Expert Systems with Applications 37 (2010) 6176–6181

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Expert Systems with Applications

journal homepage: www.elsevier .com/locate /eswa

An efficient and reliable E-DRM scheme for mobile environments

Chin-Chen Chang a,*, Jen-Ho Yang b, Da-Wei Wang c

a Department of Information Engineering and Computer Science, Feng Chia University, 100 Wenhwa Rd., Seatwen, Taichung 40724, Taiwan, ROCb Department of Information and Electronic Commerce, Kainan University, No. 1, Kannan Road, Luzhu, Taoyuan County, 33857, Taiwan, ROCc Department of Information Management and Information System, Renmin University of China, No. 59 Zhongguancun St., Haidian District, Beijing, 100872, PR China

a r t i c l e i n f o

Keywords:Enterprise Digital Right ManagementDigital contentsMobile environments

0957-4174/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.eswa.2010.02.110

* Corresponding author. Address: Department ofCommerce, Kainan University, No. 1, Kannan Road, LuTaiwan, ROC. Tel.: +886 3412500x6069; fax: +886 33

E-mail addresses: ccc@cs.ccu.edu.tw (C.-C. Chang)(J.-H. Yang), wdw@ruc.edu.cn (D.-W. Wang).

a b s t r a c t

Enterprise Digital Right Management (E-DRM) scheme is a mechanism that protects the confidentialinformation of an enterprise from illegal accesses. In 2008, Chen proposed an E-DRM scheme for mobiledevices, and Chen’s scheme has low computation costs so it is suitable for mobile environments. How-ever, we find that Chen’s scheme is insecure because the symmetric key can be easily computed by anattacker. In addition, tampering with the user’s password cannot be discovered by the mobile user. More-over, there are some redundant computations for user authentication in Chen’s scheme. To overcome theabove-mentioned flaws, we propose an efficient and reliable E-DRM scheme for mobile environments inthis paper. In the proposed scheme, the symmetric key is protected by a one-way hash function so it can-not be directly computed by an attacker. In addition, tampering with the transmitted message can bedetected by the mobile users in the proposed scheme. Besides, the proposed scheme has no redundantcomputation for user authentication. Therefore, the proposed scheme is more efficient and reliable thanChen’s scheme.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

With the rapid development of network technologies, it is easyto download and distribute digital contents (e.g., digital music,images, videos, games, e-books) on the Internet. Therefore, illegalaccesses of the digital contents become an important problem. Toprotect the digital contents from illegal usages, various DigitalRight Management (DRM) schemes (Adobe Lifecycle DocumentSecurity, 2009; Microsoft Windows Right Management ServicesSystem, 2007; Windows Media Digital Rights Management, 2009)have been proposed. The DRM scheme is a mechanism that pro-vides the access control of digital contents and prevents the digitalcontents from illegal uses. In the DRM scheme, the author canspecify the access rights of digital contents, such as reading, mod-ifying, executing, and printing. Only authorized users can accessthe protected digital contents with the access rights specified bythe author. In addition, the DRM scheme is also important forenterprises to protect their sensitive information, such as productdesigns, customer lists, and financial reports. The disclosure of con-fidential documents in the enterprises may cause a large amount offinancial losses. To solve this problem, many Enterprises Digital

ll rights reserved.

Information and Electroniczhu, Taoyuan County, 33857,412373., jenhoyang@mail.knu.edu.tw

Right Management (E-DRM) schemes (Authentica Delivers Next-Generation Enterprise Rights Management Platform, 2005; Chen,Chen, & Chen, 2009; Mulligan, Han, & Burstein, 2003; WindowsRights Management Services, 2007) have been proposed. However,these E-DRM schemes are not suitable for mobile devices becausethey have heavy computation costs.

To reduce the computation costs, Chen (2008) proposed an E-DRM scheme for mobile devices based upon one-way hash func-tions in 2008. In Chen’s scheme, users can utilize mobile devicesto download the encrypted digital content from a content server.After passing the authentication by an authorization authority, amobile user receives a symmetric key from a license server anduses it to decrypt the encrypted digital content. Finally, the mobileuser can obtain the digital content and access it by using the mo-bile device. Compared with the previously proposed works, Chen’sscheme has less computation costs. However, we find that Chen’sscheme has some security flaws.

First, Chen’s scheme is not secure because the symmetric keycan be easily computed by an attacker who eavesdrops the commu-nications between the mobile user and the license server. Second,Chen’s scheme uses a nonce to generate the one-time passwordfor user authentication. However, the mobile user does not verifythe correctness of the nonce. If the attacker tampers the transmit-ted message containing the nonce, then the mobile user cannotcompute the correct password to pass the user authentication nexttime. Third, we find that Chen’s scheme has some redundant verifi-cations. This increases the computation costs of his scheme.

Table 1The notations in Chen’s E-DRM scheme (Chen, 2008).

T; s A timestamp and a time constantCert The digital certificate of a mobile userPi The ith one-time passwordSEED The initial random seed number generated by the

authorization authorityNi The ith request random number generated by

authorization authorityIMEI A unique international mobile equipment identification

number of each mobile terminalmsgreq The authorization request message of the mobile userCID The identity of a digital contentDRM � APope The type of the DRM-enable applicationVXð�Þ=SXð�Þ The verifying/signing function using X’s public/secret keyKEYCID The symmetric key for the digital content with CIDEKEYX ð�Þ=DKEYx ð�Þ The symmetric encryption/decryption function using a

symmetric key KEYX

Hð�Þ; Fð�Þ Two collision free one-way hash functionsSigc ; SigCH The signature of the encrypted digital content C and the

content header CHSigKEYCID

The signature of the symmetric key KEYCID

C.-C. Chang et al. / Expert Systems with Applications 37 (2010) 6176–6181 6177

To overcome the above-mentioned flaws, we propose an effi-cient and reliable E-DRM scheme for mobile devices in this paper.In the proposed scheme, the symmetric key is protected by a one-way hash function with a nonce. Thus, the symmetric key cannotbe obtained directly by an attacker. In addition, the nonce needsto be verified when it is received by the mobile user. Thus, tamper-ing with the nonce can be easily discovered by the mobile user inthe proposed scheme. Moreover, we eliminate the redundant ver-ifications of Chen’s scheme so the computation costs of the pro-posed scheme can be reduced. According to the above reasons,the proposed E-DRM scheme is more efficient and reliable thanChen’s E-DRM scheme.

2. Chen’s E-DRM scheme

In this section, we introduce Chen’s E-DRM scheme (Chen,2008) as follows. There are six roles in their scheme: the authorof the digital content, the package server, the content server, the li-cense server, the authorization authority, and the mobile user. Inthis scheme, the package server encrypts the digital content andpacks it into E-DRM formatted file which is shown in Fig. 1. In addi-tion, the encrypted digital content and the symmetric key arestored in the content server and the license server, respectively.And, the authorization authority is responsible for authenticatingthe user’s access right of the digital content.

To manage the access right of the digital content, Chen’s E-DRMscheme using the E-DRM formatted file and the file structure isshown in Fig. 1. The E-DRM formatted file is divided into two parts:the content header (CH) and the encrypted content C. The contentheader contains the information as follows. The content identity(CID) is the identity of a digital content. Then, the type of theDRM-enable application ðDRM � APtypeÞ is for integrating the exist-ing applications and indicating suitable applications to access thedigital content. In addition, the content header also contains theidentity of the decryption key ðKEYCIDÞ and some attributes (Attri-butes). Moreover, the signature ðSigCÞ is a digital signature of Csigned by the package server, and URL is the uniform resource loca-tor of the authorization authority.

2.1. The notations

The notations used in Chen’s E-DRM scheme are shown in Table1.

2.2. The package phase

In this phase, the author sends the digital content to the pack-age server. Then, the package server encrypts the digital contentand packs it into the E-DRM formatted file. After that, the packageserver sends the E-DRM formatted file and the symmetric encryp-tion key to the content server and the license server, respectively.Finally, the content server stores the E-DRM formatted file in its

Fig. 1. The file structure of the E-DRM formatted file (Chen, 2008).

public directory. Now, we state the steps of this phase presentedin Chen (2008) as follows.

Step 1. The author uploads the digital content to the packageserver.

Step 2. The package server packs the digital content into M andgenerates a symmetric key KEYCID to encrypt M byC ¼ EKEYX ðMÞ. The package server also generates the con-tent header (CH). Then, the package server uses its secretkey to construct two signatures for C and CH by comput-ing SigC ¼ SPSðCÞ and SigCH ¼ SPSðCHÞ, respectively. Also,the package server packs C and CH into E-DRM formattedfile as shown in Fig. 1. Finally, the package server sendsE-DRM formatted file to the content server.

Step 3. The package server uses its secret key to construct a dig-ital signature for CID and KEYCID by computingSigKEYCID

¼ SPSðCID;KEYCIDÞ. Then, the package server sendsðCID;KEYCID; SigKEYCID

Þ to the license server. After receiv-ingðCID;KEYCID; SigKEYCID

Þ, the license server uses the pack-age server’s public key to verify the signature. If thesignature is valid, then the license server storesðCID;KEYCIDÞ into its database.

Step 4. After receiving SigC and SigCH , the content server uses thepackage server’s public key to verify the signature. If SigC

and SigCH are both valid, then the license server stores theE-DRM formatted file into its public directory.

Step 5. The mobile user downloads the E-DRM formatted fileand the DRM-AP from the content server. Then, themobile user gets the authorization authority’s URL fromthe content header and uses it to send an authorizationrequest to the authorization authority.

2.3. The registration phase

In this phase, the mobile user sends the personal certificate Certto the authorization authority for the user registration. Then, theauthorization authority creates required parameters for the mobileuser. The steps of this phase presented in Chen (2008) are stated asfollows.

Step 1. The mobile user sends IMEI and Cert to the authorizationauthority via a secure channel.

Step 2. After receiving Cert, the authorization authority startsthe authentication process. If the mobile user passes

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the authentication, then the authorization authority gen-erates a random number SEED and sends it to the mobileuser via a secure channel. The authorization authoritychooses two one-way hash functions Hð�Þ and Fð�Þ forthe mobile user.

Step 3. Finally, the authorization authority stores Cert, IMEI, andSEED into its database. Then, the authorization authoritysends Cert, IMEI, and SEED to the license server via thesecure channel.

Fig. 2 illustrates the steps of the registration phase as follows.

2.4. The authorization phase

In this phase, the mobile user sends his/her personal informa-tion and one-time password Pi to the authorization authority toauthenticate his/her identity. If the mobile user passes the authen-tication, the license server sends the symmetric key to the mobileuser. Then, the mobile user uses the symmetric key to decrypt thecipher of the digital content. The steps of this phase presented inChen (2008) are stated as follows.

Step 1. In the mobile user side, the DRM-AP uses the ith requestnumber Ni to generate the ith one-time passwordPi ¼ HiðNi � IMEI � CertkTkCIDÞ, where the symbols �and k are the bitwise exclusive-or operator and the con-catenation operator, respectively. Then, the mobile usersends the message ði;msgreq; T;CID;HiðSEEDkTjCIDÞ;Pi;CertÞ to the authorization authority.

Step 2. After receiving ði;msgreq; T;CID;HiðSEEDkTjCIDÞ; Pi;CertÞ,the authorization authority checks if T 6 s. If T 6 s, thenthe authorization authority computes HiðSEEDkTkCIDÞand checks if it is equal to HiðSEEDkTkCIDÞ which is sentfrom the mobile user in Step 1. In addition, the authori-zation authority also checks if the equation Pi ¼HiðN0i � IMEI0 � CertkTkCIDÞ holds. If the above verifica-tions are correct, then the authorization authoritycomputes Di ¼ HFðNiÞðPikSEEDkIMEIÞ � HFðNiÞðPi�1kTÞ forverification. Then, the authorization authority randomlygenerates Niþ1 and sends ðDi;H

FðNiÞðSEEDkIMEIkTÞ � Niþ1Þto the mobile user.

Step 3. After receiving ðDi;HFðNiÞðSEEDkIMEIkTÞ � Niþ1Þ, the

mobile user checks if the equation Di ¼ HFðNiÞðPikSEEDkIMEIÞ � HFðNiÞðPi�1kTÞ holds. If the equation holds, thenthe DRM-AP stores the Niþ1 and computes Niþ1 ¼HFðNiÞðSEEDkIMEIkTÞ � HFðNiÞðSEEDkIMEIkTÞ � Niþ1.

Fig. 2. The registration phase of Chen

Step 4. The authorization authority uses its secret key to gener-ate a signature as SigAA ¼ SAAðIMEI;CID; i; T;Cert;DiÞ. Then,the authorization authority sends SigAA ¼ SAAðIMEI;CID; i;T;Cert;DiÞ to the license server.

Step 5. After receiving ðIMEI;CID; i; T;Cert;Di; SigAAÞ, the licenseserver uses the authorization authority’s public key toauthenticate the message by checking if VAAðSigAAÞ ¼ðIMEI;CID; i; T;Cert;DiÞ. If the equation holds, then thelicense server stores the message. Afterward, the licenseserver computes ðDi;HðIMEI � Cert � TÞ � KEYCIDÞ andsends it to the mobile user.

Step 6. After receiving ðDi;HðIMEI � Cert � TÞ � KEYCIDÞ, theDRM-AP checks if the equation Di ¼ HFðNiÞðPikSEEDkIMEIÞ � HFðNiÞðPi�1kTÞ holds. If it holds, then the mobileuser computes the symmetric key by KEYCID ¼ HðIMEI�Cert � TÞ � HðIMEI � Cert � TÞ � KEYCID. Then, the mobileuser can obtain the digital content by computingM ¼ DKEYCID ðCÞ.

Fig. 3 illustrates the steps of the authentication phase. Accord-ing to Chen’s scheme, we find that his scheme has some securityflaws shown as follows. First, the symmetric key KEYCID is embed-ded into HðIMEI � Cert � TÞ � KEYCID to transmit to the mobile userin the authorization phase. However, IMEI, Cert, and T can be di-rectly obtained by an attacker who eavesdrops the communica-tions among the mobile user, the authorization authority, andthe license server. That is, the attacker has the ability to computeHðIMEI � Cert � TÞ, and then he/she can easily compute KEYCID

from HðIMEI � Cert � TÞ � KEYCID. Thus, Chen’s scheme is insecurebecause KEYCID can be easily computed by the attacker.

Second, Chen’s scheme does not verify Niþ1 and KEYCID in Steps 3and 6 of the authorization phase, respectively. Therefore, the at-tacker may tamper with the messages Niþ1 ¼ HFðNiÞðSEEDkIMEIkTÞ � HFðNiÞðSEEDkIMEIkTÞ � Niþ1 and HðIMEI � Cert � TÞ�KEYCID which is sent to mobile user in Step 2 and Step 5 of theauthorization phase, respectively. Without verifying Niþ1 andKEYCID, the mobile user may get the incorrect Niþ1 and KEYCID sohe/she cannot compute the correct symmetric key to decrypt theencrypted digital content.

Third, we discover that there are some redundant computationsin the authorization phase of Chen’s scheme. For example, Chen’sscheme needs to verify SEED every time when the mobile user asksto perform the user authorization for the system. However, SEED isonly used to initialize Ni and Di for user authorization in the firstsession. Thus, verifying SEED or Di in each session is unnecessaryso these redundant computations can be eliminated from Chen’sscheme.

’s E-DRM scheme (Chen, 2008).

Fig. 3. The authorization phase of Chen’s E-DRM scheme (Chen, 2008).

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3. The proposed E-DRM scheme

To overcome the above flaws of Chen’s E-DRM scheme, wepropose a new E-DRM scheme for mobile devices in this section.The roles of the proposed scheme are the same as those of Chen’sE-DRM scheme. Moreover, the proposed scheme is divided intothree phases: the registration phase, the package phase, and theenhanced authorization phase. Note that the package phase andthe registration phase of the proposed scheme are the same asthose of Chen’s E-DRM scheme. Moreover, the notations of theproposed scheme are the same as those of Chen’s scheme asshown in Table 1. Now, we present the proposed scheme asfollows.

3.1. The enhanced authorization phase

In this phase, the mobile user sends the personal informationand one time password to the authorization authority to authenti-cate his/her identity. If the mobile user is valid, then the authoriza-tion authority generates a new number as the seed of the nextrequest and sends it to the mobile user and the license server.

Then, the license server sends the symmetric key to the mobileuser. Finally the DRM-AP uses the symmetric key to decrypt the ci-pher of the digital content. The steps of this phase are shown asfollows.

Step 1. The mobile user uses ith request number Ni to generateith one-time password Pi ¼ HiðNi � IMEI � CertkTkCIDÞ,where N1 ¼ Seed. Then, the mobile user sends the mes-sage ði;msgreq; T;CID; Pi;CertÞ to the authorizationauthority.

Step 2. After receiving ði;msgreq; T;CID; Pi;CertÞ, the authoriza-tion authority checks if T 6 s. If T 6 s, then the authori-zation authority loads Ni and IMEI from its database andchecks if the equation Pi ¼ HiðNi � IMEI � CertkTkCIDÞholds. If the equation holds, then the authorizationauthority randomly generates the next nonce Niþ1 andsends ðHFðNiÞðIMEIkTkNiÞ � Niþ1;HðIMEIkNikNiþ1ÞÞ to themobile user.

Step 3. The authorization authority generates a digital signatureby computing SigAA ¼ SAAðIMEI;CID; i; T;Cert;HFðNiÞðIMEIkTkNiÞ � Niþ1Þ.Then, the authorization authority sends

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the message ðIMEI;CID; i; T;Cert;HFðNiÞðIMEIkTkNiÞ�Niþ1; SigAAÞ to the license server.

Step 4. After receiving ðHFðNiÞðIMEIkTkNiÞ � Niþ1;HðIMEIkNikNiþ1ÞÞ, the mobile user can obtain the next nonce Niþ1

by computing N0iþ1 ¼ HFðNiÞðIMEIkTkNiÞ � ðHFðNiÞðIMEIkTkNiÞ � Niþ1Þ. In addition, the mobile user checks ifHðIMEIkNikNiþ1Þ ¼ HðIMEI0kNikN0iþ1Þ holds. If it holds,then the mobile user stores N0iþ1 ¼ Niþ1.

Step 5. After receiving ðIMEI;CID; i; T;Cert;HFðNiÞðIMEIkTkNiÞ�Niþ1; SigAAÞ, the license server can also compute the nextnonce Niþ1 by N0iþ1 ¼ HFðNiÞðIMEIkTkNiÞ � ðHFðNiÞðIMEI0kTkNiÞ � Niþ1Þ. Then, the license server uses the authoriza-tion authority’s public key to check if VAAðSigAAÞ ¼ðIMEI;CID; i; T;Cert;HFðNiÞðIMEI0kTÞ � N0iþ1Þ holds. If theequation holds, then the license server uses KEYCID tocompute the message ðHFðNiÞðIMEI � Cert � T � NiÞ�KEYCID;HðIMEIkNikKEYCIDÞÞ and sends it to the mobileuser.

Step 6. The mobile user can obtain the symmetric key by com-puting KEY 0CID ¼ HFðNiÞðIMEI � Cert � T � NiÞ� ðHFðNiÞ

ðIMEI � Cert � T � NiÞ � KEYCIDÞ. Then, the mobile usercan confirm the correctness of KEYCID by checking if theequation HðIMEIkNikKEYCIDÞ ¼ HðIMEI0kNikKEY 0CIDÞ holds.If it holds, then the mobile user can decrypt the digitalcontent by computing M ¼ DKEYCIDðCÞ.

Fig. 4 illustrates the steps of the enhanced authorization phaseas follows.

According to the proposed scheme, the symmetric key KEYCID isprotected by HFðNiÞðIMEI � Cert � T � NiÞ � KEYCID with Ni so the at-

Fig. 4. The enhanced authorization pha

tacker cannot directly computed KEYCID from HFðNiÞðIMEI � Cert�T � NiÞ � KEYCID without knowing Ni.

In addition, the mobile user can verify the correctness of Niþ1

and KEYCID by computing the verification equationsHðIMEIkNikNiþ1Þ ¼ HðIMEI0kNikN0iþ1Þ and HðIMEIkNikKEYCIDÞ ¼HðIMEI0kNikKEY 0CIDÞ in Step 4 and Step 6, respectively. Thus, tam-pering the message containing Niþ1 and KEYCID can be discoveredby the mobile user.

Besides, the proposed scheme eliminates some redundant com-putations for verifying SEED and Di in Chen’s scheme. From theabove descriptions, the proposed E-DRM scheme is more efficientand reliable than Chen’s scheme.

4. Discussions

In this section, we analyze the security of the proposed E-DRMscheme. Now, we discuss some possible attacks on the proposedscheme as follows.

4.1. Outsider attack

Assume that an attacker wants to obtain the symmetric keyKEYCID, then he/she eavesdrops the communications between themobile user and the license server. Thus, the attacker can obtainHFðNiÞðIMEI0 � Cert � T � NiÞ � KEYCID. To obtain KEYCID fromHFðNiÞðIMEI0 � Cert � T � NiÞ � KEYCID, the attacker needs to knowHFðNiÞðIMEI � Cert � T � NiÞ. However, it is impossible because theattacker does not know Ni. Therefore, the outsider attack is infea-sible for the proposed scheme.

se of the proposed E-DRM scheme.

C.-C. Chang et al. / Expert Systems with Applications 37 (2010) 6176–6181 6181

4.2. Insider attacker

Assume that an attacker inside the enterprise who wants to getthe nonce Ni or Niþ1, then he/she eavesdrops the communicationsbetween the authorization authority and the license server. Thus,the attacker can obtain HFðNiÞðIMEIkTkNiÞ � Niþ1. To get Niþ1 fromHFðNiÞðIMEIkTkNiÞ � Niþ1, the attacker needs to knowHFðNiÞðIMEIkTkNiÞ. However, it is infeasible because the attackerdoes not know the nonce Ni. Besides, computing Ni fromHFðNiÞðIMEIkTkNiÞ is impossible because Ni is protected by theone-way hash functions Fð�Þ and Hð�Þ. According to the above anal-ysis, the attacker cannot derive any confidential information evenhe/she is an insider of the enterprise.

4.3. Replay attack

Assume that an attacker collects the messages once being trans-ferred between the user and the server. Then, the attacker may usethe pre-collected message Pi ¼ HiðNi � IMEI � CertkTkCIDÞ to pre-tend that he/she is a valid user. Thus, the attacker sendsði0;msgreq; T

0;CID; Pi;CertÞ to the authorization authority for askingthe symmetric key in the later transaction. However, this attackcannot work because i0 – i and T 0 – T. That is, the authorizationauthority can discover this replay attack by computing:

Pi ¼ HiðNi � IMEI � CertkTkCIDÞ – Hi0 ðNi0 � IMEI � CertkT 0kCIDÞ:

Therefore, the replay attack is infeasible for the proposed scheme.

4.4. Impersonation attack

Assume that an attacker wants to impersonate the mobile user,then he/she randomly generates a forged N0i and uses the pre-col-lected IMEI, CID, and Cert to compute P0i ¼ HiðN0i � IMEI�CertkTkCIDÞ. Then, the attacker sends ði;msgreq; T;CID; P0i;CertÞ tothe authorization authority for user authentication. However, thiskind of attack is impossible for the proposed scheme becauseN0i – Ni and HiðN0i � IMEI � CertkTkCIDÞ – HiðNi � IMEI � CertkTkCIDÞ. Thus, the authorization authority can discover thatP0i ¼ HiðN0i � IMEI � CertkTkCIDÞ is sent from an attacker. Similarly,

the attacker cannot impersonate the authorization authority andthe license server because he/she does not know the correct nonceNi in each transaction. Therefore, the impersonation attack is infea-sible for the proposed scheme.

5. Conclusions

In this paper, we pointed out the security flaws of Chen’s E-DRMscheme on mobile devices. To overcome the above security flaws,we further propose an efficient and reliable E-DRM scheme for mo-bile environments. In the proposed scheme, the symmetric keycannot be directly computed by an attacker. In addition, the pro-posed scheme can detect the message tampering by the mobileuser. Unlike Chen’s scheme, the proposed scheme has no redun-dant computations for user authorization. Therefore, the proposedscheme is more efficient and reliable than Chen’s scheme. In the fu-ture, we will investigate how to eliminate the usage of public-keycryptosystems in the proposed scheme and further design a sim-pler and more efficient E-DRM scheme for mobile devices.

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