01-10-2014, 12:51 PM
S-CRAHN: A Secure Cognitive-Radio Ad-Hoc Network
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Abstract
Cognitive-radio (CR) technology is to solve the spectrum scarcity problem, make accessible supplementary spectrum bands required for the data transmission in mobile ad hoc networks (MANET) and provide an appropriate level of security for CR networks received far less attention than other areas regarding to common key management schemes for MANET as well. Key management and authentication are two important factors in MANET security. The recent development in Identity-based cryptography has made the method to be a potential candidate for MANET. However the security in CR-MANET has attracted less attention in comparison with other regions. In this article, the authors try to propose a new security scheme for CR- MANET called as S-CRAHN which is fundamentally on the basis of a threshold identity-based cryptography. This method results in the elimination of SSDF attack trouble in Cognitive radio Ad-Hoc Networks, where the intruder sends wrong results of local spectrum sensing and leads to a wrong spectrum sensing determination in CRs consequently. Following cooperative spectrum sensing scheme, it can find out SSDF attack occurrence, limit intruders’ access to “t” numbers of neighbour nodes for the key updating or delete the intruder nodes from the network.
Introduction
The current progress of communication networks and the growth of laptops and 802.11/Wi-Fi wireless networking have led to the formation of wireless Ad-Hoc networks as self-organized ones which can be formed without infrastructure. The Ad-Hoc networks have a wide capability for supporting and covering different wireless standards. Although their current application is severely bound to industrial, scientific and medical (ISM) band (900MHz to 240GHz). With the growing proliferation of wireless devices, these bands are increasingly getting congested. There are several licensed bands accessible for operators such as 400MHz to 700MHz ranges which are occasionally used.
A Brief History of Identity-Based Cryptography
Identity-based cryptography (IBC) is a special form of a PKI cryptography considered as an asymmetric cryptography. The idea of IBC was first proposed by Shamir [12] in 1984. Shamir introduced a novel type of cryptographic scheme which enables two partners to communicate securely and to verify each other’s signatures without exchanging private or public keys, keeping key directories, and using the services of a third party as well. In such a scheme, a user’s public key is an easily calculated function of his identity, while a user’s private key can be calculated for him by a trusted authority called a Private Key Generator (PKG). The identity-based public key cryptosystem can be an alternative for certificate-based PKI, especially when efficient key management and moderate security are required. Comparing with traditional PKI, it saves storage and transmission of public keys and certificates which is especially attractive for devices forming MANETs.
For a long time after Shamir published his idea, the IBC development was very slow; however in 2000, Joux [13] showed that Weil pairing can be used in a protocol to construct three-party one-round Diffie-Hellman key agreement; afterwards Boneh and Franklin [14] presented an identity-based encryption scheme at Crypto 2001 based on properties of bilinear pairings on elliptic curves which are the first fully functional, effectual and provably secure identity-based encryption scheme. This type of identity-based cryptography is also named Pairing-based Cryptography
Master Key Generation
The master key pair is computed collaboratively by the initial network nodes without constructing the master private key at any single node. The scheme we used [17] is an extension to Shamir’s secret sharing [12] without the support of a trusted authority. In the scheme, each node C_i randomly chooses a secret x_i and a polynomial f_i (z) over GF(q) of degree t-1, such that f_i (0)=x_i. Node C_i computes his sub-share for node C_j as SS_ij=f_i (j) for j=1,…,n and sends SS_ij securely to C_j. After sending the n-1 sub-shares, node C_j can computes its share of master private key as S_j=∑_(i=1)^n▒SS_ij =∑_(i=1)^n▒〖f_i (j)〗 that is the master key share of node C_j is combined by the subshares from all the nodes, and each of them contributes one piece of that information. Similarly, any coalition of “t” numbers of shareholders can jointly recover the secret as in basic secret sharing using ∑_(i=1)^n▒s_i l_i (z) mod(q), where l_i (z) is the Lagrange coefficient. It is easy to see that the jointly generated master private key skm= ∑_(i=1)^n▒x_i =∑_(i=1)^n▒〖f_i (0)〗.
After the master private key is shared, each shareholder publishes S_i.P, where P is a common parameter used by the boneh and Franklin’s identity-based scheme [14]. Then the master public key can be computed as QM=∑_(i=1)^n▒〖S_i.P〗. When a new node joins a network, it presents its identity, self-generated temporary public key, and some other required physical proof to “t” neighbouring nodes and then asks for PKG service, the master public key and its share of the master private key subsequently. Each node in the coalition verifies the identity validity of the new node C_m. If the verification process succeeds, the private key can be generated using the method described afterwards. To initialize the share of a master key for the requesting node, each coalition node C_i generates the partial share S_(m,i)=S_i.l_i (m) for node C_m. Here l_i (m) is the Lagrange term. It encrypts the partial share using the temporary public key of requesting node and sends it to node C_m obtains its new share by adding the partial shares as S_m=∑_(j=1)^t▒S_(m,j) . After obtaining the share of the master private key, the new joining node is available to provide PKG service to other joining nodes.
Spectrum Sensing in Cognitive Radio Ad Hoc Networks
The components of the cognitive radio ad hoc network (CRAHN) architecture can be classified in two groups as the primary network and the CR network components. The primary network is referred to as an existing network, where the primary users (PUs) have a license to operate in a certain spectrum band. Due to their priority in spectrum access, the PUs should not be affected by unlicensed users. CR users are mobile and can communicate with each other in a multi-hop manner on both licensed and unlicensed spectrum bands. Usually, CR networks are assumed to function as stand-alone networks, which do not have direct communication channels with the primary networks. Thus, every action in CR networks depends on their local observations. In order to adapt to dynamic spectrum environment, the CRAHN necessitates the spectrum-aware operations. The objectives of spectrum sensing are twofold: first, CR users should not cause harmful interference to PUs by either switching to an available band or limiting its interference with PUs at an acceptable level and, second, CR users should efficiently identify and exploit the spectrum holes for required throughput and quality of service (QoS).
SSDF Attack Models in Cooperative Spectrum Sensing Schemes
In cooperative spectrum sensing, malicious secondary users may launch SSDF attacks by sending false local spectrum sensing results to others, resulting in a wrong spectrum sensing decision. Three attack models are presented as follows [10]. In the first attack model, a malicious secondary user sends out relatively high primary user energy to indicate the presence of primary users although there is no primary user and its sensed energy is low. In this case, other secondary users make a wrong decision that primary users are present and they will not use the spectrum. The intention of the malicious secondary user is to gain the exclusive access to the target spectrum. This kind of attack is called a selfish SSDF. In the second attack model, a malicious secondary user sends out relatively low primary user energy to indicate the absence of primary users although there are primary users and its sensed energy is high. In this case, other secondary users make a wrong decision that there is no primary user and they will use the spectrum. The intention of the malicious secondary user is to give interference to primary ones. This attack is named interference SSDF. In the third attack model which is called a confusing SSDF, a malicious secondary user sends out random primary user energy during the process of cooperative spectrum sensing. The intention of the malicious secondary user is to make other secondary confused, and no consensus can be reached among secondary users.
S-CRAHN: A Secure CRAHN
As described in [11] in network formation phase, network public and private keys based on their identity are obtained for all the network nodes applying the current algorithm in [14]. Then a matrix is made inside each one of the nodes as table I. To accomplish primary exchanges of (t,n) threshold key management pattern based on identity and former algorithm phases of sensing data exchanges, a common default control channel is used; as a result 16 licensed channels can be made in accordance with spectrum sensing operation if IEEE 802.11 is applied.
Decision phase of S-CRAHN
If the counter against each node is equal or bigger than [t/2], one digit will be added to the RKC and the transmitted sensing results obtained from the node will be abandoned. Considering “t out of N” rule, this section is fulfilled on the neighboring t-node not the whole network ([t/2] out of t). This quantity of threshold may also be utilized instead of [3t/4] and [t/3].
After doing binary OR operation on the received string of the nodes where RKC is less than [t/2], the resulting string will be saved inside each node. If even one single channel is found occupied, it should be considered to avoid interference. The results obtained from empty channels are used for data transmission. In the next sensing phase, if the preceding process is enforced and a node from the same former nodes with an RKC equals 1 and -in one unit- increment (RKC=2), the neighboring node itself can delete the intruder’s ID with all rows and columns relating to the node’s NID, subsequently the total key updating process is accomplished. Now the intruder node can’t be approved by all the network nodes any more. So it is to be deleted instead of a proper threshold of t; otherwise it will face a trouble in obtaining t-share of t-PKG services
Conclusion
The area of security in CR-MANETs has received far less attention than other areas. Malicious CRs can send false local spectrum sensing results in cooperative spectrum sensing. In this paper we have presented a cooperative spectrum sensing scheme according to an Identity-based threshold key management for MANET to encounter SSDF attacks in CR-MANETs. Through the suggested scheme both SSDF attack and intruder nodes can be diagnosed with an accurate probability. By the way it doesn’t require frequent repetition and long convergence time needed for consensus–based schemes. With a proper selection of threshold “t” in S-CRAHN, the false alarm probability decreases comparing with the simulation results of a consensus-based scheme in [10]. It causes the network security, confidentiality and authentication as well and finds and discharges intruders from the network. To show current channels status, binary strings are used instead of correspondent signals transmission with different distributions. This procedure decreases the utilization of common control channel, delay and energy consumption too. Moreover, a common receiver is not needed for the final decision in the proposed scheme. It’s enforced on the neighbouring “t” nodes instead of the whole network. In the case of the network node increment, the internal calculation of each node limited to its neighbouring t-number is still remained. Consequently it’s a scalable