Trusted Data Sharing over Untrusted Cloud Storage Provider

Gansen Zhao, Chunming Rong, Jin Li, Feng Zhang, and Yong Tang

 Cloud Computing Technology and Science (CloudCom), 2010 IEEE Second International Conference on

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OutlineIntroductionSecurity requirementsProgressive elliptic curve encryption schemeTrusted sharing on untrusted cloud serversSecurity analysisRelated workConclusions

Introduction

With cloud computing, data owners have only limited control over the IT infrastructure.Cloud service providers have excessive privileges.

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Introduction

The general idea of the proposal mechanism is to encrypt the data before storing on the cloud. On sharing the data, the encrypted data will be re-encrypted without being decrypted first. The re-encrypted data will then be cryptographically accessible to the authorize user only.

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Security requirements

1. Data stored on the cloud should be confidential.2. Sharing of the data can be achieved by the

authorization by the data owner.3. Permissions given by data owner cannot be

transferred to others by the permission bearer.

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Alice

Bob

Cloud Storage Provider

Trudy

Security requirements

The challenge of meeting the requirements is that secure data sharing needs to be achieved via an untrusted cloud storage provider.

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Progressive elliptic curve encryption

The PECE scheme allows a piece of data to be encrypted multiple times using different keys such that the final cipertext can be decrypted in a single run with a single key.The encryption and decryption are both based on Elliptic Curve Cryptography.

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Notation

Let m be a piece of data, U be a set of N users. For each , has the secret key ki.Let q be a random number agreed by all The encryption is performed in the order of For , it computes

Where

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Uui

Gqkmm iii 1

Uui

Uui iu

Ni uu

mm 0

Encrypt

When all has participated in the encryption process, the final encrypted data is as follows.

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Uui

N

ii

N

ii

N

tNiitN

NN

Ne

Gqkm

Gqkm

Gqkm

Gqkmmm

1

10

1

1

)(

)(

)(

)(

Decrypt

Let , then me can be decrypted by a single operation as follows.

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m

Gqkm

Gkqm

Gqkmm

N

iie

N

iie

cep

1

1

)(

)(

N

i ic kk1

Trusted sharing on untrusted cloud servers

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Notation

ka : Alice private key (Alice’s key is not used)kaG : Alice public keykb : Bob private keykbG : Bob public keykc : Cloud Storage Provider private keykcG: Colud Storage Provider public key

Assuming kc shared with Alice(the key kc can be a key that is dynamically generated by Alice and the Cloud Provider mutually) 12

Trusted sharing on untrusted cloud servers

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tGGrkmm ce )1(

Gkb)2(

tGGrkGkrGkrGt cccbbc )3(),)(4( GtGr cc

Grb)5(

GtGkrmm cccec )6(

Gkrmm bbcb )7(

Alice BobCloud Storage Provider

Random number r, t

Random number rc, rb

Algorithm Proof

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mGkr

tGGrkGkrGkrGkrtGGrkmGkrGtGkrm

Gkrmm

bb

cccbbccc

bbccce

bbcb

))()(()(

Security Analysis

Unauthorized Access to Data:1. The attacker acquires a credential that can

decrypt the data without the help of the Cloud Storage Provider.

To acquire such a credential, the attacker will need the knowledge of rkcG + tG, or the knowledge of the three secrets of r, kc and t. As m, r, kc, and t are all kept in secret, the knowledge of rkcG+tG, or the knowledge of the three secrets of r, kc and t, are not possible.

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Security Analysis

2. The attacker acquires a credential that can decrypt the data with the help of the Cloud Storage Provider.

To acquire such a credential, the attacker must have the knowledge of rb, kb, or the knowledge of rbkbG. As rb is delivered to Bob in the form of rbG, it is not possible for the attacker to calculate rb from rbG. kb is a secret that is kept in private by Bob, hence the attacker could not acquire kb.

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Security Analysis

Information Disclosure During SharingTo acquire the clear data during the sharing, an attacker must either have the decryption key for me, mc or mb. The above discussion proves that the attacker cannot decrypt me or mb. To decrypt mc, the attacker needs the knowledge of rckcG. As kc is the private secret kept by the Cloud Storage Provider, the attacker could be able to calculate rckcG from rcG.

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mc=me + rckcG + tcG

Attacker Use Case

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Conclusions

Limitations:This work assumes that the private key of the cloud provider is known to the data owner. This is a very strong assumption as no system administrators would want to share their systems’ keys with users, making it impractical to be deployed.The proposed algorithm and the protocol are less efficient than those protocols that requires only a single ECC encryption operation

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Conclusions

Contributions:1. Identify the need for implementing trusted data

sharing over untrusted cloud storage providers.2. Propose a progressive encryption scheme based on

elliptic curve encryption.3. Devise a scheme for secure sharing on the cloud.4. Perform a comprehensive security analysis of the

proposed scheme and show that the scheme achieves trusted sharing over untrusted cloud servers.

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