A lightweight identity authentication method by exploitingnetwork covert channel

Haijiang Xie & Jizhong Zhao

Received: 21 January 2014 /Accepted: 13 May 2014# Springer Science+Business Media New York 2014

Abstract The state of art authentication schemes are tightlylinked with encryption or crypto systems, which providesconcrete foundations to move towards the concept of accesscontrol by confirming the user identity. However the open-ness of the computer network makes the identity credentialsvulnerable even transmitted as cipher text especially in lotsof peer-to-peer (P2P) networks. The malicious attackerscan possibly steal and fake the user identi ty byeavesdropping, hijacking, cryptanalysis and forging. In thispaper, a novel identity authentication mechanism is pro-posed based on the reverse usage of the Network CovertChannel (NCC) which is originally designed by attackers tocreate stealth communication. Different from NCC, wherethe packet intervals can be exploited as the data carrier totransmit the unauthorized information, we exploit suchcapability in Network-Covert-Channel-based Identity Au-thentication (NCCIA) to transmit the identity tag. By vali-dating user identity in a covert manner, we provide a moresecure authentication method compared with many existingapproaches. A NCCIA demo system is designed on a FTPPlatform to verify our method. The experiments demon-strate the NCCIA can prevent the attackers fromeavesdropping while maintaining transmission efficiency.

Keywords Network security . Identity authentication .

Network covert channel . Packet intervals . Encryption

1 Introduction

Authentication severs as the cornerstone to maintain asecure network, which is always the primary concern whenestablishing a communication link between nodes in vari-ous types of networks such as lots of peer-to-peer applica-tions in order to validate the user’s identity and guaranteethe legitimacy of the communication [1, 2]. The widelyused password authentication is first proposed by Lamportin 1981 [3]. Since then a wide range of authenticationsolutions have emerged out, such as Kerberos [4],challenge-response authentication protocols [5], firewall,digital signature [6] and virtual private networks, etc.

Several more rigorous authentication methods evolves asthe growing sophistication of the attack tools and peopleattempting unauthorized access, such as smart card [7], digitalcertificate [8], biometric features [9], information hiding [10]and P2P network structure [11–13]. These state of art schemesare all tightly linked with encryption or crypto systems. Thecollaboration of cryptography and authentication providesconcrete foundations to move towards the concept of accesscontrol, which can help create user accountability and a morereliable audit trail. The key basis of these authenticationmethods is built on the share credentials, that is, the partybeing authenticated and the authenticator share the bindingrelationship of the credentials.

However, the openness of the computer network makes theshared credentials vulnerable when transmitted in the openchannel, considering a host of threats and attacks stemmingfrom the vulnerabilities within the network communicationsystem. It is of great chance that a malicious third party caneavesdrop and intercept the data packets both sent and re-ceived in the channel. The attacker can later reply all therecorded traffic, pretend to be the authorized user to gainaccess to system or even crack the authentication protocolwhen enough communication packets are captured.

H. Xie (*) : J. ZhaoDepartment of Computer Science and Technology, Xi’an JiaotongUniversity, Xi’an, People’s Republic of Chinae-mail: hjxie.xjtu@gmail.com

J. Zhaoe-mail: jizhongzhao@gmail.com

Peer-to-Peer Netw. Appl.DOI 10.1007/s12083-014-0287-x

Researchers have come up with many new solutions forauthentication without using the packets carrying identitymessage transmitted in the open network. Keystroke dynam-ics [14] and behavioral biometrics of human computer inter-action [15] are exploited to develop authentication method foridentity verification.

Covert Channel is a communication channel that violates asecurity policy by using shared resources in ways for whichthey were not initially designed [16]. A Network CovertChannel is built upon computer networks in which the sharedmedium is the network environment. It is always regarded as ahuge threat to a multi-level system in which the direct com-munication between two security levels is disallowed, sincecovert channel can find indirect ways to exploit the overtlegitimate communication to leak information to unauthorizedusers.

Originally designed as an information-leaking technique,covert channel intends to create stealth communication pro-cess to transmit unauthorized message from a high securityclearance process to a low clearance process under the coverof the legitimate communication channel. Reversely, byexploiting this intension of concealing the very existence ofinformation transmission for user validation, we can increasethe difficulty for a malicious attacker trying to steal and fakethe credentials by hijacking the communication as obscuringthe authentication process. Hiding the covert communicationfor authentication process makes a malicious third party diffi-cult to detect and obtain the message in the network covertchannel, even though the whole communication process hasbeen monitored, since there is little chance for the attacker tobe aware of the covert channel without the pre-knowledge ofits existence, which helps guarantee the communication legit-imacy and secure the network.

In this paper, a Network Covert Channel-based identityauthentication method is investigated and a demo system isimplemented. The intervals between two consecutive networkpackets are exploited as the data carrier to indicate the authen-tication tags with the identity information [17]. In our pro-posed method, two communication channels exist: the overtchannel for the legitimate communication process and thecovert channel for the embedded parasitic identity authentica-tion process. The packet sending time in the overt channel ismodulated by the authenticated party to transmit the authen-tication tag in the covert channel where the user identityvalidation process takes place. Based on our previous study,a NCCIA demo system is designed on a FTP platform toverify the proposed method, consisting of the client and theserver: at the client, the network packet intervals are manipu-lated to present authentication tag, the long delay means bit“1” and short delay means bit “0”; at the server, all the packetarriving time are recorded to decode the authentication tag.

The major innovative point of NCCIA is to integrate thenetwork security concept with an attack technique. The

network covert channel is applied for user identity validation,which is a risk in information-sensitive network in commonopinion emerges. Advocating authentication in a covert man-ner demonstrates our proposedmethod provides a more secureway to identify authorized users. Besides, NCCIA authentica-tionmethod requires no time synchronization between the twocommunication parties, which makes the authentication easilyapplicable and adapted for an open network. And the exper-iments in LAN and Campus Network verify our authentica-tion method in efficiency, robustness and stealth. The resultsprove that our approach does not degrade the overall systemperformance, which can achieve reliability against networkchannel delay and noise perturbation while maintaining trans-mission efficiency by not significantly decreasing the datarate.

The reminder of the paper is organized as follows:Section 2 is the related work; In Section 3, the framework ofour proposed method and the detailed design scheme is pre-sented; In Section 4, we describe the experimental platformrunning over TCP/IP networks, and analyze the performanceof our approach, with the discussion of encoding scheme; andSection 5 is the conclusion and future work.

2 Related work

Covert Channel is described as a communication mechanismthat allows an indirect communication for information leakagein a manner that violates the system’s security policy [14]. Thenotion is not necessarily new, which is first introduced byLampson in the 1970s as channels not intended for transmis-sion at all [18]. Various resources related methods, such as theallocation policies, management implementations and statevariables, are exploited as the carrier medium to establish acovert channel. Several elaborated mechanisms are designedfor covert communication, including the file-lock based, disk-arm based and bus-convention based covert channel [14]. Andthe basic idea of these initial methods is how to transfersensitive information to unauthorized parties by manipulatingthe shared resources in a single system.

Generally, covert channel are classified into two types:covert storage channel and covert timing channel. Covertstorage channel employs the storage location as the datacarrier, which entails the indirect or direct writing of a storagelocation by sender process and the correspondingly indirect ordirect reading of the same location by receiver process. Coverttiming channel mainly deals with the timing sequence servedas the signal that can be observed by the receiver if certainmanipulation is produced by the sender [19]. This requiresthat a sender process which signals information to another bymodulating its own use of system resources in such a way thatthe response time observed by the second process is changed.

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A Network Covert Channel is built upon the computernetworks in which the shared medium is the network environ-ment. Girling first proposes this concept in 1987 by elaborat-ing on three obvious covert channels, including two storagechannels and a timing channel, which demonstrates the pos-sibility of covert channel existence in LAN [20]. Later Wolfextends the work to the LAN protocols, presenting the realimplementation method of possible covert transmissions viathe unused bandwidth in the commonly deployed IEEE802.2,3,4,5 LAN architecture standards [21]. Handelbroadens the perspective of the general design for covertchannel by employing the network communication protocolsin the OSI model and establishes basic principles to hide datain each OSI layer [22], which paves the way for furtherresearch. Most protocols, such as TCP, UDP, HTTP, FTP,P2P and ICMP, are all exploited to set up network covertchannels [23–26]. Besides, timing sequence and patterns ofnetwork packets are also employed for covert timing channels[27, 28].

Kamran investigates the covert channel existence in TCP/IP protocol suits and proposes two design scenarios based onpacket header manipulation and packet sorting approach [24].Murdoch studies a variety of previous methods using TCP/IPpacket headers and creates a steganographic covert storagechannel as an improvement of data hiding scheme [25]. Cabukprovides a detailed and comprehensive description of covertstorage channel and covert timing channel, in which he de-velops a new family of covert timing channels that does notgenerate traffic anomaly [14]. Sellke contributes to provide asystematical encoding scheme and develop a non-detectablecovert timing channel [29]. Liu introduces a novel frameworkfor covert timing channel that preserves the traffic distributionpattern and guarantees robustness using a spreading encodingscheme [30]. And a further polynomial undetectable andreliable covert timing channel that can tradeoff between datarate and robustness for the i.i.d traffic models is proposed in[31]., which presents a high performance covert timingchannel.

However, the hardness to distinguish, the feasibility ofchoosing carrier medium and the diversity in implementa-tion makes covert channel detection and elimination a chal-lenging task. Covert channel usually arises in networks forwhich direct communication is forbidden by policy. Tocompletely eliminate such channels requires a total isola-tion system, which is impractical due to the under-utilization of network resources. A feasible solution is tolimit the covert channel using well-defined access controlstrategy for different security clearance. As to detection,basically, there are three countermeasures: documentation,audition and bandwidth limitation [14, 28, 32, 33]. ButCabuk points out that the availability and applicability ofthese approaches in high-speed networks and real-worldindustry are still questionable [14].

Thus, the exploitation of unauthorized traffic flow in thecovert channel is always designated as a threat to information-sensitive networks in which the message disclosure to unau-thorized parties may be unacceptable, as a method for authen-tication makes it hard for a malicious third party who ismonitoring the communication process to obtain useful infor-mation. Since the common communication process in theovert channel doesn’t provide any identity information, eventhough the eavesdropper captures all the network packets, it isstill difficult for him to be aware of the real authenticationprocess in the covert channels due to the difficulty of detec-tion. And we believe it is a secure way to validate the useridentity, which can be a supplement for previous authentica-tion methods

3 Methodology

Based on the reversed Network Covert Channel, we proposeand implement a novel method for user identity verification byexploiting the overt communication channel as the data carrierto transmit the authentication tag, named as the NCCIA au-thentication method. The basic idea is to apply steganographyto the overt traffic flow during the network communicationprocess which utilizes the timing sequence of the data packetsto establish a covert communication for authentication.

In our proposed method, the authentication process isattached to the licit communication, which indicates twocommunication channels exist, defined as overt channel andcovert channel. Overt channel refers to the legitimate com-munication channel which is the common wired or wirelessnetwork; while covert channel is the parasitic communicationchannel in which the authentication process takes place. Theoverall system framework is shown in Fig. 1.

As shown in Fig. 1, the sender communicates with thereceiver in the overt channel to transmit a legitimate content;meanwhile, the sender acted as the authenticated party isrequired to prove its identity to the receiver that is the authorityparty. This validation process is further completed by trans-mitting the authentication tag in the covert channel. Particu-larly, the covert channel is established via modulating thetiming sequence of the relative network packets within thecommunication flow. By utilizing the intervals between twoconsecutive data packets, the authentication tag can be indi-cated in such a way that it is unique and stealth to the authorityparty. And the authority party can decode the tag transmittedin the covert channel while receiving the legitimate datapackets in the overt channel simultaneously.

In our work, the authentication tag is predetermined by twocommunication parties and expressed as a binary stream dur-ing the transmission process. The binary string is thenencoded to generate a transmitted bit sequence which is finallyutilized by the Controller to manipulate the intervals of the

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network packets. Let Ts(k) denote the sending time of the kth

packet, the Controller can delay the packet-sending time toindicate the transmitted bit sequence such that:

TS kð Þ ¼ TS k−1ð Þ þ DNCCIA k−1ð Þ ð1Þ

WhereDNCCIA(k-1) denotes the k-1 bit in the authenticationbinary stream bit(k-1), and it is determined by the followingcriteria:

DNCCIA k−1ð Þ ¼ Dlong bit k−1ð Þ ¼ 1Dshort bit k−1ð Þ ¼ 0

�ð2Þ

Where Dlong, and Dshort are the long and short delays setprior to the communication to indicate bit “1” and “0”,respectively.

We use Tr (k) to denote the receiving time of the kth packetat the authority party, the RNCC Analyzer first monitors thecommunication process to record the packet arriving timesuch that

TR kð Þ ¼ TS kð Þ þ DChannel kð Þ ð3Þ

Where DChannel (k) is the end-to-end delay of kth packetover the running network communication, and it can beexpressed as:

Dchannel kð Þ ¼ DNCCIA kð Þ þ δ kð Þ ð4Þ

Where δ(k) is a random variable that includes the delays ofthe network channel and the processing delays on both theauthenticated and authority party.

By calculating the intervals of all the received packets, thebinary stream in the covert channel can be recovered at theNCCIA Analyzer according to the following principle:

bitR k−1ð Þ ¼ 1 TR kð Þ − TR k − 1ð Þ>ε0 TR kð Þ − TR k − 1ð Þ≤ ε

�ð5Þ

Where ε is the threshold chosen that can distinguish anytwo distinct bits transmitted.

By obtaining this transmitted sequence, the authenticationtag can then be recovered based on the predetermineddecoding scheme which leads to the final validation. Accord-ing to the authentication tag, the authority party identifieswhether the sender is authentic. And once the tag is validated,the authenticated party will be granted access to certain infor-mation binding with the security level the authority partyassigns. Otherwise, any access request from the sender thatgoes beyond the allowed security clearance will be denied.

4 Experiment and performance analysis

4.1 Experimental platform

Based on our proposed scheme, a demo system is designed ona FTP Platform exploiting NCCIA authentication methodrunning over TCP/IP networks, which consists of two mod-ules: the FTP Client Module and the FTP Server Module. TheFTP Client Module, served as the authenticated party, isapplied on the client end which incorporates three functionelements: network monitoring, packet delay controller andencoding scheme. These elements work together to embedthe authentication tag into the legitimate traffic flow throughmanipulating the intervals of the network packets. The FTPServer Module, served as the authority party, is applied on theserver which incorporates three corresponding elements: net-working monitoring, packet delay analyzer and decodingscheme. These functions coordinate with each other to vali-date the client’s identity based on the authentication tag re-covered by decoding the intervals of consecutive networkpackets.

The design scenario is that by default the client connectsthe server with a low security level using the username andpassword. For any request of sensitive information that de-mands a higher security level, the client is required to upload afile to the server running the FTPClientModule to provide theauthentication tag. And the covert authentication process willbe captured by the server running the FTP Server Modulewhich records the arrival time of each packets and computesthe packet delays to obtain the transmitted tag. This message isthen used by the server to identify the user’s authorization. Ifthe validation passes, the client’s request can be approved;otherwise, the client is forced to repeat the authentication

Fig. 1 Framework of NCCIA

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process, and would be temporarily locked after three failedtrials.

4.2 Performance analysis

4.2.1 Traffic statistic

In NCCIA authentication, we exploit the network packetintervals as the data carrier to indicate the authentication tag.The manipulation of the packet sending time adds additionaldelay to the overt communication, which may lead to a shiftand a shape change in the traffic flow distribution pattern, asshown in Fig. 2 below.

The traffic stream pattern in the covert channel appears in adiverse pattern from that in the overt channel. The distributionin the overt channel is unimodal. The delay of most datapackets (99 %) is less than 1ms, where the packet intervalswithin 0.1 and 0.3ms accounts for more than 80 %. In thecovert channel, it is a bimodal distribution, in which two peaksare Dlong and Dshort. Besides, the shape of distribution patternin covert channel varies as the Dlong and Dshort chosen totransmit the authentication tag change. And there is also anobvious departure of the packet intervals in the covert channelfrom normal ones in the overt channel, which is designatedsuch that signified bits of the authentication tag can bedistinguished.

4.2.2 Channel capacity

In our proposed method, the covert channel for authenticationis actually draw bandwidth from the overt channel, since the

information we can obtain per channel usage is the authenti-cation tag embedded in the network packets of the legitimatecommunication. Thus, the channel capacity can be defined as

Ct NCCIAð Þ ¼ limk→∞

sup Bs kð Þ ð6Þ

where Bs(k) is maximum number of error- free bits trans-mitted of the kth packet. According to the Shannon’s informa-tion theory, the covert channel capacity is determined by thetransmission rate and the bit error rate, which are mainlyaffected by the packet sending intervals. The longer delaychosen would result in a decrease in both the transmissionspeed and the bit error rate. Therefore, in order to achieve ahigh channel capacity, the selection of an appropriate packetdelays is crucial in order to balance the system performanceand efficiency.

4.2.3 Transmission rate

The transmission rate is one of the most important metricdefined to evaluate the performance of the NCCIA authenti-cation as a communication method. Usually it can be depictedas a function of software processing time, network speed,network packet size and protocol overhead, etc. Specificallyin our method, as the data carrier is a sequence of packetintervals, the data rate of the network covert communicationis relatively low such that several packets are needed totransmit one bit considering the encoding scheme.

The exploit of network covert timing channel for authenti-cation tag transmission can also impact the data rate of the

(a) LAN (b) Campus Network

Fig. 2 Traffic stream distributionpattern in LAN & campusnetwork. (a) LAN, (b) CampusNetwork

Table 1 Covert channeltransmission rate withauthentication tag ofdifferent length

Transmission rate 8 M File/11b Tag 8 M File/15b Tag

No covert channel With covert channel No covert channel With covert channel

LAN 8.05*106 bit/s 4.83*106 bit/s 8.05*106 bit/s 4.25*106 bit/s

Campus network 4.12*106 bit/s 2.94*106 bit/s 4.12*106 bit/s 2.47*106 bit/s

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overt channel. The intervals Dlong and Dshort utilized to indi-cate the tag adds additional delay for packet sending in theovert channel. Thus, in order to analyze the effect of ourproposed authentication method on the legitimate communi-cation, we conduct a series of experiments in LAN and Cam-pus Network to measure the transmission rate in the overtchannel that is calculated using the effective number of bitsconveyed per unit of time.

As listed in Table 1, the system transmission rate in LANand Campus network with and without covert channel em-bedded is summarized. Here in this table, Dlong and Dshort forthe covert channel is set as 9ms and 1ms, respectively. Asshown below, the data rate is greatly reduced when covertchannel is embedded to transmit the authentication tag. InLAN, the data rate drops nearly 40 % of the overt communi-cation with an 11bit length tag, and 47.3 % with a 15bit lengthtag. In Campus network, the data rate decreases nearly 28.7 %with the covert channel of an 11bit length tag, and 40.1 % of a15bit tag. And the length of the tag will also affect thetransmission rate such that longer tag can lead to a furtherdecline in the data rate. The data rate decreases almost 12.1 %

sending 4more bits tag using covert channel in LAN and 16%in Campus network.

Besides, Table 2 shows how the transmission rate of theovert communication will be influenced when employingvariable choice of Dlong and Dshort values. Fixing the shortinterval Dshort =0.001 s, we carry out a series of experimentsusing a set of different Dlong values. The experimental resultsdemonstrate that the data rate declines as the long intervalDlong extends. And when the long interval Dlong values dou-bles, the transmission rate drops 33.7% in LAN and 23.3% inCampus Network, which claims the selection of the packetsending intervals has a significant effect on the data rate.

4.2.4 Robustness

Unlike overt communication channels, the network coverttiming channel we exploit for the NCCIA authenticationmethod usually confronts with a lot more random errorsduring the communication process. The relative poor date ratecauses the generally low signal-to-noise ratio, which indicatesthat the authentication tag transmission can easily get dis-

Table 2 Covert channel transmission rate using different packet intervals

Transmission Rate (*106 bit/s) 8 M File/11b Tag <Dlong, Dshort>

<0.01s, 0.001 s> <0.009 s, 0.001s> <0.008s, 0.001s> <0.007s, 0.001s> <0.006s, 0.001s> <0.005s, 0.001s>

LAN 4.24 4.83 5.26 5.79 6.01 6.38

Campus network 2.76 2.94 3.06 3.18 3.37 3.59

Table 3 System bit error rate in LAN

Threshold Dlong=5msDshort=1ms

Dlong=6msDshort=1ms

Dlong=7msDshort=1ms

Dlong=8msDshort=1ms

Dlong=9msDshort=1ms

Dlong=10msDshort=1ms

Dlong -0.1 29.73 % 26.67 % 26.72 % 23.67 % 23.67 % 23.67 %

Dlong -0.2 16.80 % 10.47 % 9.73 % 8.04 % 6.02 % 6.02 %

Dlong -0.3 10.47 % 6.02 % 3.44 % 1.80 % 1.72 % 1.80 %

Dlong -0.4 8.20 % 3.44.% 0.00 % 0.00 % 0.00 % 0.00 %

Dlong -0.5 6.02 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 %

Dlong -0.6 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 %

Dlong -0.7 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 %

Dlong -0.8 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 %

Dlong -0.9 3.44 % 1.72 % 0.39 % 0.00 % 0.00 % 0.00 %

Dlong -1 5.47 % 1.80 % 1.72 % 0.16 % 0.16 % 0.00 %

Dlong -2 6.80 % 3.44 % 2.47 % 0.39 % 0.39 % 0.00 %

Dlong -3 5.47 % 3.65 % 1.72 % 1.23 % 0.00 %

Dlong -4 5.47 % 2.04 % 1.80 % 0.16 %

Dlong -5 3.85 % 2.27 % 0.39 %

Dlong -6 2.70 % 0.89 %

Dlong -7 1.80 %

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turbed by random events such as network delay, bandwidthfluctuation and packet disorder, etc. Besides, this networkcovert channel is also a one-way channel that the sender getsno feedback from the receiver considering whether the datapacket is decoded correctly. Thus, we introduce the bit errorrate to evaluate the robustness of our proposed authenticationmethod.

The authentication tag transmitted in the network coverttiming channel is decoded using the intervals of the receiveddata packets, which is given by:

TR kð Þ − TR k − 1ð Þ ¼ Ts kð Þ þ DChannel kð Þ − Ts k − 1ð Þ þ DChannel k − 1ð Þð Þ¼ DNCCIA k − 1ð Þ þ DChannel kð Þ −DChannel k − 1ð Þ

ð7Þ

Equation (7) above shows that two consecutive packets’arrival delay can be mainly divided in two parts: the channelnoise and the NCCIA delay (Dlong and Dshort). This sequenceof the receiving packets is then compared with the threshold εto recover the tag according to Eq. (5). Therefore, the bit errorrate is closely related with the channel noise Dchannel, thepacket sending delay Dlong and Dshort, and the threshold ε.

As to the channel noise, the covert timing channel for theauthentication tag is not an ideal noiseless communicationchannel. The inherent noise exists in the overt channel dueto the network jitter, packet loss and retransmission whichposes additional delay on the packet arrival time. Byconducting thousands of experiments in LAN and Campusnetwork, we conclude that average inherent channel noise isdistributed with 0.2msmean, and 0.7ms standard deviation inLAN, and 0.29ms mean with 0.98ms standard deviation inCampus Network.

The long and short packet sending delays Dlong and Dshort

emerges as the primary factor to affect the accuracy of ourproposedmethod. Theoretically, they should be set large enoughto guarantee the intervals of the reception time TR(k)-TR(k-1) isalways distinguishable to maintain the timing information evenwith channel noise and unexpected network disturbance andjamming. Once the NCCIA delays Dlong and Dshort are given,another important issue is the appropriate threshold ε to distin-guish between the two intervals, which is of great importance insuccessfully recovering the authentication tag.

We conduct a series of experiments in LAN and CampusNetwork to evaluate the impact of the Dlong, Dshort and thethreshold ε on the system robustness. We introduce a two-

Table 4 System bit error rate in campus network

Threshold Dlong=5msDshort=1ms

Dlong=6msDshort=1ms

Dlong=7msDshort=1ms

Dlong=8msDshort=1ms

Dlong=9msDshort=1ms

Dlong=10msDshort=1ms

Dlong -0.1 31.96 % 29.73 % 26.67 % 24.20 % 24.20 % 24.20 %

Dlong -0.2 24.20 % 21.93 % 16.80 % 10.67 % 8.20 % 8.20 %

Dlong -0.3 16.80 % 10.71 % 8.20 % 7.34 % 6.02 % 5.47 %

Dlong -0.4 10.47 % 6.80 % 5.47 % 3.44 % 2.27 % 1.80 %

Dlong -0.5 8.20 % 3.44 % 2.27 % 0.00 % 0.00 % 0.00 %

Dlong -0.6 3.44 % 1.70 % 0.00 % 0.00 % 0.00 % 0.00 %

Dlong -0.7 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 %

Dlong -0.8 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 %

Dlong -0.9 5.47 % 1.80 % 0.89 % 0.27 % 0.00 % 0.00 %

Dlong -1 6.02 % 3.85 % 1.80 % 0.89 % 0.39 % 0.00 %

Dlong -2 10.72 % 6.02 % 3.44 % 1.72 % 0.82 % 0.00 %

Dlong -3 8.20 % 5.47 % 3.85 % 1.34 % 0.00 %

Dlong -4 7.32 % 5.47 % 1.80 % 0.39 %

Dlong -5 6.80 % 2.23 % 0.89 %

Dlong -6 3.44 % 1.80 %

Dlong -7 2.23 %

Table 5 Covert channel accuracy with different encoding schemes in LAN

Bit error rate 8 M File/11b Tag <Dlong, Dshort>

<0.01s, 0.001 s> <0.009 s, 0.001s> <0.008s, 0.001s> <0.007s, 0.001s> <0.006s, 0.001s> <0.005s, 0.001s>

Hamming code 0.17 % 0.49 % 0.76 % 1.22 % 1.30 % 1.99 %

Cyclic code 0.13 % 0.43 % 0.75 % 1.10 % 1.31 % 1.70 %

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tuple <Dlong, Dshort> to uniquely identify the various packetintervals used in covert channel. Particularly in our experi-ments, the short delayDshort is fixed as a constant 1ms, and wetest different samples when Dlong is set as 5ms, 6ms, 7 ms, 8ms, 9ms and 10ms, respectively. Tables 3 and 4 summarize theresults of these experiments.

As shown in Tables 3 and 4, it can be concluded that theaccuracy increases by selecting a longerDlong both in LAN andCampus network, since the larger Dlong can avoid the loss ofthe timing sequence information. With the selection of a largerDlong, any two consecutive packets are not sent too close toeach other, which can spare enough timing redundancy con-sidering the queuing delay, channel noise and network jitter.

Besides, given the Dlong value, it can be found that theaccuracy increases by choosing a higher threshold, since theclose ε set to Dlong can help greatly reduce the impact ofnetwork jamming and perturbation which may lead to a wronginterpretation. However, problems may occur if the thresholdε is set too close to the long interval. As shown in Tables 3 and4 above, the accuracy drops when ε is chosen very close toDlong, since the random network noise and additional delaycaused by the covert channel of a previous packet would leadto the misjudgment of the following up packets, which couldin reverse result in a lower accuracy.

In addition, it can be concluded that we can have theoptimal accuracy (zero bit error rate) if the threshold ε is setwithin [σ, 2σ] less than the chosen long packetDlong in LAN,and within [0.5σ, σ] in Campus Network, where σ is thestandard deviation of the inherent packet delays in overtchannel. The range for choosing the threshold ε can be ex-tended to [σ, 3σ] in LAN and [0.5σ, 2σ] in Campus Networkwith 95 % accuracy.

4.3 Encoding scheme simulation

Exploiting the timing sequence of the network packets as thecarrier medium makes our proposed NCCIA- authentication

method easily disturbed by network packet disorder and chan-nel noise; even the processing delay on the client and servermachine can alter generated authentication sequence beforereaching by the NCCIA Analyzer to decode. Thus, an error-correction mechanism is needed to guarantee system robust-ness and reliability.

The current encoding scheme employed in our proposedauthentication method simply transmits one information bitthree times in order to add redundancy for error correction.The transmission of additional two bits can help verify thesequence when one error occurs. To study the effect of theencoding scheme on the system robustness, we conduct asimulation experiment applying another two widely usedmethods: Hamming Code and Cyclic Code. The simulationresults are summarized in Tables 5 and 6 as follows:

Compared with the simple retransmission encodingscheme, both Hamming Code and Cyclic Code can achievebetter system robustness by reducing the bit error rate intransmission. As shown in Tables 5 and 6, these two encodingscheme have a better performance as Dlong grows larger. Andin general case Cyclic encoding scheme outperforms theHamming encoding scheme both in LAN and Campus net-work. But this higher accuracy is achieved at the cost oftransmission efficiency and channel bandwidth, consideringthe relatively high complexity of Cyclic Code implementationcompared with Hamming Code. Thus, the selection criteriafor the encoding scheme are based on the system robustnessrequirement. The Cyclic encoding scheme is a better choicefor strong request of perfect robustness, while the HammingCode would be an appropriate choice from the perspective oftradeoff between transmission rate and system reliability.

4.4 Comparison with previous authentication methods

We compare our proposed authentication method with severalwidely used authentication schemes from the perspective ofimplementation complexity, system reliability and security,

Table 6 Covert channel accuracy with different encoding schemes in campus network

Bit Error Rate 8 M File/11b Tag <Dlong, Dshort>

<0.01s, 0.001 s> <0.009 s, 0.001s> <0.008s, 0.001s> <0.007s, 0.001s> <0.006s, 0.001s> <0.005s, 0.001s>

Hamming Code 0.43 % 0.81 % 1.06 % 1.82 % 1.91 % 2.28 %

Cyclic Code 0.37 % 0.61 % 1.15 % 1.70 % 1.99 % 2.20 %

Table 7 Comparison ofauthentication method Crypto-based

authenticationDigitalsignature

Informationhiding

Digitalwatermark

RNCC-basedauthentication

System overhead High Low High Low Low

Robustness Strong Strong Strong Strong Relative strong

Undetectability Poor Poor Good Poor Good

Peer-to-Peer Netw. Appl.

which are evaluated using the term of system overhead, ro-bustness and undetectability, respectively. The comparison areshown in the following table.

As shown in Table 7, our proposed NCCIA authenticationmethod exploits the covert communication channel as the datacarrier for user validation, which makes it attractive and easilyimplantable on the running network with relative low systemoverhead. While the crypto-based authentication method re-quires a complex algorithm and the information hiding tech-nique requires an ingenious design due to the strict require-ment of robustness, which incurs high computationconsumption.

As to robustness, compared with other authenticationmethod, the reliability of NCCIA authentication may be af-fected by the design parameters, since the timing sequenceinformation can be easily disturbed by network packet disor-der and channel noise. However, an error correction encodingscheme with an appropriate packet intervals and threshold canstill guarantee a relative strong robustness of our proposedauthentication method.

And our proposed method especially stands out in theundetectability due to the indistinguishability of the networkcovert timing channel. The ingenious legitimate mechanismand the unpredictable huge pool of available data carrier,system status and events help the covert communication forauthentication tag transmission hide under the cover of thelicit overt channel. The anomaly traffic pattern caused bymodulation of packet intervals, though may possibly leave atrace for the malicious third party, will not degrade theundetectability considering the tremendous network commu-nication flow. For an 11bit authentication tag in our experi-ment, the minimum requirement for file length used as thedata carrier is about 1.92 M. As the relative small size of thedata carrier, this covert communication will be masked by theoverwhelming legitimate overt communication. Without thepre-knowledge that the authentication process took place inthe covert channel, the malicious third party cannot be awareof the existence of the covert communication, even thoughmonitoring the network traffic flow.

5 Conclusion and future work

In this paper, the reserve usage of Network Covert Channelthat is usually considered as a threat to network security isemployed for identity authentication. It is a very importantresearch problem for secure information exchange in varioustypes of applications [34–37]. A NCCIA authentication meth-od is designed by exploiting the intervals between two con-secutive packets to embed identity tag. And we design a demosystem which implement our proposed method on the FTPplatform and conduct a series of experiments in LAN and

Campus Network. The experimental results demonstrate thatour method is a effective way to authenticate with easy im-plantation and high system robustness that can be a powerfulsupplement for traditional authentication methods.

We will further analyze the tradeoff between transmissionrate and system robustness with different encoding schemes.And it is also our future work to enhance the proposedmethodby introducing an encoding scheme also in order to bettermaintain the traffic flow patterns.

Acknowledgements This work was supported by the Doctoral Fund ofMinistry of Education of China and the Fundamental Research Funds forthe Central Universities.

References

1. Butler KRB, Ryu S, Traynor P, McDaniel PD (2009) LeveragingIdentity-Based Cryptography for Node ID Assignment in StructuredP2P Systems. Parallel and Distributed Systems, IEEE Transactionson, vol. 20, pp. 1803–1815, 2009-01-01

2. Byeong-Thaek O, Sang-Bong L, Ho-Jin P (2008) A Peer MutualAuthentication Method using PKI on Super Peer based Peer-to-PeerSystems. In Advanced Communication Technology. ICACT 2008.10th International Conference on, 2008, pp. 2221–2225

3. Lamport L (1981) Password authentication with insecure communi-cation, Communications of ACM, no. 24, pp. 770–772

4. Neuman BC, Ts’O T (1994) Kerberos: an authentication service forcomputer networks, Communications Magazine, IEEE, vol. 32, pp.33–38, 1994-01-01

5. Rhee K, Kwark, Kim S, Won D (2005) Challenge-Response BasedRFID Authentication Protocol for Distributed DatabaseEnvironment. 3450: p. 70–84

6. David Pointcheval DP, Jacques Stern JS (2000) Security argumentsfor digital signatures and blind signatures. J Cryptol 13:361–396

7. Huang K, Ou Q, Wu X, Song Y (2009) Cryptanalysis of a RemoteUser Authentication Scheme Using Smart Cards, Proc. WirelessCommunications, Networking and Mobile Computing, 2009.WiCom ’09. 5th International Conference on, pp. 1–4

8. Hwang J, Wu K, Liu D (2000) Access control with role attributecertificates. Comput Stand Interfaces 22(1):43–53

9. Upmanyu M, Namboodiri AM, Srinathan K, Jawahar CV (2010)Blind authentication: a secure crypto-biometric verification protocol.IEEE Trans Inform Forensics Secur 5(2):255–268

10. Min W, Liu B (2004) Data hiding in binary image for authenticationand annotation. IEEE Trans Multimedia 6(4):528–538

11. Chen GX, Chen HF, Xie L, Song GL, Zhuang T (2010) An identityauthentication scheme in wireless peer-to-peer network. InCommunication Technology (ICCT), 12th IEEE InternationalConference on, 2010, pp. 473–476

12. He S, Li X, Chen J, Cheng P, Sun Y, Simplot-Ryl D (2013) EMD:energy-efficient P2P message dissemination in delay-tolerant wire-less sensor and actor networks. IEEE J Sel Areas in Commun 31(9):75–84. doi:10.1109/JSAC.2013.SUP.0513007

13. Zhou YH, Lin HQ (2010) An authentication protocol without trustedthird party on P2P network. In Future Computer and Communication(ICFCC), 2nd International Conference on, 2010, pp. V2-686-V2-689

14. Monrose F, Rubin A (1997) Authentication via keystroke dynamics.In Proceedings of the 4th ACM conference on Computer and com-munications security. Zurich, Switzerland: ACM

Peer-to-Peer Netw. Appl.

15. Zhang C, Lu R, Lin X, Ho P, Shen X (2008) An Efficient Identity-Based Batch Verification Scheme for Vehicular Sensor Networks. InINFOCOM 2008. The 27th Conference on ComputerCommunications. IEEE

16. Cabuk S (2006) Network covert channels: design, analysis, detection,and elimination. West Lafayette, IN, USA

17. Sun Y, Guan X, Liu T, Qu Y (2012) An identity authenticationmechanism based on timing covert channel. in 11th IEEEInternational Conference on Trust, Security and Privacy inComputing and Communications, TrustCom-2012, June 25, 2012 -June 27, 2012, Liverpool, United kingdom, pp. 832–836

18. Lampson BW (1973) A note on the confinement problem. CommunACM 16(10):613–615

19. U.D.of Defense (1985) Trusted Computer SystemEvaluation CriteriaTCSEC. DOD5200.28-STD. Washington

20. Griling CG (1987) Covert channels in LANs[J]. IEEE Trans SoftwEng 13(2):292–296

21. Wolf M (1989) Covert channels in LAN protocols Local AreaNetwork Security. 396: p. 89–101.

22. Handel T, Sandford M (1996) Hiding data in the OSI network modelInformation Hiding. 1174: p. 23–38

23. Kamran A (2002) Covert channels analysis and practical data hidingin TCP/IP. University of Toronto, Canada

24. Kamran A, Deepa K (2002) Practical Data Hiding in TCP/IP. In Proc.ACM Workshop on Multimedia Security, vol. 2002

25. Murdoch S, Lewis S (2005) Embedding Covert Channels into TCP/IP Information Hiding. 3727: p. 247–261

26. Steven JM, Stephen L (2005) Embedding Covert Channels into TCP/IP. in Proceedings of the 7th Information Hidding Workshop

27. Wray JC (1992) An analysis of covert timing channels. J ComputSecur 1(3):219–232

28. Zhu H, Du S, Gao Z, Dong M, Cao Z (2014) A probabilisticmisbehavior detection scheme toward efficient trust establishmentin delay-tolerant networks. IEEE Trans Parallel Distrib Syst 25(1):22–32

29. Sellke SH, Wang CC, Bagchi S, Shroff N (2009) TCP/IP TimingChannels: Theory to Implementation. In INFOCOM 2009, IEEE

30. Liu Y, Ghosal D, Armknecht F, Sadeghi A, Schultz S, KatzenbeisserS (2009) Hide and seek in time—robust covert timing channels.Computer Security–ESORICS 2009: 120–135

31. Liu Y, Ghosal D, Armknecht F, Sadeghi A, Schultz S, KatzenbeisserS (2010) Robust and undetectable steganographic timing channels foriid traffic. Information hiding. Springer, Berlin/Heidelberg

32. He J, Cheng P, Shi L, Chen J (2013) SATS: secure average-consensus-based time synchronization in wireless sensor networks.IEEE Trans Signal Process 61(24):6387–6400

33. Cabuk S, Brodley CE, Shields C (2004) IP covert timing channels:design and detection. In proceedings of the 11th ACM conference oncomputer and communications security. ACM, Washington

34. Hongwei Li, Xiaodong Lin, Haomiao Yang, Xiaohui Liang,Rongxing Lu, and Xuemin (Sherman) Shen, EPPDR: An Efficient

Privacy – Preserving Demand Response Scheme with Adaptive KeyEvolution in Smart Grid, IEEE Transactions on Parallel andDistributed Systems, doi:10.1109/TPDS.2013.124

35. Hongwei Li, Rongxing Lu, Liang Zhou, Bo Yang, and Xuemin(Sherman) Shen. An Efficient Merkle Tree Based AuthenticationScheme for Smart Grid, IEEE SYSTEMS Journal, doi:10.1109/JSYST.2013.2271537

36. Dong M, Ota K, Li H, Du S, Zhu H, Guo S (2013) RENDEZVOUS:towards fast event detecting in wireless sensor and actor networks.Computing, pp. 1–16. doi:10.1007/s00607-013-0364-7

37. Dong M, Ota K, Lin M, Tang Z, Du S, Zhu H (2014) UAV-assisteddata gathering in wireless sensor networks. The Journal ofSupercomputing, pp. 1–14. doi:10.1007/s11227-014-1161-6

Haijiang Xie is currently work-ing toward the PhD. degree in theDepartment of Computer Scienceand Technology, Xi’an JiaotongUniversity, Xi’an, China. His re-search interests include networksecurity, internet measurement.

Jizhong Zhao received the BSand MS degrees in mathematicsand the PhD degree in computerscience (in 2001), with focus ondistributed systems, from Xi¡¯anJiaotong University, Xi¡¯an, P.R.China. He is a professor in theComputer Science and Technolo-gy Department, Xi¡¯an JiaotongUniversity. His research interestsinclude computer software, perva-sive computing, distributed sys-tems, and network security. He isa member of the IEEE, the IEEEComputer Society, and the ACM.

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