Access control is the security mechanism to ensure that only valid users are allowed access to the network. In the most general terms, an access control system has three elements: (1) an entity that desires to get access: the supplicant, (2) an entity that controls the access gate: the authenticator, and (3) an entity that decides whether the supplicant should be admitted: the authentication server.
Figure 7.11 shows a typical access control architecture used by service providers. Access control systems were first developed for use with dial-up modems and were then adapted for broadband services. The basic protocols developed for dial-up services were PPP (point-to-point protocol) [60] and remote dial-in user service (RADIUS) [50]. PPP is used between the
Si d e ba r 7 . 2 T h e M a t h B e h i n d A s y m m e t r i c Key E n c r y p t i o n : RSA A l g or i t hm
Asymmetric key encryption is based on the simple fact that it is quite easy to multiply two large prime numbers but computationally very intensive to find the two prime factors of a large number. In fact, even using a supercomputer, it may take millions of years to do prime factorization of large numbers, such as a 1,024-bit number. It should be noted that although no computationally efficient algorithms are known for prime factorization, it has not been proved that such algorithms do not exist. If someone were to figure out an easy way to do prime factorization, the entire PKI encryption system would collapse.
Here are the steps the RSA (Rivest-Shamin-Adleman) algorithm uses for public/private key encryption [52].
1. Find two large prime numbers p and q such that N = pq.N is often referred to as the modulus.
2. Choose E,the public exponent, such that 1 < E < N, and E and (p– 1) (q– 1) are relatively prime. Two numbers are said to be rela- tively prime if they do not share a common factor other than 1.
N and E together constitute the public key.
3. Compute D,theprivate key, or secret exponent, such that (DE– 1) is evenly divisible by (p– 1) (q– 1). That is, DE = 1{mod[(p– 1) (q– 1)]}.
This can be easily done by finding an integer X that causes
D= (X(p– 1)(q – 1) + 1)/Eto be an integer and then using that value of D.
4. Encrypt given message M to form the ciphertext C, using the function C=ME[mod(N)], where the message M being encrypted must be less than the modulus N.
5. Decrypt the ciphertext by using the function M =CD[mod(N)]. To crack the private key D, one needs to factorize N.
supplicant and the authenticator, which in most cases is the edge router or network access server (NAS), and RADIUS is used between the authenticator and the authentication server.
PPP originally supported only two types of authentication schemes: PAP (password authen- tication protocol) [37] and CHAP (challenge handshake authentication protocol) [65], both of which are not robust enough to be used in wireless systems. More secure authentication schemes can be supported by PPP using EAP (extensible authentication protocol) [38].
7.3.3.1 Extensible Authentication Protocol
EAP, a flexible framework created by the IETF (RFC 3748), allows arbitrary and complicated authentication protocols to be exchanged between the supplicant and the authentication server.
EAP is a simple encapsulation that can run over not only PPP but also any link, including the WiMAX link. Figure 7.12 illustrates the EAP framework.
Figure 7.10 Mutual authentication and shared key distribution using PKI
Figure 7.11 Access control architecture
User A User B
Send (Random Number A, My Name) encrypted with public key of B.
Send (Random Number A, Random Number B, Session Key) encrypted with public key of A.
Send (Random Number B) encrypted with Session Key.
Begin transferring data encrypted with Session Key.
IP Network Authenticator
(Network Access Server)
AuthenticationServer EAP
EAP
RADIUS/DIAMETER Link-Layer Protocol
(e.g., PPP, Wi-Fi,WiMAX) User 1
User 2
User n
EAP includes a set of negotiating messages that are exchanged between the client and the authentication server. The protocol defines a set of request and response messages, where the authenticator sends requests to the authentication server; based on the responses, access to the client may be granted or denied. The protocol assigns type codes to various authentication meth- ods and delegates the task of proving user or device identity to an auxiliary protocol, an EAP method, which defines the rules for authenticating a user or a device. A number of EAP methods have already been defined to support authentication, using a variety of credentials, such as pass- words, certificates, tokens, and smart cards. For example, protected EAP (PEAP) defines a pass- word-based EAP method, EAP-transport-layer security (EAP-TLS) defines a certificate-based EAP method, and EAP-SIM (subscriber identity module) defines a SIM card–based EAP method. EAP-TLS provides strong mutual authentication, since it relies on certificates on both the network and the subscriber terminal.
In WiMAX systems, EAP runs from the MS to the BS over the PKMv2 (Privacy Key Man- agement) security protocol defined in the IEEE 802.16e-2005 air-interface. If the authenticator is not in the BS, the BS relays the authentication protocol to the authenticator in the access service network (ASN). From the authenticator to the authentication server, EAP is carried over RADIUS.
7.3.3.2 RADIUS
The most widely used standard for communication between the authenticator and the authentica- tion server, RADIUS, is an IETF standard [50] that defines the functions of the authentication server and the protocols to access those functions. RADIUS is a client/server UDP application that runs over IP. The authentication server is the RADIUS server, and the authenticator is the RADIUS client. In addition to authentication, RADIUS supports authorization and accounting functions, such as measuring session volume and duration, that can be used for charging and billing purposes. The authentication, authorization, and accounting functions are collectively referred to as AAA functions. Numerous extensions to RADIUS have been defined to accom- modate a variety of needs, including supporting EAP.
RADIUS, however, does have a number of deficiencies that cannot be easily overcome by modifications. Recognizing this, the IETF has developed a new standard for AAA functions:
DIAMETER [13]. Although not backward compatible with RADIUS, DIAMETER does pro- vide an upgrade path to it. DIAMETER has greater reliability, security, and roaming support than RADIUS does.
7.4 Mobility Management
Two basic mechanisms are required to allow a subscriber to communicate from various locations and while moving. First, to deliver incoming packets to a mobile subscriber, there should be a mechanism to locate all mobile stations (MS)—including idle stations—at any time, regardless of where they are in the network. This process of identifying and tracking a MS’s current point of attachment to the network is called location management. Second, to maintain an ongoing session as the MS moves out of the coverage area of one base station to that of another, a mech- anism to seamlessly transition, or hand off, the session is required. The set of procedures to
manage this is called handoff management. Location management and handoff management together constitute mobility management.