Handbook of Wireless Networks and Mobile Computing, Edited by Ivan Stojmenovic´Copyright © 2002 John Wiley & Sons, Inc.ISBNs: 0-471-41902-8 (Paper); 0-471-22456-1 (Electronic)309310SECURITY AND FRAUD DETECTION IN MOBILE AND WIRELESS NETWORKSworks, or against information systems in general. However, intrusion detection in mobile telecommunication networks has received very little attention. It is our belief that this is-sue will play a major role in future generations of wireless systems. Several telecom carri-ers are already complaining about the loss due to impostors and malicious intruders.In this chapter, we will identify and describe several aspects of wireless and mobile net-work security. We will discuss the intrusion detection systems in wired and wireless net-works and identify the new challenges and opportunities posed by the ad hoc network, a new wireless paradigm for mobile hosts. Unlike traditional mobile wireless networks, ad hoc networks do not rely on any fixed infrastructure. Instead, they rely on each other to keep the network connected. Next, we will examine the authentication problem of mobile users. Finally, we discuss the problems of cloning and fraud detection in mobile phone operations 14.2NETWORK SECURITY PROBLEMSSecurity is an essential part of wired and wireless network communications. Interestingly enough, these systems are designed to provide open access across vast networked environ-ments. Today’s technologies are usually network-operation-intrusive, i.e., they often limit the connectivity and inhibit easier access to data and services. With the increasing popu-larity of wireless networks, the security issue for mobile users could be even more serious than we expect. The traditional analogue cellular phones are very insecure. The 32-bit ser-ial number, the 34-bit phone number, and the conversation in a cell can be scanned easily by an all-band receiver. The widely used advanced mobile phone system (AMPS) is an analogue phone system. Therefore, sending a password or a host name through this system can be a serious security issue. Other security issues in wireless networks that have been studied extensively are anonymity and location privacy in mobile networks; these have re-ceived a great deal of interest recently [23]. A typical situation is one in which a mobile user registered in a certain home domain requests services while visiting a foreign do-main. Concerned about security and privacy, the user would prefer to remain anonymous with respect to the foreign domain. That is, only the home domain authority should be in-formed as to the mobile user’s real identity, itinerary, whereabouts, etc. Another important issue, namely cloning phones, raises a number of concerns to many telecom carriers. In-deed, many telecommunication companies are losing money due to the use of clones or genuine mobile phones by impostors. One might argue that although it is rather easy to clone an AMPS phone, it is much trickier to clone a D-AMPS, a GSM, or an IS-95 phone. However, the security issue remains, and needs to be resolved in the next wireless network generation. Consequently, there has been a great deal of interest recently in designing mo-bile phones using new technologies, such as Boot Block flash technology used by Intel Corporation, that will make it much more difficult to clone cellular phones. However, to the best of our knowledge there is very little work being done at the software level. To combat cloning, cellular operators analyze usage to check for unusual patterns. Most obvi-ously, they know that genuine phone cannot be in two places at once. If a phone is making more than one call at a time, it has definitely been cloned. Furthermore, to verify if a call is out of the client patterns, current software (i) does not have an efficient automatic process to warn clients about the impostors using their mobile phones; in most of these14.4INTRUSION DETECTION SYSTEMS (IDS)311 systems, human staff are used to do that (only lists of large bills are reviewed to identify cloned phones); (ii) has no efficient ways to control/identify impostors; and (iii) uses an “experimental satisfaction” to prove the correctness of the security framework. Some sys-tems provide the billing process via the Web. H owever, the identification of a cloned phone is done only at the end of the month. This, unfortunately, is not quite efficient and may lead to a big loss of revenue for the carrier.The wireless Web opens up many new business opportunities, the most important of which use location-based technology. Ever since the mobile Internet was first suggested, antivirus companies have warned that viruses could attack cellular phones and PDSs. Tim-ofonica was among the first viruses that attacked cell phones. Timofonica was an ordinary virus programmed to send abusive messages to random users of Spanish Telefonica mo-bile systems. Viruses are a threat to any computing platform and may be a threat to wire-less terminals that include processing and memory akin to those of modern computers. 14.3NETWORK SECURITY MANAGEMENT PLANAn adequate security system management policy has long been an important issue. A comprehensive network security plan must also consider losses of privacy when we define authentication and authorization as well as losses of performance when we define key management and security protocols. Therefore, a security plan must encompass all of the elements that make up the wireless and/or wired network, and provide important services such as:1.Access control, i.e., authorization by capability list, wrappers, and firewalls (accesscontrol matrix)2.Confidentiality, i.e., we must ensure that information and transmitted messages areaccessible only for reading by authorized parties3.Authentication, i.e., the receiver must be able to confirm that the message is indeedfrom the right sender4.Nonrepudiation, i.e., the sender cannot deny that the message was indeed sent byhim/her5.Integrity, i.e., the message has not been modified in transit6.Availability, i.e., making sure that the system is available to authorized parties whenneeded7.Security administration, i.e., checking audit trails, encryption and password man-agement, maintenance of security equipment and services, and informing users of their responsibilities.14.4INTRUSION DETECTION SYSTEMS (IDS)Intrusion is most probably one of the key issues that wireless and mobile systems will have to deal with. The nature of wireless ad hoc networks makes them very vulnerable to312SECURITY AND FRAUD DETECTION IN MOBILE AND WIRELESS NETWORKSan adversary’s malicious attacks. Generally speaking, an intrusion can be defined as an act of a person or proxy attempting to break into or misuse your system in violation of an es-tablished policy. Very little research work dealing with the intrusion problem has been done for wireless networks.In this section, we shall describe the intrusion problem in general. We hope that re-searchers will pick up what has been done in related areas, and find efficient approaches on how to deal with this problem in an ad hoc network environment.14.4.1Current IDS TechniquesGenerally speaking, intrusion can be classified as: (i) misuse intrusions, i.e., well-defined attacks against known system vulnerabilities; and (ii) anomaly intrusions, i.e., activities based on deviation from normal system usage patterns. Intrusion detection systems (IDS) are one of the latest security tools in the battle against these attacks. As is well known, it is very difficult to determine exactly which activities provide the best indicators for the es-tablished (normal) usage patterns. Thus, researchers have turned to using expert systems or knowledge-based intrusion detection to search for activities known to be indicative of possible intrusive behavior [16]. The motivation behind this approach is to seek a proper behavior as opposed to a normal one. Knowledge-based intrusion detection schemes apply the knowledge they have accumulated about specific attacks and system vulnerabilities. Using this knowledge database, any action that is not explicitly recognized as an attack is considered acceptable. Otherwise, an alarm is triggered by the system.There are many different intrusion systems available in the marketplace. Expert sys-tems are based on knowledge-based intrusion detection techniques. Each attack is identi-fied by a set of rules. Rule-based languages [13] are used for modeling the knowledge that experts have accumulated about attacks/frauds. Information regarding some intruders has also been added to these systems. A major drawback of knowledge-based intrusion sys-tems is the difficulty of gathering the information on the known attacks (which should be updated regularly) and developing a comprehensive set of rules that can be used to identi-fy intrusive behaviors. Some systems use a combination of several approaches to cover both the normal and proper behavior schemes [17]. We refer to them as behavior-based in-trusion detection. Their basic characteristic is that any action that does not match with a previously learned behavior triggers an alarm. The action is considered as intrusive. The main advantages of these systems are that they can exploit new and unforeseen attacks, and contribute to automatically discovering new attacks. However, their high false alarm rate is generally cited as a main drawback of these systems, due basically to the accuracy of the behavior information accumulated during the learning process.14.5SECURING DATA TRANSFER IN DIGITAL MOBILE SYSTEMSAll digital mobile systems provide security through some kind of encryption. Data can be encrypted in many ways, but algorithms used for secure data transfer fall into two cate-gories: symmetric and asymmetric. Both rely on performing mathematical operations us-ing a secret number known as a key. The difficulty with symmetric algorithms is that both14.6SECURING WIRELESS AD HOC NETWORKS313 parties need to have a copy of the key. On the other hand, asymmetric techniques use two separate keys for encryption and decryption. Usually, the encryption key can be publicly distributed, whereas the decryption key is held securely by the recipient.The most widely used symmetric algorithm in DES (data encryption standard), devel-oped by IBM in 1977. It uses a 56-bit key, which seemed unbreakable at that time. In 1997, a group of Internet users managed to read a DES-coded message. Most organization now use triple-DES, which uses 112 bits. The basic idea is that larger keys mean more possible permutations, and so better encryption. GMS encrypts all data between the phone and the base station using a code called A5 (The A stands for algorithm). The details of the code are kept secret to make it harder to crack. Unfortunately, details have been leaked out over the years and have been posted on hackers’web sites. Thus, we believe there is still much work to be done in the cloning mobile phone area.Several different asymmetric algorithms have been developed, each using a different type of “one-way” mathematical function. Rivest et al. [32] proposed an efficient algo-rithm, which they refer to as RSA, that relies on the fact that factorization is more difficult than multiplication. Indeed, multiplying two prime numbers together is easy for a comput-er, but recovering those two numbers from the product is not. The main drawback of asymmetric schemes is that they use a lot of CPU, and so cannot be used to encrypt an en-tire message through a mobile phone. Instead, A5 encrypts the message itself using a sym-metric algorithm, with a key randomly generated by the network and sent to the handset using an asymmetric algorithm.14.6SECURING WIRELESS AD HOC NETWORKSMany WLANs in use today need an infrastructure network. Infrastructure networks not only provide access to other networks, but also include forwarding functions, medium ac-cess control, etc. In these infrastructure-based wireless networks, communication typical-ly takes place only between the wireless nodes and the access point, but not directly be-tween the wireless nodes. Ad hoc wireless networks, however, do not need any infrastructure to work. Each node can communicate with another node; no access point controlling medium access is necessary. Mobile nodes within each other’s radio range communicate directly via wireless links, whereas those that are far apart rely on other nodes to relay messages as routers. Node mobility in an ad hoc network causes frequent changes of the network topology.Since an ad hoc network can be deployed rapidly at relatively low cost, it becomes an attractive option for commercial uses such as sensor networks or virtual classrooms. How-ever, before an ad hoc network becomes a commodity, several security issues must first be resolved. On one hand, the security-sensitive applications of ad hoc networks require a high degree of security; on the other hand, ad hoc networks are inherently vulnerable to security attacks. Therefore, security mechanisms are indispensable for ad hoc networks.As in any wireless or wired network, traffic across an ad hoc network can be highly vulnerable to security threats. Thus, to secure an ad hoc network, one should consider not only the attributes described in Section 14.3, i.e., availability, confidentiality, integrity, au-thentication, and nonrepudiation. but also new types of threats that are extended even to314SECURITY AND FRAUD DETECTION IN MOBILE AND WIRELESS NETWORKSthe basic structure of the networks. The salient characteristics of ad hoc networks pose both challenges and opportunities in achieving these security goals.Since ad hoc networks use wireless links, they are susceptible to link attacks ranging from passive eavesdropping to active impersonation, message replay, and message distor-tion. Active attacks might allow the adversary to delete messages, inject erroneous, modi-fy messages, and impersonate a node, thereby violating availability, integrity, authentica-tion, and nonrepudiation.14.6.1Intrusion Detection in Wireless Ad Hoc NetworksMost of the IDS systems developed for wired networks described in previous section can-not be applied to wireless networks. This is mainly due to the fact that today’s network-based IDSs, which rely on real-time traffic analysis, can no longer function in the wireless and mobile environments such wireless ad hoc networks. When compared with wired net-works, in which traffic monitoring is usually done at switches, routers, and gateways, a wireless ad hoc network does not have traffic concentration points at which IDS can col-lect audit data for the entire network. Recall that in a wireless ad hoc network, each node can communicate with another node, and no access point controlling medium access is necessary. Mobile nodes within each other’s radio range communicate directly via wire-less links, whereas those that are far apart rely on other nodes to relay messages as routers.Recently, Zhang and Lee [31] examined the vulnerability of a wireless ad hoc network. They described an intrusion detection and response mechanism. In their approach, each node is responsible for detecting signs for intrusion locally and independently, but neigh-boring nodes can collaboratively investigate in a broader range. Individual IDS agents are placed on each and every node. Each IDS agent runs independently and monitors local ac-tivities such as user/system activities, communication activities, etc. These IDS agents collectively form the IDS system to protect the wireless ad hoc network against malicious attacks. If an IDS agent detects an intrusion from local data, neighboring IDS agents will collaborate in the global intrusion detection actions. Intrusion detection responses are pro-vided by both the local response initiated by the IDS agent, and global response modules. The type of intrusion response depends on the type of network protocols and applications, and confidence (or certainty) in evidence. For example, the IDS agent can send a “reau-thentication” request to all nodes in the network to prompt the end users to authenticate themselves (end hence their wireless nodes), using out-of-bound mechanisms (e,g., visual contacts). Only the reauthenticated nodes may collectively negotiate new communication channels, which in turn recognize each other as legitimate. Thus, the compromised and/or malicious nodes can be excluded. Last but not least, the authors use a secure communica-tion module in their IDS system and provide a high-confidence communication channel among IDS agents. However, this work is still at an early stage, and no experimental data were provided to study the effectiveness of their scheme.14.6.2Securing Routing Protocol in Wireless Ad Hoc NetworksSecurity for any routing protocol [24, 29] is a very difficult problem to deal with. One can take advantage of the redundancies in the network topology, i.e., multiple routes between14.7AUTHENTICATION OF MOBILE USERS315 nodes, to achieve availability. The security of routing protocols is closely tied to the prop-er distribution of some keys that allow the creation of unforgeable credentials. Thus, de-signing secure key distribution in ad hoc networks is a challenging problem. Diffie–Hell-man key exchange may indeed help to establish some temporary security between particular endpoints. However, they are also vulnerable to the man-in-the-middle attacks that are hard to defeat in an ad hoc network.Recently, Zhang and Lee [31] defined trace data to describe, for each node, the normal (i.e., legitimate) updates of routing information. Since a legitimate change in the route table can basically be caused by the physical movement(s) of node(s) or network membership changes, and each mobile node should use only reliable information that it can trust, the au-thors have decided to use data on a node’s physical movements and the corresponding change in its routing table as the basis of the trace data. A normal profile on the trace data in effect specifies the correlation of physical movements of the nodes and the changes in the routing table. A classification algorithm is used to compute the classifier and to describe the changes measured by the percentage of changed routes and the percentage of changes in the sum of hops of all routes. A detection model that uses deviation scores distinguishes abnor-mal from normal updating of the routing table. Unfortunately, no experimental data was provided to study the performance and effectiveness of their scheme.Public key protocols and symmetric key methods are also devilishly difficult, and with-out an infrastructure it is very hard to conceive of the use of certificate-based protocols. Multicast data distribution in ad hoc networks poses new types of security problems. In-deed, one should not forget that there will always be many different trust relationships that are hard to maintain between neighbors in a large community. Quality of service (QoS) control could be used to provide a reasonable solution to the multicast data distribution in ad hoc networks.14.7AUTHENTICATION OF MOBILE USERSSome wireless communications systems protocols such as GSM [27, 28] and IS-41 [18, 22] use the secret key crypto-system for authentication. Although the authentication of these systems is only unilateral, and the user’s identity and location are not anonymous, the protocols [13, 20] provide more security functions, such as identity, confidentiality, and mutual authentication. The drawback of the above schemes is that they all need a third party, i.e., a third trusted server such as the home location register (HLR) and old visitor location register (VLR). Although HLR creates a record that contains the mobile station’s (MS) directory number, profile information, current location, and validation period, etc., whenever the MS subscribes to the service of a mobile system, VLR records the temporal information for the MS when it visits a mobile system other than the home system. HLR acts as the CA; VLR is responsible for authenticating the MS.Several public key and secret key hybrid schemes have also been proposed in the litera-ture. Brown [7] proposes a hybrid technique to provide privacy and authentication in per-sonnel communication systems. Other authors [10] present a protocol based on Diffie–Hellman scheme to generate a session key, and a mechanism to detect a cloned phone.Several certificate-based protocols have been proposed for wireless authentication,where the traffic between VLR and HLR is minimized [2, 15, 21]. The basic idea behind these protocols is that when roaming across a domain becomes more frequent, the fre-quency of long-distance signaling across domains is dramatically reduced. The drawback of these schemes is that incorrect use of certificates in protocols may result in security flaws. For instance, it might be easy to clone a user if its certificate is used as secret infor-mation.To remedy to these drawbacks, other authors [30] propose an authentication procedure consisting of two protocols: a certificate-based authentication (CBA) protocol and a tick-et-based authentication (TBA) protocol. Only two parties—mobile station (MS) and visi-tor location register (VLR)—are involved in executing their scheme. The CBA protocol is used in registration, handover, and when the ticket is invalid.The certificate contains the user public key and other information and signature in-formation provided by the CA (certificate authority). The ticket is a message authentica-tion code (MAC) {TID, Date, L, (TID, Date, L ) K VLR }, whereK VLR is the secret code key of VLR, TID is the temporary identify for MS, Date is the issue date of the ticket, and L is the lifetime of the ticket. Only the user owning the secret key can make a ticket and verify its validity. VLR will save the secret key K , and (.)K indicates a secret key crypto-system.In the authentication suite of protocols [30], HLR issues the certificates Cert VLR and Cert MS to VLR and MS; MS stores the following data: Cert MS , Cert HLR , and KR MS . VLR saves the following data: Cert VLR , Cert HLR , KR VLR , and KR HLR ; where Cert X represents the certificate of entity X , and KR X represents a private key of entity X .Let us denote by two random numbers R 1and R 2. Let KU X be a public key of entity X ,K 0the old session key, and TID a temporary identity. The basic idea of the CBA protocol (shown in Figure 14.1) is the exchange of three type of messages which are described as follows:316SECURITY AND FRAUD DETECTION IN MOBILE AND WIRELESS NETWORKSFigure 14.1CBA and TBA protocols.1.Cert_Auth_Ask: Cert VLR, R12.Cert_Auth_resp: (K s)KUVLR , [Cert MS||(R2||R1)KRMS]Ks3.Cert_Auth_Ack: (K s)KUVLR , [Ticket||(R1||R2)KRVLR]Kswhere the session key is R1 R2.If the MS stays at the same cell and requests the service several times, we use the TBA protocol. The TBA protocol uses the ticket in the authentication procedure and is de-scribed as follows:1.Ticket_Auth_Com: TID, (Ticket||R1)K2.Ticket_Auth_Result: (R2||R1)K)where the session key is R1 R2.If the ticket is out of date, the MS runs the CBA protocol to get the current ticket. TBA protocol uses a symmetric crypto-system in authenticating the MS. Therefore, the compu-tational cost is lower and the efficiency is higher than that of the CBA protocol.Since the CBA/TBA suite of protocols do not need a trusted party, entities in these pro-tocols store more information. Nevertheless, Tzeng and Tzeng [30], have proven that the message complexity exchanged between entities the CBA/TBA protocols is less than that of previous UMTS schemes.Last but not least, an authentication architecture for ad hoc networks has been proposed by Jacob and Corsen [14]. Their scheme presents the formats of all messages together with protocols that are supposed to achieve authentication.14.8SUBSCRIPTION AND FRAUD DETECTION IN MOBILEPHONE SYSTEMSWith the increasing popularity of wireless networks, the security issue for mobile users could be even more serious than we expect [5, 12]. Before the mobile phones became widely popular, the greatest threat to the network security in most organizations was dial-up lines. While dial-up lines still merit attention, the risks they pose are minor when com-pared to wireless and mobile connections. To break the system, one need only buy a piece of portable radio equipment, such as a scanner, to program a mobile cloned to debit calls from genuine mobile phone, and register the frequencies at which mobile phones operate in surrounding areas. Then the person committing the fraud may, for example, park his car in a shopping mall, jot down various frequencies, transfer the data to clones, and then pass them to whomever may be interested in these cloned mobiles. Mobile phones will change many aspects of our lives, but not until potential users become convinced of the security of the mobile networks.This author and others [4, 26] have presented a distributed security system supported by CORBA that uses on-line telecommunication databases (i.e., CallsFile), as well as database files (i.e., Baseline) created during the training process of the system for the classification of the clients. It uses neural network/pattern recognition techniques.14.8SUBSCRIPTION AND FRAUD DETECTION IN MOBILE PHONE SYSTEMS317318SECURITY AND FRAUD DETECTION IN MOBILE AND WIRELESS NETWORKSCORBA and Web/Java security components were added to further secure the system. LOTOS formal specification and validation techniques were embedded in the system to prove its correctness and validate it. The basic framework consists of three main com-ponents. The first part protects the security system against cellular cloning (SSCC). The second part uses the security of the Internet and the Web (SETWeb). Finally, the third component, SIPI, protects the system from future impostors that might try to use the mobile phones improperly.SSCC can be viewed as a black box that interacts with the users via mail or phone, which we refer to as gate-mail, and gate-phone, respectively. The first gate is used by the SSCC to send alarms of possible frauds to the users by “surface” mail, and the second gate allows the SSCC to use mobile phones to send the same alarms. The main purpose of sending alarms by phone is for an immediate notification of possible fraud. Although the “surface” mail is more secure, it is still slower than the notification by phone. The most abstract specification of the SSCC system corresponds to a formalization of the user re-quirements of this system, and it might be used as a basis for future refinements of the project. Furthermore, using this requirement specification, it will be possible to prove—formally—that it is equivalent to the future final and complete specification. In order to validate the SSCC system, we make use of the CADP tool (Caesar Aldbaran Development Package) [11] available in the Eucalyptus toolbox. The procedure used to obtain the cor-rection proofs between refinements generates the following two automata: SSCC.AUT and SSCC_DET.AUT. These two automata aim at proving the correctness of the system in conformation with ISO 8807 [6] and US DoD ClassA1.SETWeb, a system phone bill on line via the Web, has been developed to allow clients to consult their phone bill online at any time. The client can then observe if a call from a clone just arrived in his bill, thus avoiding losses. Our system ensures the secu-rity and the privacy of the client when he or she tries to access to his/her file [19]. In what follows, we will present the security policy scheme to ensure the security of the carrier site and protect the privacy of the client. Several issues must be maintained, such as access controlling, logging, confidentiality, authentication, and administration of the systems resources, just to mention a few. In their design Notare et al. [26] used all these services to implement the Java security API with the overall goal of protecting the user’s information from eavesdropping and tampering. To avoid spoofing attacks and ensure that the user is accessing the right phone carrier site, we made use of digital certificates on the server side.In this system, the tool “policytool” creates and manages a text file that stores security policy definitions, known as “mypolicy.” Those definitions can give special privileges to users having some form of authentication, such as a digital signature. Many security poli-cies can be defined for an environment and its many resources.14.8.1SIPI—System to Identify Probable ImpostorsThe SIPI system has been designed to identify “probable” impostors using cloned phones. Boukerche et al.’s [5, 10, 33] approach to identifying fraudulent calls is to classify the mo-bile phone users into a set of groups according to their log files [10]. They assume that all relevant characteristics that identify the users will be stored in these files; i.e., where, at。
