism. The network authenticates the subscriber through the use of a challenge-response method:
2.3. Value of SIM Based Authentication
Authentication allows an operator to ensure that only legitimate subscribers in the possession of an operator supplied SIM card (i.e., using a handset that is not stolen) are the only ones allowed to make calls on the network. Authentication ensures that the network is being used by a paying customer and the call ends up generating actual revenue for the operator.
There are some inherent weaknesses in the GSM security system and algorithms that makes it vulnerable to fraud. Two types of fraud are possible - a) making "free" calls using a stolen SIM and/or equipment and b) making "free" calls using a cloned SIM. ETSI (European Telecommunications Standards Institute), the GSM standards body, has been making several improvements in the GSM security (improved cryptographic algorithm, etc) while GSM operators have setup sophisticated fraud detection and management system. For example, GSM networks prohibit duplicate SIMs to be active simultaneously, GSM handsets will not work without a SIM, and handsets are verified against a database to determine if they are stolen and are then restricted to emergency calls only.
3. SIM Based Authentication for WLAN Networks
3.1. WLAN Authentication Process
SIM based WLAN authentication requires the use of a SIM reader attached to the computing device, so that the authentication software can use the SIM credentials.
The EAP-SIM protocol, resident on the client, specifies the Extensible Authentication Protocol (EAP) mechanism for authentication and session key distribution using GSM SIM. In EAP-SIM several RAND challenges are used for generating several 64-bit ciphering keys (Kc), which are combined to constitute a longer session key. EAP/SIM also enhances the basic GSM authentication mechanism by accompanying the RAND challenges with message authentication code in order to provide mutual authentication. The EAP-SIM client starts the authentication process by connecting to the AAA server. The AAA server issues a challenge over the 802.11b radio interface, which is then forwarded to the SIM reader by the EAP-SIM client. The EAP-SIM client communicates to the SIM through the SIM reader, the SIM calculates the response that contains the SRES and Kc, which is sent to the EAP-SIM client. The EAP- SIM client then forwards the response to the AAA server, which then checks the response and provides access appropriately. In this scenario it is assumed that the AAA server has a secure connection to GSM backbone network components like HLR.
3.2. Use of SIM cards on open platforms
An open platform, for the purposes of this paper, is defined as a general purpose computing device with an open operating system. A notebook PC is considered to be an open platform since users can download/write software for execution in a notebook PC without any restrictions.
There are various types of SIM readers available as an external attachment to personal computers as described below:
4. Security Considerations in SIM Use for WLAN Authentication
4.1. SIM Security
SIM card are a subset of smart cards—actually, SIM is an application on a smart card. In general, smart card security is guaranteed by four components, which are: card body, chip hardware, operating system and application. The chip hardware, operating system and application protects the data and programs in smart card micro controller. Also, in the GSM system, SIM is manufactured, provisioned, distributed, and managed in trusted environments. This results in the SIM being a tamper resistant device in which the access credentials of a mobile network subscriber can be securely stored. However, even if the key data (Ki) inside the SIM cannot be directly obtained, its opacity depends on the algorithms used to hide it from the outside world. Using cryptanalysis, hackers can find a way to calculate the value of the secret data by analyzing a huge number of command-response pairs. The risks of such attacks in cellular systems are, however, relatively low since the cell phone is "closed" to the outside world and also depends on the weakness of the cryptographic algorithm (e.g. Comp 128-1) used. One possibility open to hackers is to eavesdrop on the data being transmitted between the cell phone and the base station. This approach requires having access to the cellular infrastructure, such as base station stations, for capturing the triplets (RAND, SRES, Kc) while they are transmitted over the air. Since the infrastructure equipment is quite expensive, this approach is not quite realistic. Another approach is for the hacker to have physical possession of a stolen or cloned SIM. For this reason, operators use software to detect cloned SIM usage and block their use, without impacting the customer service. Also GSM systems have fraud detection mechanism in place to deter usage of stolen or cloned SIM usage.
4.2. Open Platform Security
The use of SIM cards for authentication of users with open platforms in WLAN networks requires consideration of the openness of data paths used to access the SIM data. Possible open paths are Path A and Path B:
Path B issues are being addressed by the standards bodies with EAP-SIM and PEAP (Protected EAP) protocols to address data security. However, the data pathway security for Path A in open platforms has not yet been addressed in standard bodies.
External SIM card readers are attached over an open bus, providing potential exposure of SIM card data. Given access to a SIM card, it is possible to obtain any number of GSM triplets and hence attack GSM security. Possible types of types of SIM attacks could be:
Vulnerability due to these threats varies with the cryptographic algorithm used in SIM based authentication. Comp-128-1 cryptographic algorithm is weaker than the newly defined Comp-128-3. In the future, as more operators use Comp-128-3 algorithm, Path A security may become less of an issue.
Since the deployment of stronger forms of cryptographic algorithms is not widespread yet, PATH A security needs to be addressed. Possible ways to mitigate PATH A security issues are:
Utilization of SIM for WLAN authentication could be very advantageous to operators. They can leverage existing business processes and network infrastructure. There are various ways described in this paper for attaching a SIM to a notebook PC. However, accessing the SIM from a notebook PC requires consideration of some security threats. With the appropriate mitigation approaches and in light of the advantages a SIM brings to WLAN, the industry should work on building the strength of the algorithms used in authentication and the definition of independent data structures, while enhancing the existing data that allows identification, authentication, control and service provisioning.
 GSM Association WLAN Task Force, "Security Objectives to be included in PRD AA.39: WLAN/GSM Roaming User Scenarios, V 2.0.0," WLAN Doc 122/02, 2002.
 Mike Hendry, Smart Card Security and Applications, Artech House Publishers, 2001.
 GSM Association Wireless LAN Task Force, "Advantages of using a SIM as an authentication token for Wireless LANs," SD Doc 407/02, 2002.
 Jane Dashevsky, Edward C. Epp, Jose Puthenkulam, and Mrudula Yalemanchi, "SIM Trust Parameter," Intel Developer Services, (http://developer.intel.com), 2003.
 Micehel Mouly and Marie-Bernadette Pautet, The GSM System for Mobile Communications, Self Published, 1992.
 W. Rankl and W. Effing, SmartCard Handbook, John Willey Publication, 1999.
 European Telecommunication Standards Institute, GSM 11.11, Version 5.0, 1995.
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