R378 2017 11 choi CS

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P2P

Ϋϥ΢υετϨʔδʹ͓͚Δసૹίετ࡟ݮํ͓ࣜΑͼಗ໊ԽʹΑ

ΔϓϥΠόγอޢํࣜ

ቐɹ૬Ⴣ

ேൺಸɹܒ

࡫੉ɹ؜

ܚጯٛक़େֶཧ޻ֶ෦৘ใ޻ֶՊ

ԣ඿ࢢߓ๺۠೔٢

3-14-1

E-mail:

choi@sasase.ics.keio.ac.jp

͋Β·͠ ۙ೥ɺ΢ΣϒΫϥ΢υετϨʔδαʔϏεͷधཁ͕ߴ·͍ͬͯΔɻطଘͷ΢ΣϒετϨʔδαʔϏεͰ͸ɺ

αʔόʹશϢʔβσʔλ͕อ؅͞ΕΔͨΊɺ֎෦͔Βͷ߈ܸʹΑͬͯେྔͷϢʔβσʔλ͕ྲྀग़͢ΔڪΕ͕͋Δɻ͜

Εʹର͠ɺP2P(Peer to Peer)ωοτϫʔΫͷϊʔυΛετϨʔδͱ͠ɺϒϩοΫνΣʔϯʹΑͬͯσʔλͷॴ༗ऀ

ͷਖ਼౰ੑΛอূͨ͠P2PΫϥ΢υετϨʔδํ͕ࣜఏҊ͞Ε͍ͯΔ͕ɺଟ਺ͷϊʔυΛதܧͯ͠σʔλΛอ؅͢Δ

͜ͱʹΑΓ஗ԆɺτϥώοΫྔͱ͍ͬͨσʔλసૹίετ͕૿Ճ͢Δͱ͍͏՝୊͕͋Δɻ·ͨɺϒϩοΫνΣʔϯ͸ શϊʔυ͕ӾཡͰ͖ΔͨΊσʔλͷॴ༗ऀͳͲϢʔβͷϓϥΠόγʔ͕৵֐͞ΕΔڪΕ͕͋Δɻͦ͜Ͱຊ࿦จͰ͸ɺ σʔλͷมߋස౓ʹجͮ͘ετϨʔδϊʔυબ୒๏͓ΑͼϥϯμϜʹબ୒ͨ͠ϦϨʔϊʔυΛϓϩΩγͱͯ͠༻͍ͯ ૹ৴ऀΛಗ໊Խ͢Δ͜ͱʹΑΔϓϥΠόγΛอޢͷํࣜΛఏҊ͢ΔɻγϛϡϨʔγϣϯධՁʹΑΓɺఏҊํ͕ࣜैདྷ ํࣜͱൺ΂σʔλసૹίετ͕௿ݮͨ͜͠ͱɺϢʔβͷϓϥΠόγΛޮՌతʹอޢͰ͖Δ͜ͱΛࣔ͢ɻ

Ωʔϫʔυ P2PετϨʔδɺΫϥ΢υετϨʔδɺϒϩοΫνΣʔϯ

Low Transmission Cost P2P Storage Scheme with Privacy Protection

Sanghun CHOI

, Hiromu ASAHINA

, and Iwao SASASE

Dept. of Information and Computer Science, Keio University

3-14-1 Hiyoshi, Kohoku, Yokohama, Kanagawa 223-8522, Japan

E-mail:

choi@sasase.ics.keio.ac.jp

Abstract Although the WCS(Web Cloud Storage) Services are quite convenient and usually used these days, there is a problem that the storage server cloud be targeted by the attacker. To safely store the user’s data without the central server, the P2P(Peer to Peer) storage methods have been studied. Even though the P2P storage relieves the risk of attacks, these have two challenges: reducing the transmission and the delay which are the fundamental problems of P2P, and preventing the user’s privacy from the malicious nodes which store the user’s data. In this paper, in order to solve these challenges mentioned above, we propose an efficient P2P storage scheme with privacy protection. The main ideas of our scheme are that the storage nodes are selected according to the updating frequency of data for reducing the transmission and the delay, and the relay nodes can be acted as the owners of data instead of the real owner of data for protecting the user’s privacy. Furthermore, by using the DIFF method which generates the difference between two data, the user can effectively modify the stored data with the lower transmission and delay. We demonstrate the reduction of the transmission and the delay, and the privacy protection by the computer simulation.

Key words P2P storage, Cloud storage, Blockchain.

1.

Introduction

In these days, with the growth of the network and the

portable devices, many people can easily access to the

Inter-net and enjoy a lot of interInter-net services. One of the most

pop-ular services is the WCS(Web Cloud Storage) service such as Google Drive [1] and Drop Box [2]. If a user accesses to the

Internet by its own devices, the user can save new data and

― 15 ―

This article is a technical report without peer review, and its polished and/or extended version may be published elsewhere.

一般社団法人 電子情報通信学会 信学技報

THE I NSTI TUTE OF ELECTRONI CS,         I EI CE Tec hni c al Repor t I NFORMATI ON AND COMMUNI CATI ON ENGI NEERS CS2017- 57 ( 2017- 11)  

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modify the data anytime and anywhere by saving the data to

the WCS. However, it is impossible to guarantee the user’s

privacy. Firstly, since the attacker can easily detect the

tar-get server, the user’s data is always exposed to a risk of the attacks. Secondly, the WCS itself can become an attacker

who utilizes the user’s data without the agreement. To solve

these problems, the schemes which encrypt the user’s data

have been proposed [3]−[5]. However, since these still adopt

the central server, they have a risk of the undefeatable attack anytime [6].

To store the user’s data without the central server, Peer to

Peer(P2P) storage methods have been investigated [7]−[9].

The P2P storage can provide the same usability as the WCS

does while relieving the difficulty of privacy protection. The user’s data are encrypted and divided into multiple pieces

and stored in free space of the storage nodes and the

au-thority of the data is guaranteed by the blockchain which

records all history of data transfers. Since the node except

the owner of data cannot obtain all the pieces of data, the privacy protection in the P2P storage scheme is easier than

that of the WCS. Also, the user can restore the lost data

from the storage nodes through the error correction, even

though some storage nodes and the user’s data are lost.

Fur-thermore, the P2P storage’s storage space is able to get lager than the WCS. According to the P2P scheme [10], if the users

of the whole world begin to share the storage space on their

devices for using the P2P storage, the P2P storage can have

the storage space of 250,000 PB. Compared to the Google

Drive which recently has the storage space of 8,000 PB, the P2P storage’s space is much lager. To prove the

reliabil-ity of the P2P storage scheme, the blockchain is adopted [9].

By using the blockchain, every storage node in P2P network

checks each other whether other storage nodes safely stored

the user’s data. If the user’s data are damaged or changed, every storage node can detect this and share the malicious

situation in the P2P network. Despite of these advantages,

there are two agendas to be considered. First, the

communi-cation cost of the user which is the number of transmission

and the delay for exchanging is high. Since the user ran-domly selects the storage nodes in the P2P network, some of

the user’s divided data might be saved in the distant storage

nodes. This causes the high communication cost. Second,

there is still the possibility of the attacks. Since all nodes in

the network can trace the data transfers, an attacker can nar-row the target nodes which store the specific user’s data [11].

In this paper, in order to develop the P2P storage scheme

mentioned above, we propose an efficient P2P storage scheme

with privacy protection. To improve the first agenda, the

user selects the closest storage nodes based on the updating frequency which is the rate how often the user reads and

modifies the data. Since a user can obtain the data which

is often accessed with low delay and few transmissions, the

average communication cost is reduced. In order to reduce

the communication cost further, the methods calledDIF F

which generates the script for converting an old data to a new data is adopted when the user tries to modify the stored data

in the storage nodes. As the second agenda, to prevent the

owner of the data and the user’s privacy from being identified

by tracing the blockchain, the randomly selected relay nodes

are recorded as the owner of the data in the blockchain in the P2P storage.

The rest of this paper is constructed as the follows. Section

2. describes the system model. The related work is

intro-duced in Section 3.. The proposed scheme is described in

Section 4.. The analysis and the results are shown in Section 5.. Finally, we conclude our research in 6..

2.

P2P Storage

2. 1 Blockchain

The blockchain is a digital ledger in which transactions are

made in bitcoin scheme for protecting the nodes and

detect-ing the malicious nodes [14]. For operatdetect-ing the blockchain in

the P2P storage scheme, the user shares the block includ-ing the user’s data list and the storage nodes which have

the user’s data to the P2P network. Because this block is

connected like the chain and it records the flow of the data

with the agreement of all nodes, the user’s data are safely

stored in the storage nodes in the P2P network. Also, any node cannot harm the data because all nodes can detect the

malicious situation such as the abnormal modification of the

data. Through the blockchain, all of the nodes can know the

user’s information included in the block.

3.

Related work

3. 1 Cloud storage with the edge computing

To reduce the overload of the cloud central server, the edge

computing schemes have been proposed [4],[5]. The edge

computer is the distributed information technology architec-ture in which client data are processed at the periphery of

the network as closest to the originating source as possible.

According to the user’s preference of the data such as the

movie, the preferred data will be stored in the edge

com-puter nearby the user. Therefore, the user accesses to the closest edge computer, not to the cloud central server, to

utilize the data. This is able to reduce the communication

cost as the overload. However, if user’s preference of the data

is changed and the number of the users becomes large, the

communication cost will be increased. Moreover, the many edge computers are required to cover many users. To

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as below.

3. 2 P2P storage scheme

To maintain the P2P storage scheme, the user who is the

one of nodes in the P2P network shares some space via the Internet [7]−[9]. The user can store the data in the storage

nodes in P2P network regardless of the server. In addition,

the use can access to the P2P storage without any

restric-tion as the P2P storage is composed of many users. For

storing the data in the storage nodes, first the user divides the data into some pieces of data. The user encrypts

di-vided data by using the secret key. Secondly, to store the

encrypted data to the nodes in the P2P network the user

randomly selects the nodes to be the storage nodes whose

number is equal to that of the divided data. In this case, the user’s data might be located in the distant nodes. When

the user wants to modify the data, the user receives the

di-vided data from the storage nodes. Then, the user decrypts

them and combines them into the one original data. After

converting the original data, the user divides the data and encrypts them again. The divided data are stored in the

storage nodes again. To improve P2P storage scheme, the

blockchain method is adopted [9]. Through the P2P storage

with the blockchain, the user’s data are safely stored in the

storage nodes. The storage nodes cannot try to change the user’s data at their will, because the blockchain system

de-tects it. Despite of this advantage, there are two points to

improve. Firstly, the communication cost of the user which is

the transmission time and the delay of the exchanging data

in the network is high when the user stores the data and reads the stored data for modifying from the distant storage

nodes. Secondly, the user’s privacy can not be guaranteed.

Because all of the storage nodes can know who is the owner

of stored data via the blockchain method. If Bob who is the

real user stores the data in the storage nodes: S1, S2 and S3, he will share the block, including the user’s data list and the

storage nodes which have the user’s data, to all of the nodes

in the P2P network. Then, the storage nodes S1, S2, and

S3 can know that they have Bob’s data. Therefore, they are

possible to decode the Bob’s data.

3. 3 Modification with the difference of data

To modify the data with the small data size, the schemes

which modify the data by using DIF F, the difference

be-tween the original data and modified data, have been

pro-posed [12],[13]. DIF F can adopt all of the data types since the data are composed of the binary ones. Also,DIF F can

be extracted from the encrypted data. By usingDIF F, the

data are effectively modified. For example, ifDIF F is

gen-erated betweenA which is the original one andB which is the modified one,C which is the copied one fromAcan

ob-tainB by usingDIF F. To adopt this to the data storage

scheme, the user can change the stored data by usingDIF F

without sending the modified data to the storage.

4.

Proposed Scheme

In order to overcome the shortcomings mentioned in

Sec-tion III, we propose the efficient P2P storage scheme with

privacy protection. The main idea of our scheme is classified

into three categories. Firstly, to reduce the communication

cost when the user stores the data in storage nodes, the clos-est storage nodes in P2P network are selected by the

differ-ence of the updating frequency of the data. The updating

frequency is the rate how often the user reads and modifies

the data. Secondly, to reduce the communication cost when

the user modifies the stored data, DIFF method is utilized. Thirdly, to ensure the user’s privacy from the storage nodes

in the blockchain, the random relay nodes pretend to be the

owner of the data instead of the real user. In our proposed

P2P storage scheme, the user can safely use the P2P storage

with the low communication cost and the guarantee of the privacy.

4. 1 Selection of the closest storage nodes

Here, to reduce the user’s communication cost when the

user wants to store the data, the user chooses the closest

storage nodes by the difference of updating frequency, which is the rate how often the user reads and modifies the data,

of the data. Then, the user makes the policy of updating

frequency of the data as shown in Table I-(a) and checks the

state of all nodes in Table I-(b) before storing the data to

the storage nodes. The user can set the updating frequency of the data and the policy of the updating frequency how

to work in the P2P storage before storing the data to the

storage nodes. Also, the updating frequency of stored data

in the storage nodes is arranged by the user. In this step, the

variables are defined as: D is the user’s data which will be stored in the storage nodes,Lis the list of all of the storage

nodes in P2P network,T is the close storage nodes which is

chosen fromL,Sis the number of close storage nodes which

the user’s data will be stored and the data to be divided.

First, the user inspects the updating frequency of D in the policy as shown in Table I-(a). If D’s updating frequency

has been set high, the user selectsT. Then, the user divides

and encryptsDby the user’s secret key as the number ofS.

Finally, the user sendsD toT.

4. 1. 1 Exchanging the storage nodes

In this step, if the updating frequency of the stored data

is changed in the storage nodes, the stored data’s storage

nodes are will be exchanged. For example, if the document

frequency becomes to be low or the video frequency becomes

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ද1: Initialization of the user

(a) Updating Frequency

High Low Document Audio

Image Video Graphic Record

..

. ...

(b) List of the nodes in P2P

Node Hop M/S Capacity(GB) 1 1 2 20 2 2 10 10 ..

. ... ... ...

n n n n

the other hand, the video data will be replaced in the closest

storage nodes.

4. 2 Modification of the data with the low cost

To reduce the communication cost when the user wants to modify the stored data in the P2P storage scheme, we

take note of DIF F method. In the conventional scheme,

the user has to receive all of the stored data from the

stor-age nodes for modification and resends the modified data to

store. This causes the high communication cost due to the exchanging data in the network. On the other hand, in the

proposed scheme the user can effectively modify the stored

data with the low communication cost throughDIF F

meth-ods. DIF F methods are the following two cases.

4. 2. 1 Modification with the original data in the user’s

devices.

In this step, if the original data is kept in the user’s device,

the user is not necessary to receive all of the divided-stored

data from the storage nodes for modifying the stored data.

We assume that A is the original one, B is the modified data, C is the stored data in the storage nodes, and D is

DIF F which is the difference betweenAandB. In the con-ventional scheme, after generating B the user divides and

encrypts B to store in the storage nodes. However, in the

proposed scheme, the user does not need to send the divided

B to the storage nodes. ByDIF F methods, after generat-ing B the user makes D betweenA andB and sendsD to

the storage nodes to modify C. The number ofD as same

as the number of C. For example, if the number of stored

data in storage nodes are three, the user has to divide the original data and the modified data into three divided ones.

And the user sendsD to the storage nodes. Then, by using

D, the storage nodes can changeC to B. In the proposed

scheme, the user does not need to send all of dividedBto the

storage nodes. The user only sendsDto the storage nodes. Therefore, the user can effectively modify the stored data in

storage nodes by usingDIF F.

4. 2. 2 Modification without the original data in the

user’s devices

If the user does not have the original data in the device, the user needs to receive all of the divided-stored data from

ද2: Simulation parameters

Parameter Value The space of one storage node 10GB∼200GB

The size of the user’s data 1GB∼5GB The size ofDI F F 1020%

The number of the user’s data to be divided 3 The number of the stored data to be exchanged 2 The delay rate of the hops 2m/s The number of the storage nodes 10000

The type of P2P network Mash Topology

ද3: Average hop per user in P2P

Hop 1 2 3 4 5 6 7 8 9 Node 1 7 89 870 3779 4228 1551 137 9

the storage nodes are similar to the conventional scheme for

the modification. The user restores the divided-stored data

into the one original data. Then, the user starts to modify

the original data. After modifying the data, the user follows the same process mentioned above 4.2.1.

5.

Analysis And Result

Our analytical method has two purposes: Fist purpose

is that the comparison of the communication cost between the conventional scheme [9], and our proposed scheme with 4

cases, Second purpose is that the protection of the user’s

pri-vacy from the malicious storage nodes in the blockchain. To

compare the communication cost between the conventional

scheme and our proposed scheme, the data set of Gnutella P2P network is adopted [15]. Gnutella is a decentralized

P2P network that allows users to share data via the Internet

without the central server. The parameter of the simulation

is shown in Table II. The average hops per user in this data

set is shown in Table III. This analysis is carried by increasing 500 nodes from 1000 nodes to 10000 nodes by the MATLAB.

5. 1 Case 1

Fig. 1-(a) shows the comparison of the communication

cost of the delay rate of hops in the P2P storage scheme

when the user sends data to the storage nodes. It is shown that our proposed scheme’s cost is lower than the

conven-tional scheme. The user sends three data to the storage

nodes in both schemes as shown in Table II. In the

conven-tional scheme, the user randomly selects the storage nodes

three times in the range shown in Table III. In our proposed scheme, the user chooses the closest storage nodes three times

by considering the updating frequency. Also, if the space of

the closest storage nodes is full, the user stores the data in

the next hop’s storage nodes. Therefore, the delay due to

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be-a Cbe-ase 1 b Case 2

c Case 3 d Case 4

ਤ1: Comparison of the communication cost

cause the space of the storage nodes is full in range of 1∼2

hops. So, the user needs to store the data to the storage nodes of 3 hops. The cost is increasing little by little from

2500 to 8000 users because the user’s data can be stored in

the storage nodes in range of 5 hops. In addition, the cost is

decreasing from 8000 users, the user’s data can be stored in

the storage nodes in the range of 4 hops.

5. 2 Case 2

This part reflects the additional cost to the case 1. In the

proposed scheme, we assume that the two storage nodes are

exchanged according to change in the updating frequency of

the two data as shown in Table II. The communication cost of the conventional scheme is as same as the case 1. In the

pro-posed scheme, the communication cost of the transmission

delay is generated from exchanging the storage nodes when

the updating frequency of the data is changed by the user.

The storage nodes are exchanged two times in the range of Table III since the user’s data which will be exchanged in the

storage nodes are two. Fig. 1-(b) shows that our proposed

scheme’s cost is lower than the conventional scheme despite

of the additional process of exchanging the storage nodes.

Because the closest storage nodes are selected by the policy of the updating frequency when the user stores the data to

the storage nodes for the first time.

5. 3 Case 3

Fig. 1-(c) shows the comparison of the communication cost

generated from the size of exchanging the data when the user modifies the stored data. Here, we assumes the user has the

original data in the user’s devices. Ais the original one,Bis

the modified data,Cis the stored data in the storage nodes

and D isDIF F which is the difference between A and B.

In the conventional scheme, after generatingB, the user di-vides and encrypts B to store in the storage nodes. Then,

the user has to send the divided B to the storage nodes to

store. However, in the proposed scheme, the user does not

need to send the divided B to the storage nodes. The user

only sendsDto the storage nodes. We assume the size ofD is 1020% ofB by the experiment. Therefore, in our

pro-posed scheme, the communication cost is much lower than

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5. 4 Case 4

Fig. 1-(d) shows another comparison of the

communica-tion cost generated from the size of exchanging data when

the user modifies the stored data. Here, it is assumed that the user has not the original data in the user’s devices. It

re-flects the additional cost, which is the size of receiving data

from the storage nodes, to the case 3. After receiving all

of the stored data from the storage nodes, the user has to

restore them to the original one in both schemes. Also, by using the original one, the user modifies the data. But, as

we mentioned above, the sending cost is different, because

the user only sends the size ofDIF F to the storage nodes.

Therefore, our proposed scheme’s cost will be lower than the

conventional scheme.

Through these analysis, we can show that the user can use

the P2P storage scheme with the low communication cost in

our proposed scheme.

5. 5 The protection of the user’s privacy

To protect the user’s privacy in the blockchian in the P2P storage scheme, the relay nodes are adopted in our proposed

scheme. The relay nodes are the mediators between the user

and the storage nodes. In the conventional scheme, the user’s

privacy is leaked from the malicious storage nodes in the

blockchain. However, in our proposed scheme by using the relay nodes the user’s data will be protected from the the

malicious nodes in the blockchain. For this, firstly the user

selects the storage nodes: S1, S2 andS3, and chooses the

relay nodes: R1,R2 andR3. If the user’s data are divided

and encrypted into three, the user sends each data to R1,

R2 andR3 respectively. The number of relay nodes as same as the storage nodes. R1, R2 and R3 know the owner of

the data, but they can not inspect whether the other relay

nodes have received the data from the same user. They will

be the owner of data on behalf of the user. And they send the data to the storage nodes: S1, S2 and S3. After the

data are stored in the storage nodes,S1,S2 andS3 generate

the block one by one and share the block to the blockchain

in P2P network. Then, S1, S2 and S3 will recognize R1,

R2 andR3 as the owner of the data, not the real user. The user’s data will be safe from the storage nodes. Therefore,

if the storage nodes and the relay nodes are malicious, they

cannot get the user’s data since they cannot know the real

owner of the data despite the blockchain scheme is adopted

in our scheme. Also, the the relay nodes cannot damage the user’s data while mediating. Because, if the relay nodes

damage the user’s data at will, the user can know this

situ-ation through the block which is made by relay nodes in the

blockchain. Moreover, the relay nodes and the storage nodes cannot conspire in order to obtain the user’s data. Because

the user’s data are divided one by one respectively in P2P

network, they only have one piece of the user’s whole data.

Through this process, the user can safely maintain and use

the P2P storage scheme with the protection of user’s privacy

in the blockchain.

6.

Conclusion

To reduce the communication cost and protect the user’s

privacy in the P2P storage scheme by using the blockchain,

we have proposed the efficient P2P storage scheme with pri-vacy protection. By selecting the storage nodes according to

the updating frequency of the data, the communication cost

is reduced. Furthermore, the user’s privacy is protected from

the storage nodes through the relay nodes in the blockchain.

In our proposed scheme, the user can safely use the P2P storage with the low cost.

7.

Acknowledgment

This work is partly supported by the Grant in Aid forɹ Scientific Research (No.17K06440) from Japan Society for

Promotion of Science (JSPS).

จ ݙ

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