The UMTS-AKA Protocols for Intelligent Transportation Systems

1.1. Characteristics of ITS

Signals are delivered to the different elements of ITS with the help of the communications infrastructure. The communications infrastructure consists of an array of diverse systems, which may be classified into wired networks and wireless networks. These networks may be either public or private, such as public land mobile networks, personal communication networks, public land mobile communications systems, private networks, packet switched data networks, integrated services digital networks, public switched telephone networks, and broadcast networks. These network forms should have the following characteristics so as to be suitable for ITS [4]: (1) support communication services including: voice, data, image, video, and signaling; (2) accommodate a wide variety of terminals, for example, fixed, portable mobile, and in-vehicle mobile; (3) preserve upward/downward terminal compatibility; (4) allow mobile and fixed users to utilize the services without geographical barriers (i.e., seamless communication); (5) provide service flexibility so that any combination of services may be used; (6) make efficient and economical use of the spectrum; (7) provide user authentication and billing functions; (8) provide varied degrees of network security that preserve user privacy; (9) have modular structures that allow the systems to start from small and simple configurations and then grow as needed in size and complexity; (10) generally use open architectures that permit easy introduction of advanced technology and support new applications.

Since communication services involve exchanging information between different systems, it is important to integrate the different types of communications systems rather than design an exclusive one. The hierarchical structure for communication services is as shown in Figure 2 [4].

1.2. MANET and VANET

A major part of current research concerning the communications network in transport systems surrounds Vehicle Ad hoc NETwork (VANET) [4–10], VANET focuses on messaging, broadcasting, and multicasting, which many think are services difficult to be implemented or accomplished with the existing telecommunications techniques, namely GSM and UMTS. One of the key objects of this research is to identify a communication system that is suitable for ITS. Our focus is to extend on the existing UMTS-AKA (Authentication and Key Agreement) protocol with a view to making it compliant with ITS.

Although the ITS communication environment is classified into wired and wireless networks, much of development bottlenecks surround the wireless side. The current wireless networks mainly take IEEE 802.11 (especially 802.11p is still active by IEEE 802.11 working group and scheduled to be published in July 2008) as the standard. Infrastructure and Ad hoc are widely used. All mobile nodes working within the infrastructure must connect to the access point for transmission. Unlike the infrastructure mode, devices in the Ad hoc mode do not need access points for transmission. Ad hoc relies on point-to-point networks that they constitute along with the members of mobile nodes. One such network using Ad hoc framework is MANET. The ITS communication environment can be simply classified into "car-to-car communication" (C2CC, or intervehicle communication, IVC) and "car-to-infrastructure communication (C2IC)" [11, 12]. The Ad hoc network is most suitable for C2CC. An improved version of MANET called VANET has also been proposed [13, 14].

Its security architecture can be found in [15–25]. VANET has five characteristics that meet the needs of intervehicle communication: high mobility, large number of nodes, no centralized infrastructure, user privacy, and no user interaction. VANET must suit increased movement, more nodes, and higher computational overhead (since its mobile node is the vehicle's computer rather than a PDA or a mobile phone). VANET can deliver information over the Ad hoc network formed with neighboring vehicles. Although VANET was designed exclusively for ITS, it comes with innate drawbacks. One is the scalability program [4]. In a large and distributed environment, scalability is crucial. This is especially the case for vehicles that travel very far apart from each other, in which case signal delivery cannot continue between them. Additionally, vehicle obstruction can lead to increased bandwidth overhead. Furthermore, the ease of disclosure of sensitive information over wireless networks [5, 10], general privacy management [9, 26, 27], and location program [28, 29] are subjects relating to VANET that must be urgently resolved.

1.3. Communication Types in ITS

The ITS communications infrastructure can be classified into two major types: wireless and wired communication systems. Wireless communication can be further classified into wide-area and short-range communication systems. Short-range communication has two subdivisions: dedicated short-range (formerly vehicle-to-roadside) and vehicle-to-vehicle communications [30].

Wide-area communication systems are currently used for cellular phone communication, and short-range communication systems are used in VANET. At present, cellular phone communication systems are very widely used, and network coverage is nearly universal. It is much easier to use the existing cellular phone networks and improve upon them rather than develop a new VANET. The cellular phone network is reliable and will continue to evolve and mature. In addition to being popular and stable, the benefit of cellular phone networks is that the infrastructure is already in place. However, VANET will still be needed for vehicle-to-vehicle communication.

1.4. The Rests of the Article

The focus of this research is on developing a cell phone-based system to replace VANET and thus eliminate the bottleneck that has developed with regard to improving this form of ITS communication. The remainder of this article is organized as follows. In Section 2, we review the UMTS technology, AKA protocol, and MBMS (Multimedia Broadcast/Multicast Service). In Section 3, we extend the UMTS-IMS-AKA (IMS: IP Multimedia System) protocol which is based on a group key. This proposed protocol is compliant with ITS. In addition, we also propose a dedicated vehicle-to-vehicle communication system in this section. Related discussion and analysis will be presented in Section 4. Finally, we give our conclusions in Section 5.

2.1. UMTS-AKA and UMTS-IMS-AKA Protocols

The UMTS-AKA protocol [32] is an authentication and key agreement protocol. It is equipped by the 3GPP (3rd Generation Partnership Project). The objective is to meet the requirements of UMTS so that the mobile device can stay secure both during the authentication process and during the telecommunication session. As shown in Figure 3, the UMTS-AKA protocol has two phases. One is the phase of distribution of authentication vectors from HE (Home Environment) to SN (Service Network). The other is the phase of authentication and key establishment. Table 1 defines the relevant symbols.

MS, SN, HE	Mobile Station, Service Network, Home Environment
VLR,HLR	Visitor Location Register, Home Location Register
AuC, SQN	Authentication Centre, Sequence Number
USIM	Universal Subscriber Identity Module
IMSI	International Mobile Subscriber Identity
TMSI	Temporary Mobile Subscriber Identity
AV, AUTN	Authentication Vector, Authentication Token
K	Secret Key which share between USIM and AuC
MAC	Messages Authentication Code
AMF	Authentication Management Field
Rand, RES	Random Number, User Response
XRES	Expected User Response
CK, IK, AK	Cipher Key, Integer Key, Authentication Key
f1 f5	Authentication and Key Generation Function
UE	User Equipment
IMPI, IMPU	IP Multimedia Private Identity , IP Multimedia Public Identity
P-CSCF	Proxy Call Service Control Function
I-CSCF	Intergating Call Service Control Function
S-CSCF	Service Call Service Control Function
P-CSCF	Proxy Call Service Control Function

When an MS enters into the service domain of the SN, or VLR (Visitor Location Register), for the first time, it is executing the phase of distribution of authentication vectors from HE to SN and completing a registration procedure. This procedure, in addition to making MS's HE aware of the MS location, can let the SN obtain the AVs (Authentication Vectors) from MS's HE (for authentication with the MS in the future). AVs include the set of authentication vectors and can provide time authentication between MS and SN. If MS is always registered and it wants to use a service in SN, it can execute the phase of authentication and key establishment leading to mutual authentication. Thus they confirm the legitimacy of each other. In this protocol, the basis of authentication is a secret key that is shared between MS's HE and MS. Through USIM (Universal Subscriber Identity Module) protection, this key can be recognized only by MS and its HE. SN and HE may be different system operators; this characteristic helps UMTS to expand its service range (and thus make our application ITS-compatible). Once authenticated, both parties can establish the cipher key and the integer key. Using these keys allows their messages to remain private.

UMTS is a huge system constituted by many subsystems to keep abreast of the demands. The UMTS-AKA protocol is the main mechanism for authentication and key establishment. The different subsystems operate a bit differently because of varied demands; however, the same concept is reused for the IP multimedia core network subsystems, where it is called the UMTS-IMS-AKA protocol [33]. Figure 4 illustrates this procedure, while Table 1 defines the relevant symbols.

Basically UMTS-IMS-AKA and UMTS-AKA are alike and merely run in different environments. IMPI (IP Multimedia Private Identity) corresponds to IMSI (International Mobile Subscriber Identity); IMPU (IP Multimedia PUblic Identity) to TMSI (Temporary Mobile Subscriber Identity); UE (User Equipment) to MS; P-CSCF (Proxy Call Service Control Function) to I-CSCF (Interrogating Call Service Control Function); S-CSCF (Service Call Service Control Function) to SN; HSS (Home Subscriber Server) to HE—from all these, one can find that UMTS-IMS-AKA and UMTS-AKA are alike with regard to their logic and concepts. Although the method of calculating the parameters of UTMS-AKA and IMS-AKA are identical, their parameters are transported in slightly different ways. Since messages are delivered on IP networks in the UMTS-IMS-AKA protocol, additional parameters must be developed.

2.2. UMTS Multimedia Broadcast/Multicast Service

The MBMS (Multimedia Broadcast/Multicast Service) [34, 35] has been standardized in the 3GPP. It provides a point-to-multipoint service of transmitting multimedia data via the existing UMTS cellular networks. MBMS offers two service modes: broadcast mode and multicast mode. Both of them can share the same data from a single source to multiple recipients. Figure 5 illustrates their architecture [35]. The security architecture of MBMS has been defined elsewhere [36].

BM-SC is a Broadcast/Multicast Service Centre which is a source for MBMS data, or scheduling and receiving MBMS data from third parties. It offers an interface for content providers to deliver the requested data to allocated users. SGSN performs user individual service control functions and provides MBMS data transmissions to UTRAN/GERAN. It also provides support for intra- and intermobility procedures and indicates its MBMS support to the UE. Moreover, SGSN maintains a single connection with the source of MBMS data and concentrates all users of the same MBMS service into a single MBMS service. The role of GGSN within the MBMS architecture is that of a gateway for MBMS data. It links the tunnels from SGSN with MBMS data source via IP multicast.

With respect to the security, BM-SC uses the HTTP digest authentication mechanism [37, 38] to authenticate the UEs, and the GBA ( Generic Bootstrapping Architecture) [39] to establish the secret share key with the UEs. HTTP digest authentication mechanism is defined in RFC 2617. UMTS-MBMS is specified in the clause "procedures using the bootstrapped Security Association" [37]. It is run between BM-SC and ME just as the UMTS-IMS-AKA protocol is executed between SN and MS, and their methods are the same in essence. In order to keep the privacy of MBMS data while conveying, four keys, MRK (MBMS Request Key), MUK (MBMS User Key), MSK (MBMS Service Key) and MTK (MBMS Traffic Key), must be established. MRK is used to authenticate UE to BM-SC when performing key requests; MUK is the MBMS user individual key used by BM-SC to protect MSK transferred to UE; MSK is used to protect the delivery of MTK; MTK is used to decrypt the received MBMS data on UE.

3.1. Group Key Extension of the UMTS-IMS-AKA Protocol

IMS is an extension subsystem of the UMTS to support IP-based multimedia services. It has been designed to support the point-to-point connection with unicasting transmission. Moreover, MBMS facilitates the point-to-multipoint transmission of broadcast and multicast. The earlier release of the 3GPP standard did not provide the integration of IMS and MBMS. Although IMS and MBMS are separate from the system, MBMS must be used with IMS. This will cause duplication of resources wasted. Fortunately, it has been considered and integrated in the latest release. Moreover, some reports [40–42] provided the integration of IMS and MBMS. The release and the literatures favor the integration of their functionalities and not their security mechanism. However, in the ITS environment, vehicles move so fast that special focus need to be given to the response and performance on communication. For this reason, the proposed protocol attempts to expand on the existing IMS-AKA protocol to enable it to support the group key on MBMS. In addition, considering the compatibility of the original system, we try to minimize the movement of integration.

First, observe the special situations for MBMS in ITS environment and classify them into two kinds. One is the transmission of multimedia, such as audio, video, or movie. This kind of transmission is very suitable for the original UMTS-MBMS because the source is fixed and the connection is continuing. Another one is the transmission of messages, such as traffic information, travel information, traffic control messages, and traffic management messages. This kind of transmission has a feature that the contents of the messages are regional. That is, the contents of the messages change with the location of the vehicle. The original UMTS-MBMS is unable to meet this demand because of the following.

For the above reasons, this article proposes to combine a group key with the UMTS-IMS protocol to making it compatible with ITS. The proposal, however, does not affect compatibility with the original protocol since it only increases some parameters with the original behind. Figure 6 outlines our proposal.

Our protocol is very similar to the original IMS-AKA protocol; an obvious characteristic is that our proposal joins a vehicle key, VK, and a random number, SN. However,VK is an exclusive key allocated to each vehicle that enters the P-CSCF service range; it is used to protect the delivery of GK, and GK is a group key used to decrypt the received data on the vehicle. Also, SN is a random number selected by the vehicle; it is used to generate VK. Their relationship is

(1)

Here is a key-generating function, like the on the UMTS. It may replace or on the UMTS. The details of our proposal are as follows.

Upon completing the above steps, the vehicle and the CSCF can initiate communication in two ways. Private data is encrypted by the original and , and the group data is encrypted by the . This scheme governs the private communication provided by the original UMTS-AKA protocol, and provides the group communication scheme. Moreover, it can be applied to environments that use wide-area wireless and vehicle-to-roadside communications.

3.2. Vehicle-to-Vehicle Communication Systems for UMTS

To provide a more complete scheme for ITS, we devised a new AKA protocol to be applied to the vehicle-to-vehicle communication systems. Figure 7 illustrates this protocol.

When neighboring vehicles want to communicate, they must obtain the IMPU of the other party first, as in Stage 1. IMPU resembles alias that can temporarily identify the user and can hide his true identity to achieve anonymity. In this stage, vehicles obtain the IMPU of the other party by other techniques such as sensor networks or wireless networks. UMTS also provides location techniques [46] that are helpful in this case. In Stage 2, the two parties exchange IMPU and convert them to CSCF. Since the vehicles always stay within the service range of the CSCF, they will pass through Phase 1 of our IMS-AKA protocol. CSCF has already confirmed both parties' IMPI, and selected a meeting key, , which encrypts the messages exchanged between both parties. Then, take both parties' to encrypt the and send back during Stage 3. The purpose of encrypted with the respective is to hide the of both parties; so they cannot be obtained by outside parties while allowing both authorized parties to decrypt the encrypted with his .

We have proposed two AKA protocols that allow UMTS to configure the ITS environment. One applies to wide-area and vehicle-to-roadside communication systems. The other applies to vehicle-to-vehicle communications. In other words, our proposal helps apply the UMTS to ITS environments.

4.1. Property

According to the recipient of the message, ITS wireless communication can be classified into two types.

(1)Vehicle-to-roadside communication: it indicates that the vehicle and ITS deliver data by means of wireless infrastructure. Generally it is the ITS that delivers relevant data to the vehicle. Those data may regard road conditions, video, or audio.

(2)Vehicle-to-vehicle communication: it indicates that the relevant data is delivered between vehicles. In this mode, one can view the vehicle as the router. It resembles the Ad hoc network as it can automatically search and link the neighboring vehicles to form a topology. Moreover, neighboring vehicles also can link themselves together to exchange messages.

ITS wireless communication can also be classified according to the data contents.

These characteristics help us understand the suitability of our scheme.

4.2. Realization

Currently, developments in wireless communication on ITS concern an exclusive network called VANET. VANET was constituted by an Ad-hoc network which automatically links the neighboring vehicles to form a topology. However, Ad-hoc is an exclusive network technique under development. The biggest problem is that it is not stable. Its topology is constituted by mobile nodes (vehicle) that may change at any time. An inherent drawback is that vehicles that are too close or too far cannot communicate. To solve this defect, fixed access points have been used to strengthen the signal, since the outdoor wireless network's coverage is still not comprehensive. However, it will be quite expensive to build the necessary infrastructure to solve this problem. These problems are completely avoided with UMTS. More importantly, its infrastructure is nearly complete, thus avoiding the problems of lack of signal coverage.

Seamlessly UMTS is the best platform to use with ITS. It can support vehicle-to-roadside as well as vehicle-to-vehicle communications when our modifications are used. The original UMTS was supported by IMS and MBMS to transfer private and common messages. However, when applied to the environment of ITS, it will have some additional considerations, especially the fast movement caused by rapid changes in message routing and multicasting members of the frequent changes. Both of them will cause a heavy load on the MBMS. For this, our protocol introduces the concept of group keys. Moreover, in the UMTS, IMS and MBMS are two separate systems, and the MBMS must be used with the IMS. This has caused wastage of resources and authentication delays. For this, our protocol integrates them into a single protocol. However, to solve the bottlenecks, our protocol is an expansion of the existing IMS-AKA protocol using support group key. This does not only combine IMS and MBMS but also solves the problems of MBMS in the ITS.

Two protocols are proposed in this paper. One is an improvement over the IMS-AKA protocol; another is a novel V2V-AKA protocol. The improvement over the IMS-AKA protocol was achieved by integrating a vehicle key on the UMTS-IMS-AKA protocol. The V2V-AKA protocol is an innovative design. It makes direct communication between vehicles possible on the UMTS. The suitable collocation of our IMS-AKA protocol with our V2V-AKA protocol can get all-round development on ITS. Generally messages delivered by our IMS-AKA protocol and interaction with vehicles are shared with our V2V-AKA protocol. Some of the applications can be accomplished with slight revision. For example, ETC (Electronic Toll Collection) system can to regard our protocol as their charging solution. In this scenario, P-CSCF corresponds to the tollbooth. When a vehicle passes through the tollbooth, the tollbooth sends the authentication request to the vehicle. Our IMS-AKA protocol can be adopted in this scenario. With our protocol, authentication and authorization are assured, and the billing program can also be solved.

4.3. Security Analysis

Our proposal has the structure of the original IMS-AKA protocol and inherits its security features. All the fundamental conditions of security [31] on the IMS-AKA protocol are also attained in our protocol, including anonymity and untraceability. Moreover, our protocol can achieve three objectives (confidentiality, integrity, and availability) [47] of ITS to resist four general threats (deception, disruption, usurpation, and unauthorized disclosure) [47]. Only authorized vehicles can have the secure key and pass the authentication and get the relational key and . All unauthorized vehicles intercept unreadable secret content. Moreover, as in the IMS-AKA protocol, our protocol has the enhanced feature of mutual authentication.

On vehicle-to-roadside communication, in order to retain compatibility with the original IMS-AKA protocol, we have fine-tuned the original protocol. The main change is that we have joined a vehicle key, . However, is generated by and ; is a random number selected by the vehicle; is a secret key shared between the vehicle and HSS. By the way, the vehicle has participated in the decision of because is offered by him. The advantage is that a vehicle can very easily identify fresh , and prevent a malicious attacker from stealing and reusing it. The group key, , used for encrypting the group messages is encrypted by and delivered to the corresponding vehicle. Therefore, will not leak it in the process of transaction, and group messages can also maintain secrecy.

On the vehicle-to-vehicle communication, our protocol combines UMTS and Ad-hoc. Both the technologies rely on the UMTS authentication mechanism that verifies the identity of vehicles and uses the Ad-hoc network architecture to communicate between the vehicles. It has the advantage of UMTS's security and Ad-hoc network's convenience. Moreover, vehicles know each other, and only the IMPU can ensure the anonymity of the two sides. Using to encrypt the meeting key, , and the transaction can guarantee that will not leak during the transmission.

Keeping with these discussions, a conclusion can be derived that our proposed is based on the original IMS-AKA protocol and continues to develop. In our expanding function, the random number, , and the secret key, , making the vehicle key, , has privacy and security. Since the is secure, and protected by are also secure; therefore, our protocol is secure.