Supervisor: Kaisa Sere
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- 1.2 Security
1.1 Ad Hoc NetworksWireless multi-hop1 ad hoc networks comprise of lightweight mobile computers, also called mobile hosts (the synonym node may also be used), equipped with radio enabled network interface cards to meet communication needs. Every host can communicate directly with hosts to which the radio signal can be “heard”. These nodes are called the neighbors of mobile host. Direct communication between neighbors is facilitated by point-to-point radio links which are also called one “hop” in routing terminology. Radio frequency bands are a scarce resource and often shared among all hosts, some Medium Access Control (MAC2) protocol has to be used to coordinate access to the medium. This can be done in a centralized or distributed fashion. For instance, Bluetooth uses a centralized scheme where master of a network assigns turns for transmission to slaves. IEEE 802.11 is a MAC protocol that can utilize both schemes, but in ad hoc networks the distributed scheme is used. In this scheme nodes contend for transmission turns, but if more than one host wants to transmit at the same time, a random host is chosen for transmission. MAC protocols will be discussed in chapter 2 in detail. In ad hoc networks, mobile hosts serve as user terminals as well as routers, implying that every host taking part in an ad hoc network has to be prepared to forward other host’s traffic. Some protocols are more flexible and allow hosts to omit routing functions when its batteries are showing weakening levels of power. Although a very important issue by itself, battery consumption is not within the scope of this text and only touched briefly upon in section 3 in this chapter. As with fixed networks, a routing protocol (OSI layer 3) has to be used in order to enable forwarding. This protocol performs routing which is a topology discovering process done by the nodes of the network. The type of the routing protocol specifies how much of this information has to be up-to-date (i.e. reactive or proactive). The output of the routing phase is a routing table, which can be consulted as forwarding is performed. This text will, to the largest part deal with routing protocols. Chapter 3 discusses existing routing protocols and chapter 5, 6, 7 and 8 presents a routing protocol developed as a case study for this text and results of the protocol simulation. As with routing protocols, transport protocols (OSI layer 4) also have to be adapted for ad hoc networks. For instance, the Internet Transmission Control Protocol (TCP) for fixed networks has a congestion control mechanism that can misinterpret link breakage for congestion and slow down data rates in erroneous ways. Transport protocols are usually, in fixed networks, associated with end-to-end communication without intervention of intermediate routers, that is, the routers have no transport layer. In ad hoc networks the intermediate routers (normal nodes) must also participate in the transport layer functions in order to achieve optimal results [Chandran+98]. Security in wireless networks is a enormously intricate issue, requiring a complete text on its own, so an in-depth discussion of the issue will be beyond the scope of this text. A section with a summary of the issue can be found in section 1.2. Different security concerns exist on different layers, for instance at the MAC layer one has to guard against eavesdropping and signal sabotage whereas the network layer must see to it that no node tries to “free-ride” by refusing to forward traffic from other nodes. The network and transport layers may also implement various encryption techniques to hide sensitive data. A last issue covered in this introductory chapter includes applications for ad hoc networks. 1.2 SecurityIn wireless networks, one has to be extra mindful of the fact that everyone that is in the same area of the network could in theory have access to the medium. If no precaution is taken to prevent this, an unauthorized user could tune in to the right frequency and “listen” to data addressed to other users. The schemes that are briefly talked about in chapter two, called spread spectrum (SS) frequency hopping and SS direct sequence can prevent unauthorized access of the medium. These techniques see to it that the radio signal never stays on the same frequency for more than a fraction of a second and then jumps to another. Only an authenticated receiver is aware of the frequency sequence that the sender uses, so this can at least in theory block outsiders from “overhearing” secret data. In addition, because every node serves as a router as well, it is of outmost importance that a user cannot access data that is being forwarded for another node. Because it is still possible for unauthorized users to monitor network activity provided that they know the pseudo random sequence, additional encryption techniques have to be provided. In 802.11b wireless networks this is provided with an encryption algorithm called “wireless equivalent privacy” (WEP). The WEP algorithm is, as they put it in [IEEE99] “reasonably strong”, which means that it is difficult enough to make it discouraging to try to discover the secret key with brute force methods in practice. WEP takes care of two things:
The process of encryption begins by generating a 64-bit key k, by concatenating a random 24-bit Initialization Vector (IV) and a 40-bit secret key. The strength of the encryption correlates to the length of the secret key and the frequency of its alteration. K is then fed to a pseudo random number sequence generator whose output r is used to encrypt data d by executing r d = e. E will have equal length of d plus four bytes containing a Integrity Check Value (ICV). Thus, both the data and ICV is encrypted by r. The four byte ICV is generated by a CRC-32 algorithm based on the plaintext data d. The message that is sent to the other party begins with the IV (as plaintext) followed by e. For deciphering e, the IV in the message is again concatenated with the pre-distributed3 secret key and feed into the pseudo random number sequence generator to regenerate the r. The lifetime of the secret key will be prolonged since nodes only have to change the IV and new keys (k) can be generated. Consequently, executing r e will generate d. To implement proper authentication, stations must implement WEP because the same shared secret keys are utilized in both. This works by requiring that a station wanting to connect to the network can encrypt a text string generated by a receiver, belonging to the network. If the requesting station encrypted the random text string so that the receiver can decrypt it, this means that the requesting station “knows” the secret key and is therefore authenticated. Download 298.62 Kb. Share with your friends:
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