SeC

: SECURE MOBILE SOLUTION FOR DISTRIBUTED PUBLIC

CLOUD STORAGES

Juraj Somorovsky

, Christopher Meyer

, Thang Tran

, Mohamad Sbeiti

, J

org Schwenk

and

Christian Wietfeld

Horst G

ortz Institute for IT-Security (HGI), Ruhr-University Bochum, Bochum, Germany

Communication Networks Institute (CNI), Faculty of Electrical Engineering and Information Technology

TU Dortmund University, Dortmund, Germany

Keywords:

Cloud Security, XML Encryption, Seamless Networking, Virtual Private Storage.

Abstract:

Cloud Computing is an emerging technology that attracts increasing attention as a high-performance and

low-cost solution to process arbitrarily huge data and share them among different users and organizations.

Nonetheless, this technology rises new questions on security and privacy, especially when working with highly

conﬁdential data. Existing solutions offering end-to-end security such as TLS are of no help since the stored

data is only encrypted during their transport. Therefore, a message-level security must at least be applied to

protect those data during and after the storing process. This paper investigates a novel solution for secure data

storage in the cloud. It presents a security concept allowing each client to encrypt outgoing data on one’s

mobile device and share it among a deﬁned user group while using a seamless service provision. As our

concept is used transparently as well as independently on each device, users keep full control over their data

and almost no changes on the existing infrastructure of cloud storage services are needed.

1 INTRODUCTION

Cloud Computing has become a well-used paradigm

in the recent years as reported by Zhang et al. (Zhang

et al., 2010). It brings advantages to both clients and

services. The emerging technology allows users for

private and business purposes to access all their ap-

plications and documents at anytime and anywhere

using different mobile devices (e.g. laptops or smart

phones). Companies, on the other hand, do not

need to spend a great amount of time and money

on the management of their IT-infrastructure for ap-

plication processing and data storage anymore. Us-

ing cloud computing technology, the time spent on

lower-value activities can be minimized and thereby

companies can focus more on strategic activities with

greater impact on their business. These considera-

tions were provided by Molnar and Schechter (Mol-

nar and Schechter, 2010).

To increase the user acceptance of cloud services,

the fulﬁllment of different requirements concurrently,

such as security and high availability in a heteroge-

neous network, is still a challenge. This has also been

observed by Wu et al. (Wu et al., 2010). In the past,

data could be secured by means of, e.g., personal ﬁre-

walls or assignment of permissions. However, with

increasing mobility of end-user devices as well as ser-

vices, the requirement for storing data has changed

these days. Due to this trend the outsourcing of data

to cloud storages is always fraught with uncertainty

as users have absolutely no control over their data.

This, in particular, represents a huge problem regard-

ing storage of conﬁdential information. Encryption

services which allow to encrypt users’ conﬁdential

data on their servers (e.g. Dropbox

) are of no help.

Although they allow a secure storage of users’ data in

the cloud, the clients must still build trust relation-

ships with the encryption services provided by the

cloud (proper encryption on server side, no backup

keys, backdoor-free systems, no data disclosure to

third parties, ...). Therefore, the security processing

must be moved to the cloud client in order to give him

the possibility to take full control over his data.

Besides the security aspects, today’s mobile com-

munication is available anywhere where service

providers have to ensure a high availability of their

http://www.dropbox.com

555

Somorovsky J., Meyer C., Tran T., Sbeiti M., Schwenk J. and Wietfeld C..

SeC2: SECURE MOBILE SOLUTION FOR DISTRIBUTED PUBLIC CLOUD STORAGES.

DOI: 10.5220/0003903205550561

In Proceedings of the 2nd International Conference on Cloud Computing and Services Science (CLOSER-2012), pages 555-561

ISBN: 978-989-8565-05-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

provided cloud services. On the one hand, this re-

quires reliability solutions for the cloud services in

the back-end as described by Wang et al. (Wang et al.,

2010). On the other hand, as reported by Tran et

al. (Tran et al., 2010), a deployable and capable ver-

tical handoff (VHO) solution for mobile devices is

needed in the front-end. The use of VHO allows mo-

bile clients seamless access to services across differ-

ent mobile access technologies.

The main contribution of this paper is the proposal

of a cost-efﬁcient and feasible security framework for

mobile devices that provides a holistic security so-

lution with full data control for users. These secu-

rity goals are achieved by a so called Sec

middle-

ware that encrypts and decrypts the communication

between mobile devices and cloud servers. The Sec

middleware is based on the XML Encryption stan-

dard (Eastlake et al., 2002) and allows the applica-

tions to securely store XML data in the cloud as for

example on XML databases. This gives the clients the

possibility of a seamless and secure storage solution

for their data without establishing trust relationships

with cloud services.

We present a system of group keys, which allows

to create different user groups and authorize them for

decryption of dedicated parts of documents.

Besides the security capabilities, our framework

includes a mobility support that allows seamless ser-

vice provisioning in a heterogeneous network.

2 RELATED WORK

The topic Storage Services is not new and is already

offered by several technologies like Microsoft Share-

point

, OpenText

and Dropbox that are mostly ac-

cessible by web interfaces. Except for the occa-

sional one, e.g., Brainloop

, OceanStore

and Drop-

box, current systems support cryptographic protection

for stored data, which is, however, saved on untrusted

cloud servers. Moreover, these approaches focus on

some kind of trust relationship between the mobile

client and the cloud service. Clients have to trust the

service provider not to disclose stored data and prop-

erly secure its infrastructure against potential attack-

ers.

In terms of data encryption systems, there exist

some wide-spread solutions such as TrueCrypt

http://sharepoint.microsoft.com

http://www.opentext.com

http://www.brainloop.com

http://oceanstore.cs.berkeley.edu

http://www.truecrypt.org

eCryptFS

that usually ensure only local protection

and are therefore not suitable for distributed cloud

storages. Further proposals such as the one intro-

duced by Kamara and Lauter (Kamara and Lauter,

2010) rely on token based searchable encryption

schemes which causes a dispensable huge overhead.

In contrast, the Sec

middleware concept allows

an efﬁcient, seamless and ﬁne grained data encryp-

tion on client devices. Therefore, trust relationships

between mobile clients and cloud storage services are

not necessary since content is protected before leav-

ing the device.

Securing the wire can be done by using

TLS (Dierks and Rescorla, 2008) or IPSec (Kent

and Seo, 2005) which will perfectly ﬁt for this pur-

pose. However, encrypting the transport medium is

just half-way towards secure document storage. Since

storage means making data persistent it is insufﬁcient

to only focus on the way of deliverance. Durable en-

cryption concepts for data storage have to be taken

in account. At least when it comes to ﬂexibility and

hybrid documents, with encrypted and plaintext parts

combined together in one single document, the intro-

duced technologies such as Truecrypt reach their ap-

plication limitations. These technologies are designed

for different usage scenarios and thus are not appro-

priate for partial encryption.

3 SECURITY REQUIREMENTS

IN DISTRIBUTED CLOUD

STORAGES

Without a satisfactory level of security, users lack mo-

tivation to subscribe to cloud storage services. Thus,

security is one of the main boundaries which still pre-

vents a service from wider deployment. On the other

hand, a security system must also deal with efﬁciency

and real-time capability requirements.

We have identiﬁed the following goals that a se-

curity system must cope with:

User Transparency. The conﬁdentiality of the data

at the cloud storage and during the transport should

be kept as transparent as possible to the user.

Seamless and Cost-effective Integration and In-

stallation. The costs for integrating the security sys-

tem at both, client and network side, should be as low

as possible. Besides, the system should seamlessly

operate and not interfere with parallel running appli-

cations

Efﬁciency. The security system should be based on

https://launchpad.net/ecryptfs

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</EncryptedKey>

</EncryptedData>

</Document>

Figure 1: Example of an XML Encryption metadata block.

light-weight schemes. Required resources should be

kept minimal.

Message Security. As a security goal for messages

our approach focuses on conﬁdentiality.

Secure Key Storage. It must be possible for trusted

code to securely store persistent data so that their con-

ﬁdentiality and integrity can be assured.

Scalability. The overhead of any security scheme tar-

geting the emerging cloud storage technology should

satisfactorily scale by an increasing number of users

and by a growing amount of stored data.

Tagging of Encrypted Data. Although the payload

of documents stored in the cloud is content protected

by XML Encryption, the documents have to remain

searchable. It should remain possible to some users

to look up for a particular information without knowl-

edge of a particular key. Searching particular infor-

mation can be realized by including meta data on top

of the encrypted payload.

4 FOUNDATIONS

4.1 XML Encryption

Since XML documents often contain conﬁdential

and reliable data, the W3C consortium developed

standards that describe the XML syntax for apply-

ing cryptographic primitives to arbitrary XML data:

XML Encryption (Eastlake et al., 2002) and XML

Signature (Eastlake et al., 2008). In comparison to

other security standards such as PGP (Elkins et al.,

2001) or S/MIME (Ramsdell, 2004), XML Encryp-

tion and XML Signature can be applied to arbitrary

data in the document. Therefore, these security stan-

dards offer high scalability and ﬁne granularity of se-

cured data.

Figure 1 gives an example of an XML message

containing a ciphertext. This message consists of two

main parts: The <EncryptedKey> part includes the

symmetric session key. The <EncryptedData> part

contains the payload data encrypted with the symmet-

ric key.

4.2 SAML

The Security Assertion Markup Language (Cantor

et al., 2005) represents a framework for modeling

and deﬁning security assertions (i.e., SAML asser-

tion), authentication, and authorization data. The

framework is XML-based and thus highly extensible.

Included in the speciﬁcation are means to use pre-

deﬁned protocols covering typical communication se-

quences. Moreover, the users can deﬁne their own

assertions according to their needs and model new

SAML-powered protocols. In general, assertions de-

ﬁne statements about particular subjects. These state-

ments may address the subject’s identity, authoriza-

tion, or additional attributes and may be interpreted

by other parties.

5 SeC

: THE PROPOSED

SOLUTION

This section describes the SeC

system architecture

and its operational details, focusing on the illustration

of the security architecture.

Please note that in our concept we neither analyze

the trust relationships between the clients nor address

the infrastructure necessary to manage permissions.

We assume that the clients are registered on the Key

Server by a key administrator and are in possession

of valid keys, which give them rights to access the

desired documents. Moreover, during our description

we assume the mobile device as trusted and free of

malware, which could steal the decrypted data.

5.1 System Architecture

In order to fulﬁll the application developers’ and

users’ requirements described in previous sections,

we designed a modularized Sec

system architecture.

This architecture is depicted in Figure 2 and consists

of multiple independent components decoupled

from each other. This high modularization enables

to beneﬁt from the most possible ﬂexibility and

changeability of components. The contents of the

Sec

system are discussed in the following.

XML Encryption Engine. This unit is responsible

for en- and decryption of XML payload and key data

by relying on the concept of XML Encryption (East-

lake et al., 2002). To fulﬁll its purpose, the compo-

nent utilizes components such as a crypto library and

a microSD Card.

Key Manager. To keep track of keys already gathered

and their usage in the en-/decryption process, the Key

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Seamless Mobility Support for Data-Stream

UMTSGSM

WLAN

LTE

Web Services

Applications

App-Sec²

Interface

VHO-Sec²

Interface

XML Encryption

Engine

Key Manager microSD Card

Sec

-Middleware

Mobile Device

Untrusted Cloud

Storage Services

Trustworthy Key Server

Insecure

Channel

IP based Network

(Internet)

Figure 2: Sec

application scenario is based on three parties:

Trustworthy Key Server, Untrusted Cloud Storage Services,

and a mobile device. The encryption processing and key

management within the mobile device is handled by the so

called Sec

Middleware.

Manager stores a list of available keys. These keys

are stored unextractable on the microSD Card.

microSD Card. It provides a highly secure stor-

age for conﬁdential key data such as the asymmet-

ric private and symmetric secret keys as well as ac-

celerates cryptographic operations. In our solution

the microSD Card functions as a kind of trust anchor

securely encapsulating key material unaccessible for

unauthorized persons. Stored key material is not al-

lowed to leave the device. In technical terms the used

microSD Card represents a conventional SmartCard

with additional storage capabilities. In our project we

use the Mobile Security Card SE (Standard Edition)

1.0 manufactured by Gieseke & Devrient

Trustworthy Key Server. A major component, and

the only external one that a client has to trust, is the

Key Server. This server stores all cluster keys (dis-

cussed in 5.2), extractable only in encrypted form, and

is able to provide them to authorized clients. Each

cluster participant may request for an associated key

(see 5.2, for details), which will only be delivered en-

crypted.

Seamless Mobility Support. To proﬁt from seam-

less communication and to guarantee secure commu-

nication, we integrate a Client Based Secure Hand-

off Solution (CSH-MU) proposed by Tran et al. (Tran

et al., 2010) into this module. Thereby, our frame-

work supports a seamless service provisioning across

a heterogeneous network consisting of UMTS/HSPA

and WLAN. If one of the networks fails during the

www.gd-sfs.com/the-mobile-security-card/mobile-

security-card-se-1-0/

transmission process, another network link seam-

lessly continues with the transmission.

Untrusted Cloud Storage Services. As place for

data storage, existing cloud storage providers can be

used without adjustment or further extensions. The

lack of trust relationship between service provider and

client presents no issue since all the conﬁdential data

stored on this service is strictly encrypted and thus un-

usable for potential attackers without key knowledge.

Moreover, the ﬁne granularity of the XML Encryption

allows to encrypt arbitrary document data. Therefore,

the client can decide which data is conﬁdential and

must be encrypted. The plain text data can afterwards

be used as metadata for queries.

As can be seen the security handling components

(XML Encryption Engine, Key Manager, and mi-

croSD Card) are integrated in a central component

called Sec

middleware. It should be noted that the

security system can be separated from other mod-

ules and reused in many applications. The developers

implementing their applications do not have to care

about the key management or data encryption and de-

cryption; they only have to integrate the Sec

middle-

ware between their application and the mobility layer.

Thus, the users’ data can be seamlessly encrypted, se-

curely sent over the insecure channel, and stored on

the untrusted cloud storages.

5.2 Multi Stage Key Concept

In order to cover all the proposed usage scenarios in

a secure manner, different key types are introduced.

Each key type is used for a special purpose to pro-

vide conﬁdentiality beyond user boundaries. The con-

cept empowers the usage of commonly shared keys

between legitimate participants. In particular the ap-

proach suggests three different keys as illustrated in

Figure 3r:

(asymmetric) Private/Public Keypair. Each par-

ticipant of the system owns a private/public keypair.

These keys are used for proper authentication and se-

cure key transport. The public key is deposited at the

Key Server and the private key is stored in the secure

part of the microSD Card at client side. Since public

keys are not conﬁdential, the Key Server may store

these type of keys in plaintext in a database or lookup

table.

(symmetric) Cluster Key (CK). A cluster key is a

symmetric secret key that protects the particular clus-

ter domain it belongs to. Users authorized to use the

cluster key (and thereby authorized to decrypt all the

documents in the cluster) are stored in an authoriza-

tion table on the Key Server. Compromise of this key

leads to a major break of conﬁdentiality for all ﬁles

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Trustworthy Key Server

Insecure Cloud Storage Services

CK1

(doc)

doc

CK1

CK3

CK2

DK1

(XML data)

DK1

(XML data)

DK1

(XML data)

CK1

(DK1)

microSD Card

Private and

Public Key

Cluster

Keys

CK1

CK2

CK5

Document

Keys

DK1

DK2

DK3

DK4

Sec

-Middleware

Mobile Client

Client 1

Client 2

Public Key

Client 1

Client 2

CK1, CK2

CK3

Public Key Table

Authorization Table

DK1

CK := Cluster Key E := Encryption

DK := Document Key doc := Document

Legend:

HSM

Figure 3: Our key concept is based on three key types: pri-

vate/public keypairs belonging to the clients, cluster keys

(CK) used for encryption of documents within speciﬁc clus-

ters, and document keys (DK) for encryption of the docu-

ments.

protected by the associated cluster domain.

(symmetric) Document Key (DK). In order to

protect the documents stored in the cluster, the cluster

keys encrypt document keys. The document keys

encrypt the content of the XML documents and

thereby the payload of the cluster domain. Hereby,

every document has its own document key, so that

in case of compromise of a single document key,

only the particular XML ﬁle containing this key is

concerned. All other documents of the cluster domain

remain safe. To achieve the highest security level,

the document keys are also kept inside the secure key

storage and are not extractable.

The usage of different keys provides a major ad-

vantage in contrast to the native approach of deliver-

ance of particular keys for documents: Each partici-

pant needs effectively only a cluster key belonging to

one’s cluster domain to decrypt all the ﬁles protected

by this domain (cluster). Since all documents contain

their individual encryption keys, a cluster domain par-

ticipant is able to decrypt the document parts by de-

crypting the document key and use it for decrypting

the desired document parts.

Splitting key accountabilities aims to minimize

the impact of key compromises. Adversaries who

manage to take possession of a document key are not

able to break the whole protection domain.

The described key concept offers a scalable key

and user authorization management. It allows simple

creation of new clusters and assignment of rights for

new users. Additionally, the high scalability of the

XML Encryption standard makes it possible to en-

crypt the document keys with different cluster keys.

Therefore, XML documents belonging to different

clusters can also be implicitly created. Combining all

keys results in a key hierarchy that includes an inte-

grated authorization concept. Only authorized users –

users belonging to the cluster domain – can obtain the

CK which is necessary to gain access to DKs. DKs

are used to provide conﬁdentiality of the several docu-

ment sections. Authorization at the key server is done

by XML signature generated with the user speciﬁc

private key.

Our security concept relies on a few corner stones:

(1) Security of the Key Server and local Key Storage,

(2) Security of the used encryption and signature al-

gorithms, (3) Trust to all cluster members, (4) Proper

user-to-group tables at the Key Server.

5.3 Security and Usability Goals

The Sec

concept addresses a couple of major security

goals which will be brieﬂy discussed in the following:

Conﬁdentiality. This goal is being achieved by using

XML Encryption secured documents. The content is

only accessible by those who are aware of a valid key.

Authenticity. To uniquely identify and authenticate

users when trying to obtain cluster keys it is essential

to provide means of secure authentication. In partic-

ular authentication at the Key Server side is done by

providing signed SAML requests.

Reliability (at client side). Best possible service con-

tinuity, and thus reliability at client side, is realized

by the Vertical Handoff Module which manages to

keep the connection seamlessly alive beyound media

boundaries.

Integrity (optional). An optional integrity protection

can be achieved by applying XML Signatures. Se-

cure and best practice applications are e.g. discussed

by (Jensen and Meyer, 2011). The usage of Message

Authentication Codes may also be a way to provide

integrity.

Tagging of Encrypted Data. In order to provide the

users with efﬁcient searching abilities, only parts of

the stored data are encrypted. The plaintext parts are

used for searching purposes. Please note that we do

not use the term searchable encryption as it rather im-

plicates a cryptographic approach.

6 A PROOF OF CONCEPT

SCENARIO

In order to make the idea of our concept clear we in-

troduce a concrete communication process involving

all the introduced parties from the previous section.

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Trustworthy Key

Server

Cloud Storage

Services

microSD

Card

Mobile Client

Request data

Send data

data

Signed SAML

key request

Authorization check

and

key search

SAML key

response

Encrypted

cluster key

Cluster key

decryption

and storage

Document

key decryption

Encrypted

document key

Send data for

decryption and

processing

Figure 4: Communication process sequence of Sec

key

concept.

Afterwards, we show the data storage and their struc-

ture using a simple calendar application with note

functionality.

6.1 Communication Process Example

This section describes a simple communication pro-

cedure example between the Cloud Storage Service,

Key Server, and the client attempting to get XML

data. We assume that the client is in possession of

a key pair, which is registered at the Key Server, and

is authorized to access the cluster (data within the user

group).

The communication example is depicted in Figure

4 and consists of the following steps (for the simplic-

ity we depict only the microSD Card module inside

of the Sec

middleware):

1. First, the client sends his search request with pub-

lic metadata to the Cloud Storage Service.

2. The Cloud Storage Service evaluates the request

and searches for the XML data.

3. The located XML data is sent to the client.

4. After the data arrives, the Sec

middleware ex-

tracts the cluster key information and creates a

SAML request for the appropriate cluster key.

Then, it signs the SAML request using the XML

Signature standard with its private key (stored on

the microSD Card) and sends it to the Key Server.

5. The Key Server veriﬁes the signed SAML request

and authorizes the client. Afterwards, it encrypts

the requested cluster key and inserts it to a newly

created SAML response.

6. The Key Server sends the generated SAML re-

sponse to the client.

7. The Sec

middleware extracts the encrypted clus-

ter key and provides it to the microSD Card.

8. The microSD Card decrypts the cluster key using

the client’s private key and stores it for subsequent

processing.

9. The Sec

middleware extracts the encrypted docu-

ment key from the already gathered document and

forwards it to the microSD Card.

10. The microSD Card decrypts the encrypted docu-

ment key using the cluster key.

11. As the microSD Card is in possession of a valid

document key, the Sec

middleware can now use

it for data decryption. Afterwards, the decrypted

data is sent to the client application for further

processing.

In order to achieve the highest security level, the

cluster key cannot leave the microSD Card and all

operations using this key must take place within the

module. Once the client’s microSD Card is in posses-

sion of a valid cluster key, the encryption process is

straight-forward and the steps 4–8 can be skipped.

6.2 Calendar Application with Note

Functionality

A calendar application with note functionality serves

as a proof of concept scenario that will be brieﬂy in-

troduced. The demonstration platform on mobile de-

vices side is Android

and the payload is stored at a

cloud storage service providing XML database.

By using this proof of concept application, one is

able to manage events such as meetings with notes in

a secure manner. The conﬁdential data of the events

written in a note (in this case the event description) is

encrypted, whereas date, time, and additional location

are left unencrypted for searchability and collision de-

tection purposes. After entering a PIN, the user can

write notes for a certain event. Conﬁdential parts of

the note are marked for encryption and are, therefore,

only visible for authorized cluster groups.

Given this scenario, users outside the cluster can

detect event and location collisions, but conﬁdential

event details are only visible to cluster members.

A data ﬂow example for decryption between the

Application, Sec

middleware, and cloud storage is

illustrated in Figure 5. In this scenario, the queried

event data is delivered to the client. The Sec

mid-

dleware decrypts the conﬁdential data, allowing the

client application to process them in a decrypted rep-

resentation. After the client’s processing is done, the

http://www.android.com

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...

</EncryptedKey>

<Event>

...

</EncryptedData>

</Event>

</calendar>

Encrypted

Document Key

Cloud

Client

XML Encryption Engine

microSD Card

Cluster 1

<Event>

</Event>

</calendar>

Calendar Application

Figure 5: An example data ﬂow.

data is sent again through the middleware to the XML

server.

In order to encrypt the appropriate data before

sending to the XML server, the data dedicated for the

encryption process has to be indicated. For this pur-

pose, the application adds speciﬁc attributes to the el-

ements that will be encrypted. In our example, we use

the attribute enc="CK1" meaning that the marked part

of the note must be encrypted with cluster key CK1.

7 CONCLUSIONS & FUTURE

WORK

The proposed solution provides a convenient way to-

wards secure data storage in the cloud without the

necessity of trust relationships between the storage

provider and the mobile client. Furthermore, the Sec

concept integrates transparent into existing architec-

tures with little to no changes concerning cloud in-

frastructures. Control over stored data is given back

to the cluster domains, instead of relying on trust re-

lationships and cloud storage encryption.

With upcoming steps, we are researching on op-

tions for extending the concept to a scenario without

the need of a trustworthy Key Server.

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APPENDIX

Our work has been conducted within the Sec

project

, which is funded by the German Federal

Ministry of Education and Research (BMBF).

We would like to thank Benedikt Driessen and

Florian Kohlar for their contributions.

http://www.sec2.org

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