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Security is an important topic in telecommunications. It is even more important when wireless systems are used because it is generally perceived that wireless systems easier to attack than wireline systems. The IEEE 802.16 standard, commonly known as WiMAX, is the latest technology that has promised to offer broadband wireless access over long distance.
In this paper, we first give an overview of security architecture of mobile WiMAX network. We analyze the security of Mobile WiMAX, point out several potential security threats and vulnerabilities. In this paper, we describe different security vulnerabilities found in IEEE 802.16e.
Keywords- Authentication, DDOS attack, EAP, MAC Layer, RSA Algorithm
Security is an important concern for the network operator and the network user. Each user want to ensure that privacy is protected, that the integrity of the data is not compromised. The network operator wants to know that the authenticate user connected to their network. Recently, the use of the IEEE 802.16 standard to build Metropolitan Area Networks has gained a great deal of interest. The standard offers high throughput broadband connections and coverage with respect to WLANs, and provides a security sublayer which is responsible for secrecy, authentication and secure key exchange. Several versions of 802.16 networks were released. While the first versions have shown some security weaknesses that were later corrected by the recently released versions, the security mechanisms of 802.16 still remain vulnerable and the limited deployment of such technology is insufficient to satisfy the demands of security.
IEEE 802.16e-based Mobile WiMAX is one of the most promising technique for future communications. As we all know, security support is mandatory for any communication networks especially for wireless systems, since wireless medium is available to all, the attackers can easily access the network and the network becomes more vulnerable for the user and the network service provider. .
MOBILE WIMAX: MAC LAYER
The 802.16 MAC layer in figure 1is divided into three sublayers the service specific convergent sublayer , which interfaces to higher layers; the MAC common part sublayer , which carries out the key MAC functions; and the privacy sublayer , which provides authentication, secure key exchange, and encryption.
Service Specific Convergence Sublayer: The IEEE 802.16 standard defines multiple convergences sublayers. The WiMAX network architecture framework supports a variety of convergence sublayer types, including Ethernet, Internet Protocol version 4 (IPv4), and Internet Protocol version 6 (IPv6) .
MAC Common Part Sublayer: The MAC is connection-oriented. For the purposes of mapping to services on SSs and associating varying levels of QoS, all data communications are in the context of a transport connection. A transport connection defines both the mapping between peer convergence processes that utilize the MAC and a service flow. The service flow defines the QoS parameters for the PDUs that are exchanged on the connection. Service flows provide a mechanism for uplink and downlink QoS management. It provides bandwidth allocation and establishes and maintains connections .
Fig. 1. 802.16 MAC Layer 
Security Sublayer: The security sublayer provides subscribers with privacy, authentication, or confidentiality across the broadband wireless network. It does this by applying cryptographic transforms to MPDUs carried across connections between SS and BS. Almost all the security issues in Mobile WiMAX are considered in the security sub-layer which is shown in Figure 2. It plays its role based on three essential functions: authentication, authorization and encryption .
Fig. 2. Security Sublayer 
SECURITY MECHANISM OF MOBILE WIMAX
The Mobile WiMAX system based on the IEEE 802.16e-2005 amendment has more improved security features than previous IEEE 802.16d-based WiMAX network system. Almost all the security issues in Mobile WiMAX are considered in security sub-layer, and are shown in Figure 3. WiMAX systems provide secure communications by performing three steps: authentication, authorization, and data encryption. The authentication procedure provides common keying material for the SS/MS and the BS and facilitates the secure exchange of data encryption keys that ensure the confidentiality of WiMAX data communications.
Authentication comes in two forms:
â€¢ Unilateral authentication where the BS authenticates the MS and
â€¢ Mutual authentication where the BS authenticates the MS and the MS authenticates the BS
Authentication in Mobile WiMAX is based on the Privacy Key Management (PKM) protocol, which allows three types of authentication :
RSA based authentication - X.509 digital certificates together with RSA encryption
Extensible Authentication Protocol (optional unless specifically required)
RSA based authentication followed by EAP authentication
The PKM RSA authentication protocol uses X.509 digital certificates [IETF RFC 3280], the RSA public key
encryption algorithm that binds public RSA encryption keys to MAC addresses of SSs.
A BS authenticates a client SS during the initial authorization exchange. Each SS carries a unique X.509 digital certificate issued by the SS's manufacturer. The digital certificate contains the SS's Public Key and SS MAC address. When requesting an AK, an SS presents its digital certificate to the BS. The BS verifies the digital certificate, and then uses the verified Public Key to encrypt an AK, which the BS then sends back to the requesting SS.
In the case of EAP based authentication the MS is authenticated either through a unique operator issued credential, such as a SIM or though an X.509 certificate as described above. The choice of authentication method depends on the operator's choice of type of EAP as follows :
â€¢ EAP-AKA (Authentication and Key Agreement) for SIM based authentication,
â€¢ EAP-TLS for X.509 based authentication
â€¢ EAP-TTLS for MS-CHAPv2 (Microsoft-Challenge Handshake Authentication Protocol)
Following authentication is the authorization process. In this process, SS requests for an AK as well as an SA (Security Associations) identity (SAID) from BS. The Authorization Request message includes SS's X.509 certificate, encryption algorithms and cryptographic ID. In response, the BS interacts with an AAA (Authentication, Authorization and Accounting) server in the network to carry out the necessary validation and sends back an Authorization reply that contains the AK encrypted with the SS's public key, a lifetime key and an SAID.
Application data are encrypted by Traffic Encryption Key (TEK), which is generated as a random number in the BS using the TEK encryption algorithm. The Key Encryption Key (KEK) is used for encrypting the TEK before the key transfer from BS. The KEK is 128 bits long, which is derived directly from the 160 bits long AK.
SECURITY THREATS: PHYSICAL AND MAC VULNERABILITY
In computer security, vulnerability is a weakness which allows an attacker to reduce a system's information assurance. Vulnerability is the intersection of three elements: a system susceptibility or flaw, attacker access to the flaw, and attacker capability to exploit the flaw. To be vulnerable, an attacker must have at least one applicable tool or technique that can connect to a system weakness.
WiMAX security is implemented in the security sub-layer which is above the PHY layer. Therefore the PHY is unsecure  and it is not protected from attacks targeting at the inherent vulnerability of wireless links such as jamming or scrambling.
Jamming is described by M. Barbeau as an attack "achieved by introducing a source of noise strong enough to significantly reduce the capacity of the channel". Jamming can be either intentional or unintentional. It is not difficult to perform a jamming attack because necessary information and equipments are easy to acquire .
In Scrambling, Attackers can selectively scramble control or management information in order to affect the normal operation of the network. Slots of data traffic belonging to the targeted SSs can be scrambled selectively, forcing them to retransmit. It is more difficult to perform an scrambling attack than to perform a jamming attack due to "the need, by the attacker, to interpret control information and to send noise during specific intervals" .
It describe main the main security weaknesses described by the literature regarding the 802.16e version. The latter are related mainly to problems with the security and authenticity of management communication messages and key sharing in a multicast and broadcast service.
Attacks on Authorization
An initial analysis of the authorization phase of the PKMv2 protocol shows that the first message (Authorization Request) is not protected against modification or forgery. This weakness had existed in the PKMv1. If an attacker captures such message, when it is sent by a legitimate MS, and repeatedly sends it, it could burden the BS and force it to deny access to a legitimate MS.
However, Mobile WiMAX standard fails to provide any security mechanism to keep the SBC negotiation parameters confidentiality. Thus, there exist the possibilities that, through intercepting and capturing message in this entry procedure, attacker camouflages himself as the legitimate MS and send tamped SBC-RSP message to serving BS while interrupting the legitimate MS's communication with the legitimate BS. The spoofed message may contain false message about the security capabilities of the legitimate MS. For instance, the attacker may send messages to inform the BS that the MS only supports low security capabilities or even no security capabilities. In this situation, if the BS supports this kind of MS, the communication between the MS with the serving BS will not be encrypted. As a result, the attackers would wiretap and tamper all the information transmitted .
Vulnerability to Denial of Service Attacks
In 802.16e networks, the network entry procedure, executed by a MS to attach itself to a BS, remains unprotected. Attackers can listen to the exchanged traffic and use the accessed information to forge ranging request (RNG-REQ) or ranging response (RNGRSP) messages and manipulate, in consequence, different MS settings. Since such message is unauthenticated, the MS cannot determine its real source. An attacker may intercept and forge a RNG-REQ message by modifying the specified preferred downlink burst profile. In addition, the management communication between a MS and a Bs involves the sending of plaintext messages, and the origin of some management frames, sent in unicast or broadcast, is not authenticated .
The Mobile neighbor advertisement (MOB_NBR-ADV) message, which is sent by the currently serving BS to state the characteristics of the neighbor base stations, is not authenticated. The attacker may exploit this leak to forge neighbor advertisement management message in which the information about the victim MS's neighbor BS is omitted. Thus, attacker can successfully prevent MS from handovering to BS. As a result, the victim MS's service quality decreases gradually while it moves away from the serving BS, and finally it will be out of service .
MS can set the sleep mode in the bandwidth request and uplink sleep control messages that are not authenticated. The attacker can send the bandwidth request and uplink sleep control message with the identifier of victim MS. As a result, the BS will stop transmitting messages to that MS, so performing a DoS attack. Besides, the BS can also send Traffic Indication Message to indicate a sleeping MS to notify MS that there is traffic destined to it. Accordingly, the MS is waked up from sleep mode. An adversary could generate this message to frequently wake up MS and exhaust victim MS's battery .
Attacks on handover
While the Handover optimization can be used to reduce latency, it also affects the security of the network once the handover is performed. For instance, setting bit#1 and bit#2 of Handover optimization bits in the RNG-RSP message equal to 1 and 1, respectively, forces the network to keep using the same secret keys before and after the handover and prevents it from ensuring backward and forward secrecy. In fact if a malicious mobile station has compromised the security of the serving BS, it could also compromise the security of all the previous and following BSs .
The IEEE 802.16e based WiMAX network provides better security architecture, compared to 802.16d, and basically secures the wireless transmission using different components such as X.509 certificates, PKMv2, the Security Associations, encryption methods and the Encapsulation Protocol. As with all wireless technologies, operational countermeasures may not provide protection against general wireless threats such as jamming, DoS, eavesdropping, and message replay . Operational controls often require highly specialized expertise and rely upon both management and technical controls. Countermeasures need to be devised for networks using the security options with critical or major risks. More research is needed in this direction.