28-01-2016, 11:31 AM
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. The licensing of the wireless spectrum is currently undertaken on a long-term basis over vast geographical regions. In order to address the critical problem of spectrum scarcity, the FCC has recently approved the use of unlicensed devices in licensed bands. This new research area has led to CR networks progress. Unlicensed users named as cognitive radio or secondary users, are obliged to empty the band as soon as they face licensed or primary users. The cognitive radio enables the usage of temporarily unused spectrum, which is referred as spectrum hole or white space. If this band is utilized by a licensed user later, the cognitive radio moves to another spectrum hole or stays in the same band, altering its transmission power level or modulation scheme to avoid interference.
Cognitive radio (CR) networks can be classified as the infrastructure-based CR network and the Cognitive-radio Ad- Hoc Networks (CRAHNs) [1]. Since CRAHNs have no infrastructure, a CR user can communicate with other CR users through ad hoc connection on both licensed and unlicensed spectrum bands. The spectrum sensing is a key factor in cognitive radio technology, for it avoids harmful interference with licensed users and finds existing spectrum holes for the CRs. In the infrastructure-based CR networks, the observations and analysis performed by each CR user feeds the central CR base-station, so that it can make decisions on how to avoid interfering with primary networks. According to this decision, each CR user reconfigures its communication parameters. On the contrary, in CRAHNs, each user needs to have all CR capabilities and is responsible for determining its actions based on the local observation.
The cooperative spectrum sensing scheme investigated in [2] is necessary because a CR user in CRAHNs is not able to foresee his behavior’s effect on all the networks merely on the basis of local observations and it’s not possible for all the CRs to experience receiver uncertainty or fading altogether because of the place or local differences as well. So if the majority of users observe a primary user, they will be able to share their results. Consequently, the overall detection performance can be greatly improved. The cooperation among CR users raises new concerns for the reliability and the security in cooperative sensing. This is because, when multiple CR users cooperate in sensing, a few CR users who report unreliable or falsified sensing data can easily influence the cooperative decision. This common security threat in CRs is named as spectrum sensing data falsification (SSDF) attack. Some techniques have been mentioned in [3]-[9] to clarify SSDF attack occurrence in CR networks and a novel bio-inspired consensus-based cooperative spectrum sensing scheme is presented in [10] to encounter SSDF attacks in CR- MANETs. Their scheme is based on recent advances in consensus algorithms that have taken inspiration from self- organizing behavior of animal groups such as birds, fish, ants, honeybees and others. Unlike the existing schemes, there is no need for a common receiver to do the data fusion for reaching the final decision to counter SSDF attacks. The scheme has self-configuration and self-maintenance capabilities. Moreover, in order to further improve the security of CR-MANETs, the authors present an authentication scheme using identity-based cryptography with threshold secret Sharing.
As a new idea in a cognitive radio ad-hoc network, 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.
In previous schemes, several authors have applied this idea and proposed a new security scheme for CRAHNs. Also they presented a cooperative spectrum sensing scheme considering the threshold and Identity- based cryptography parameters which cannot only recognize SSDF attack occurrence but also find an intruder and delete it from the network or at least harden its access to the “t” number of neighboring keys. In this article, we investigate the efficiency of the proposed model of [11] for a wide variety of compromised malicious nodes and consequently modify some of the detection parameters for improvement of the scheme comparing with [10]. The new scheme is named S-CRAHN.
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.
Threshold and Identity-Based Key Management in MANETs
Mobile AD-Hoc networks encounter with much more security problems in comparison with wired networks for some reasons like the lack of a network infrastructure or dynamic topology of the network and also wireless link damages. The common security techniques are usually effective for several security threats while they are not appropriate enough for a combination of the former ones. Cryptography is then used to provide a general design framework. Cryptography techniques used in MANETs can be classified into two categories named as Symmetric Key based and symmetric Key based. In Symmetric key based schemes, if an attacker compromises the symmetric key of a group of users, all encrypted messages relating to that group will be exposed. Asymmetric key schemes have more functionalities than symmetric ones e.g. key distribution is much easier, authentication and non-repudiation are much more available, and a private key compromise of a user does not reveal messages encrypted for the others in the group; however, they are estimated expensive. Asymmetric cryptography is usually based on PKI. Since PKI success is bounded to a CA4 possessing a central control unit, it is not proper for MANET.
The application of IBC as a form of a PKI and asymmetric cryptography in MANET was proposed as a significant research subject because it doesn’t require issuing licenses and public keys. In the standard application of an Identity-based encryption in an e-mail system the master-key stored at the PKG must be protected in the same preservation way of a CA private key. One way for key protection is to distribute it among different sites using techniques of threshold cryptography [14]. Most of the Identity-based cryptography systems apply Shamir threshold cryptography [12] in which they contribute a secret quantity among a number of users. Shamir threshold cryptography explains how to divide D to n- parts in such a way it can be easily reconstructed from t-parts, and even in the case of being completely informed about t-1 parts, D still remains secret and not any information reveals about it. Shamir proposes a (t, n) threshold scheme to solve this problem based on polynomial interpolation. He suggests picking a random t-1 degree polynomial q(x)= a_0+a_1 x+⋯+a_(t-1) x^(t-1) in which, a_0=D, and each piece is the polynomial at the n points. Thus any subset of “t” of the piece can determine the coefficients of the polynomial (using e.g. Lagrange interpolation) and thus the secret data at a certain point.
The idea of distributed CA has been subsequently adopted for distributed PKG in many IBC proposals in MANETs later. Khalili [15] suggested to apply IBC for a secure AD-Hoc network and create a mechanism for an effective key distribution in MANET with contribution of both IBC and threshold cryptography techniques. References [16], [17] presented fit and proper schemes on the mentioned subject too. All methods relating to the application of IBC in MANET have been investigated in [18]. The proposed approach in [17] consists of two components: distributed key generation and identity-based authentication. The key generation component provides the network master key pair and the public/private key pair to each node in a distribute way. The generated private keys are used for authentication.
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.
Distributed PKG
Using the identity-based cryptography, a mobile node’s public key can be any arbitrary string. In our scheme, the public key is computed as Q_ID=H(ID∥Expire-time)where H(.) stands for a hash function defined in identity-based encryption (Random oracles are used to model cryptographic hash functions) [14], ID represents the identity of the node, and Expire_time is a time stamp protecting from the private key loss. The nodes also need to obtain their corresponding private keys. The ways to obtain the private key is to contact at least “t” number of neighbor nodes, present the identity and request private key generation (PKG) service. Each of the “t” PKG service nodes generates a secret share of new private key S_k and sends to the requesting node. The process of generation of a share of the new secret key S_k can be represented by SK_i=s_i.Q_ID, where s_i (i=1,….,t) is the share of the master private key of the serving node, ID is the identity of the requesting node, Q_ID is its public key, and S_k, denotes the generated private key share for the requesting node. By collecting the “t” shares of its new private key, the requesting node would compute its new private key SK=∑_(i=1)^n▒〖S_i.Q_ID 〗.
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).