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Cloud computing security from single to multi clouds


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Cloud computing security from single to multi clouds

  1. 1. 2012 45th Hawaii International Conference on System Sciences Cloud Computing Security: From Single to Multi-Clouds Mohammed A. AlZain #, Eric Pardede #, Ben Soh #, James A. Thom* # Department of Computer Science and Computer Engineering, La Trobe University, Bundoora 3086, Australia. Email: [maalzain@students., E.Pardede@, B.soh@] * School of Computer Science and Information Technology RMIT University, GPO Box 2476, Melbourne 3001, Australia. Email: [] Abstract Cloud providers should address privacy and The use of cloud computing has increased rapidly security issues as a matter of high and urgent priority. in many organizations. Cloud computing provides Dealing with “single cloud” providers is becoming many benefits in terms of low cost and accessibility of less popular with customers due to potential problems data. Ensuring the security of cloud computing is a such as service availability failure and the possibility major factor in the cloud computing environment, as that there are malicious insiders in the single cloud. In users often store sensitive information with cloud recent years, there has been a move towards “multi- storage providers but these providers may be clouds”, “intercloud” or “cloud-of-clouds”. untrusted. Dealing with “single cloud” providers is This paper focuses on the issues related to the data predicted to become less popular with customers due security aspect of cloud computing. As data and to risks of service availability failure and the information will be shared with a third party, cloud possibility of malicious insiders in the single cloud. A computing users want to avoid an untrusted cloud movement towards “multi-clouds”, or in other words, provider. Protecting private and important information, “interclouds” or “cloud-of-clouds” has emerged such as credit card details or a patient’s medical recently. records from attackers or malicious insiders is of This paper surveys recent research related to single critical importance. In addition, the potential for and multi-cloud security and addresses possible migration from a single cloud to a multi-cloud solutions. It is found that the research into the use of environment is examined and research related to multi-cloud providers to maintain security has received security issues in single and multi-clouds in cloud less attention from the research community than has computing are surveyed. the use of single clouds. This work aims to promote the The remainder of this paper is organized as follows. use of multi-clouds due to its ability to reduce security Section 2 describes the beginning of cloud computing risks that affect the cloud computing user. and its components. In addition, it presents examples of cloud providers and the benefits of using their services. Section 3 discusses security risks in cloud General Terms computing. Section 4 analyses the new generation of Security cloud computing, that is, multi-clouds and recent solutions to address the security of cloud computing, as Keywords well as examining their limitations. Section 5 presents Cloud computing, single cloud, multi-clouds, cloud suggestions for future work. Section 6 will conclude storage, data integrity, data intrusion, service the paper. availability. 2. Background 1. Introduction NIST [1] describes cloud computing as “a model for The use of cloud computing has increased rapidly in enabling convenient, on-demand network access to a many organizations. Subashini and Kavitha [49] argue shared pool of configurable computing resources (e.g., that small and medium companies use cloud computing networks, servers, storage, applications, and services) services for various reasons, including because these that can be rapidly provisioned and released with services provide fast access to their applications and minimal management effort or service provider reduce their infrastructure costs. interaction”.978-0-7695-4525-7/12 $26.00 © 2012 IEEE 5490DOI 10.1109/HICSS.2012.153
  2. 2. 2.1 Cloud Computing Components available for a particular group, while a community cloud is modified for a specific group of customers. The cloud computing model consists of five Hybrid cloud infrastructure is a composition of two orcharacteristics, three delivery models, and four more clouds (private, community, or public cloud)deployment models [1]. The five key characteristics of [51]. This model represents the third layer in the cloudcloud computing are: location-independent resource environment architecture.pooling, on-demand self-service, rapid elasticity, broad Kamara and Lauter [25] present two types of cloudnetwork access, and measured service [51]. These five infrastructure only, namely private and public clouds.characteristics represent the first layer in the cloud The infrastructure that is owned and managed by usersenvironment architecture (see Figure1). is in the private cloud. Data that is accessed and controlled by trusted users is in a safe and secure private cloud, whereas the infrastructure that is Layer Cloud Computing Components managed and controlled by the cloud service provider On-demand is in a public cloud. In particular, this data is out of the self-service user’s control, and is managed and shared with unsafe and untrusted servers [25]. Broad network Five access 2.2 Cloud Service Providers Examples Characteristics Resource Rapid In the commercial world, various computing needs pooling elasticity are provided as a service. The service providers take Measured care of the customers needs by, for example, Service maintaining software or purchasing expensive hardware. For instance, the service EC2, created by Three Delivery Amazon, provides customers with scalable servers. As IaaS PaaS SaaS models another example, under the CLuE program, NSF joined with Google and IBM to offer academic institutions Four Private access to a large-scale distributed infrastructure [4]. Public There are many features of cloud computing. First, Deployment Community cloud storages, such as Amazon S3, Microsoft Hybrid models SkyDrive, or NirvanixCLoudNAS, permit consumers to access online data. Second, it provides computation resources for users such as Amazon EC2. Third, Figure 1: Cloud Environment Architecture. Google Apps or versioning repositories for source code are examples of online collaboration tools [12]. The three key cloud delivery models are Cloud service providers should ensure the securityinfrastructure as a service (IaaS), platform as a service of their customers’ data and should be responsible if(PaaS), and software as a service (SaaS). In IaaS, the any security risk affects their customers’ serviceuser can benefit from networking infrastructure infrastructure. A cloud provider offers many servicesfacilities, data storage and computing services. In other that can benefit its customers, such as fast access towords, it is the delivery of computer infrastructure as a their data from any location, scalability, pay-for-use,service. An example of IaaS is the Amazon web data storage, data recovery, protection against hackers,service [25]. In PaaS, the user runs custom applications on-demand security controls, and use of the networkusing the service provider’s resources. It is the delivery and infrastructure facilities [49].of a computing platform and solution as a service. An Reliability and availability are other benefits of theexample of PaaS is GoogleApps. Running software on public cloud, in addition to low cost [25]. However,the provider’s infrastructure and providing licensed there are also concerning issues for public cloudapplications to users to use services is known as SaaS. computing, most notably, issues surrounding dataAn example of SaaS is the CRM integrity and data confidentiality. Any customer will beapplication [25],[49],[51]. This model represents the worried about the security of sensitive informationsecond layer in the cloud environment architecture. such as medical records or financial information[25]. Cloud deployment models include public, private,community, and hybrid clouds. A cloud environmentthat is accessible for multi-tenants and is available tothe public is called a public cloud. A private cloud is 5491
  3. 3. 3. Security Risks in Cloud Computing addition, the path for the transmitted data can be also affected, especially when the data is transmitted to Although cloud service providers can offer benefits many third-party infrastructure devices[41].to users, security risks play a major role in the cloud As the cloud services have been built over thecomputing environment [53]. Users of online data Internet, any issue that is related to internet securitysharing or network facilities are aware of the potential will also affect cloud services. Resources in the cloudloss of privacy [12]. According to a recent IDC survey are accessed through the Internet; consequently even if[16], the top challenge for 74% of CIOs in relation to the cloud provider focuses on security in the cloudcloud computing is security. Protecting private and infrastructure, the data is still transmitted to the usersimportant information such as credit card details or through networks which may be insecure. As a result,patients’ medical records from attackers or malicious internet security problems will affect the cloud, withinsiders is of critical importance [34]. Moving greater risks due to valuable resources stored withindatabases to a large data centre involves many security the cloud and cloud vulnerability. The technology usedchallenges [55] such as virtualization vulnerability, in the cloud is similar to the technology used in theaccessibility vulnerability, privacy and control issues Internet. Encryption techniques and secure protocolsrelated to data accessed from a third party, integrity, are not sufficient to protect data transmission in theconfidentiality, and data loss or theft. Subashini and cloud. Data intrusion of the cloud through the InternetKavitha [49] present some fundamental security by hackers and cybercriminals needs to be addressedchallenges, which are data storage security, application and the cloud environment needs to be secure andsecurity, data transmission security, and security private for clients [49].related to third-party resources. We will address three security factors that In different cloud service models, the security particularly affect single clouds, namely data integrity,responsibility between users and providers is different. data intrusion, and service availability.According to Amazon [46], their EC2 addressessecurity control in relation to physical, environmental, 3.1 Data Integrityand virtualization security, whereas, the users remainresponsible for addressing security control of the IT One of the most important issues related to cloudsystem including the operating systems, applications security risks is data integrity. The data stored in theand data. cloud may suffer from damage during transition According to Tabakiet al. [51], the way the operations from or to the cloud storage provider.responsibility for privacy and security in a cloud Cachinet al.[12] give examples of the risk of attackscomputing environment is shared between consumers from both inside and outside the cloud provider, suchand cloud service providers differs between delivery as the recently attacked Red Hat Linux’s distributionmodels. In SaaS, cloud providers are more responsible servers [40]. Another example of breached datafor the security and privacy of application services than occurred in 2009 in Google Docs, which triggered thethe users. This responsibility is more relevant to the Electronic Privacy Information Centre for the Federalpublic than the private cloud environment because the Trade Commission to open an investigation intoclients need more strict security requirements in the Google’s Cloud Computing Services [12]. Anotherpublic cloud. In PaaS, users are responsible for taking example of a risk to data integrity recently occurred incare of the applications that they build and run on the Amazon S3 where users suffered from data corruptionplatform, while cloud providers are responsible for [50]. Further examples giving details of attacks can beprotecting one user’s applications from others. In IaaS, read in [12],[40],[50].users are responsible for protecting operating systems Cachinet al.[12]argue that when multiple clientsand applications, whereas cloud providers must use cloud storage or when multiple devices areprovide protection for the users’ data [51]. synchronized by one user, it is difficult to address the Ristenpartet al. [41] claim that the levels of security data corruption issue. One of the solutions that theyissues in IaaS are different. The impact of security [12] propose is to use a Byzantine fault-tolerantissues in the public cloud is greater than the impact in replication protocol within the cloud. Hendricks et al.the private cloud. For instance, any damage which [23] state that this solution can avoid data corruptionoccurs to the security of the physical infrastructure or caused by some components in the cloud. However,any failure in relation to the management of the Cachinet al. [12] claim that using the Byzantine fault-security of the infrastructure will cause many tolerant replication protocol within the cloud isproblems. In the cloud environment, the physical unsuitable due to the fact that the servers belonging toinfrastructure that is responsible for data processing cloud providers use the same system installations andand data storage can be affected by a security risk. In are physically located in the same place. 5492
  4. 4. Although this protocol solves the problem from a modified by Amazon S3. These technologies protectcloud storage perspective, Cachinet al. [12] argue that users from Amazon data modification and fromthey remain concerned about the users’ view, due to hackers who may have obtained access to their emailthe fact that users trust the cloud as a single reliable or stolen their password [19].domain or as a private cloud without being aware ofthe protection protocols used in the cloud provider’sservers. As a solution, Cachinet al. [12] suggest that 4. Multi-Clouds Computing Securityusing Byzantine fault-tolerant protocols across multipleclouds from different providers is a beneficial solution. This section will discuss the migration of cloud computing from single to multi-clouds to ensure the3.2 Data Intrusion security of the user’s data. According to Garfinkel[19], another security risk 4.1 Multi-Clouds: Preliminarythat may occur with a cloud provider, such as theAmazon cloud service, is a hacked password or data The term “multi-clouds” is similar to the termsintrusion. If someone gains access to an Amazon “interclouds” or “cloud-of-clouds” that wereaccount password, they will be able to access all of the introduced by Vukolic [54]. These terms suggest thataccount’s instances and resources. Thus the stolen cloud computing should not end with a single cloud.password allows the hacker to erase all the information Using their illustration, a cloudy sky incorporatesinside any virtual machine instance for the stolen user different colors and shapes of clouds which leads toaccount, modify it, or even disable its services. different implementations and administrative domains.Furthermore, there is a possibility for the user’s Recent research has focused on the multi-cloudemail(Amazon user name) to be hacked (see [18] for a environment [3],[8],[10],[11] which control severaldiscussion of the potential risks of email), and since clouds and avoids dependency on any one individualAmazon allows a lost password to be reset by email, cloud.the hacker may still be able to log in to the account Cachin et al. [11] identify two layers in the multi-after receiving the new reset password. cloud environment: the bottom layer is the inner-cloud, while the second layer is the inter-cloud. In the inter-3.3 Service Availability cloud, the Byzantine fault tolerance finds its place. We will first summarize the previous Byzantine protocols Another major concern in cloud services is service over the last three decades.availability. Amazon [6] mentions in its licensingagreement that it is possible that the service might be 4.2 Introduction of Byzantine Protocolsunavailable from time to time. The user’s web servicemay terminate for any reason at any time if any user’s In cloud computing, any faults in software orfiles break the cloud storage policy. In addition, if any hardware are known as Byzantine faults that usuallydamage occurs to any Amazon web service and the relate to inappropriate behavior and intrusion tolerance.service fails, in this case there will be no charge to the In addition, it also includes arbitrary and crash faultsAmazon Company for this failure. Companies seeking [54]. Much research has been dedicated to Byzantineto protect services from such failure need measures fault tolerance (BFT) since its first introduction [28],such as backups or use of multiple providers [19]. Both [38]. Although BFT research has received a great dealGoogle Mail and Hotmail experienced service down- of attention, it still suffers from the limitations oftime recently [12]. If a delay affects payments from practical adoption [27] and remains peripheral inusers for cloud storage, the users may not be able to distributed systems [54].access their data. Due to a system administrator error, The relationship between BFT and cloud45% of stored client data was lost in LinkUp computing has been investigated, and many argue that(MediaMax) as a cloud storage provider [12]. in the last few years, it has been considered one of the Garfinkel[19] argues that information privacy is not major roles of the distributed system agenda.guaranteed in Amazon S3. Data authentication which Furthermore, many describe BFT as being of onlyassures that the returned data is the same as the stored “purely academic interest” for a cloud service [9]. Thisdata is extremely important. Garfinkel claims that lack of interest in BFT is quite different to the level ofinstead of following Amazon’s advice that interest shown in the mechanisms for tolerating crashorganizations encrypt data before storing them in faults that are used in large-scale systems. Reasons thatAmazon S3, organizations should use HMAC [26] reduce the adoption of BFT are, for example,technology or a digital signature to ensure data is not 5493
  5. 5. difficulties in design, implementation, or understanding cloud. The DepSky data model consists of threeof BFT protocols [54]. abstraction levels: the conceptual data unit, a generic As mentioned earlier, BFT protocols are not data unit, and the data unit implementation.suitable for single clouds. Vukolic [54] argues that oneof the limitations of BFT for the inner-cloud is that DepSKy System model. The DepSky system modelBFT requires a high level of failure independence, as contains three parts: readers, writers, and four clouddo all fault-tolerant protocols [45]. If Byzantine failure storage providers, where readers and writers are theoccurs to a particular node in the cloud, it is reasonable client’s tasks. Bessani et al. [8] explain the differenceto have a different operating system, different between readers and writers for cloud storage. Readersimplementation, and different hardware to ensure such can fail arbitrarily (for example, they can fail byfailure does not spread to other nodes in the same crashing, they can fail from time to time and thencloud. In addition, if an attack happens to a particular display any behavior) whereas, writers only fail bycloud, this may allow the attacker to hijack the crashing.particular inner-cloud infrastructure [54]. Cloud storage providers in the DepSky system4.3 DepSky System: Multi-Clouds Model model. The Byzantine protocols involve a set of storage clouds (n) where n = 3 f +1, and f is maximum This section will explain the recent work that has number of clouds which could be faulty. In addition,been done in the area of multi-clouds. Bessani et al. [8] any subset of (n – f) storage cloud creates byzantinepresent a virtual storage cloud system called DepSky quorum protocols [8].which consists of a combination of different clouds tobuild a cloud-of-clouds. The DepSky system addresses 4.4 Analysis of Multi-Cloud Researchthe availability and the confidentiality of data in theirstorage system by using multi-cloud providers, Moving from single clouds or inner-clouds to multi-combining Byzantine quorum system protocols, clouds is reasonable and important for many reasons.cryptographic secret sharing and erasure codes [8]. According to Cachinet al. [12] “Services of single clouds are still subject to outage”. In addition, Bowers4.3.1 DepSky Architecture et al. [10] showed that over 80% of company management “fear security threats and loss of control The DepSky architecture [8] consists of four clouds of data and systems”. Vukolic [54] assumes that theand each cloud uses its own particular interface. The main purpose of moving to interclouds is to improveDepSky algorithm exists in the clients’ machines as a what was offered in single clouds by distributingsoftware library to communicate with each cloud reliability, trust, and security among multiple cloud(Figure 2). These four clouds are storage clouds, so providers. In addition, reliable distributed storage [15]there are no codes to be executed. The DepSky library which utilizes a subset of BFT techniques waspermits reading and writing operations with the storage suggested by Vukolic [54] to be used in multi-clouds.clouds. A number of recent studies in this area have built protocols for interclouds. RACS (Redundant Array of Cloud Storage) [3] for instance, utilizes RAID-like techniques that are normally used by disks and file systems, but for multiple cloud storage. Abu-Libdeh et al. [3] assume that to avoid “vender lock-in”, distributing a user’s data among multiple clouds is a helpful solution. This replication also decreases the cost of switching providers and offers better fault tolerance. Therefore, the storage load will be spread among several providers as a result of the RACS proxy [3]. HAIL (High Availability and Integrity Layer) [10] Figure 2:DepSky Architecture [8]. is another example of a protocol that controls multiple clouds. HAIL is a distributed cryptographic system thatDepSky Data model. As the DepSky system deals permits a set of servers to ensure that the client’s storedwith different cloud providers, the DepSky library data is retrievable and integral. HAIL provides adeals with different cloud interface providers and software layer to address availability and integrity ofconsequently, the data format is accepted by each the stored data in an intercloud [10]. 5494
  6. 6. Cachin et al. [11] present a design for intercloud that this is an active area in research on cryptographicstorage (ICStore), which is a step closer than RACS methods for stored data authentication. Cachinet al.and HAIL as a dependable service in multiple clouds. [12] argue that although the previous methods allowCachin et al. [11] develop theories and protocols to consumers to ensure the integrity of their data whichaddress the CIRC attributes (confidentiality, integrity, has been returned by servers, they do not guaranteereliability and consistency) of the data stored in clouds. that the server will answer a query without knowing As mentioned before, Bessani et al. [8] present a what that query is and whether the data is storedvirtual storage cloud system called DepSky consisting correctly in the server or not. Proofs of Retrievabilityof a combination of different clouds to build a cloud- (PORs) and Proofs of Data Possession (PDP) areof-clouds. Bessani et al. [8] discuss some limitations of protocols introduced by Juels and Kaliski [24] andthe HAIL protocol and RACS system when compared Ateniese et al. [7] to ensure high probability for thewith DepSky. HAIL does not guarantee data retrieval of the user’s data. Cachinet al. [12] suggestconfidentiality, it needs code execution in their servers, using multiple cloud providers to ensure data integrityand it does not deal with multiple versions of data. in cloud storage and running Byzantine-fault-tolerantNone of these limitations are found in DepSky [8], protocols on them where each cloud maintains a singlewhereas the RACS system differs from the DepSky replica [14],[23]. Computing resources are required insystem in that it deals with “economic failures” and this approach and not only storage in the cloud, such avendor lock-in and does not address the issue of cloud service provided in Amazon EC2, whereas if onlystorage security problems. In addition, it also does not storage service is available, Cachin et al. [12] suggestprovide any mechanism to ensure data confidentiality working with Byzantine Quorum Systems [32] byor to provide updates of the stored data. Finally, the using Byzantine Disk Paxos[2] and using at least fourDepSky system presents an experimental evaluation different clouds in order to ensure users’ atomicitywith several clouds, which is different from other operations and to avoid the risk of one cloud failure.previous work on multi-clouds [8]. As mentioned earlier, the loss of availability of There are a number of studies on gaining constancy service is considered one of the main limitations infrom untrusted clouds. For instance, similar to DepSky, cloud computing and it has been addressed by storingDepot improves the flexibility of cloud storage, as the data on several clouds. The loss of customer dataMahajan et al. believe that cloud storages face many has caused many problems for many users such as therisks [30]. However, Depot provides a solution that is problem that occurred in October 2009 when thecheaper due to using single clouds, but it does not contacts, photos, etc. of many users of the Sidekicktolerate losses of data and its service availability service in Microsoft were lost for several days [44].depends on cloud availability [8]. Other work which Bessani et al. [8] use Byzantine fault-tolerantimplements services on top of untrusted clouds are replication to store data on several cloud servers, so ifstudies such as SPORC [17] and Venus [48]. These one of the cloud providers is damaged, they are stillstudies are different from the DepSky system because able to retrieve data correctly. Data encryption isthey consider a single cloud (not a cloud-of-clouds). In considered the solution by Bessani et al. [8] to addressaddition, they need code execution in their servers. the problem of the loss of privacy. They argue that toFurthermore, they offer limited support for the protect the stored data from a malicious insider, usersunavailability of cloud services in contrast to DepSky should encrypt data before it is stored in the cloud. As[8]. the data will be accessed by distributed applications, the DepSky system stores the cryptographic keys in the4.5 Current Solutions of Security Risks cloud by using the secret sharing algorithm to hide the value of the keys from a malicious insider. In order to reduce the risk in cloud storage, In the DepSky system, data is replicated in fourcustomers can use cryptographic methods to protect the commercial storage clouds (Amazon S3,Windowsstored data in the cloud [12]. Using a hash function Azure, Nirvanix and Rackspace); it is not relayed on a[35] is a good solution for data integrity by keeping a single cloud, therefore, this avoids the problem of theshort hash in local memory. In this way, authentication dominant cloud causing the so-called vendor lock-inof the server responses is done by recalculating the issue [3]. In addition, storing half the amount of data inhash of the received data which is compared with the each cloud in the DepSky system is achieved by thelocal stored data [12]. If the amount of data is large, use of erasure codes. Consequently, exchanging datathen a hash tree is the solution [35]. Many storage between one provider to another will result in a smallersystem prototypes have implemented hash tree cost. The DepSky system aims to reduce the cost offunctions, such as SiRiUS [20] and TDB [31]. using four clouds(which is four times the overhead) toMykletun et al. [36] and Papamanthou et al. [37] claim 5495
  7. 7. twice the cost of using a single cloud, which is a Rocha and Correia [42] classified four types ofsignificant advantage [8]. attacks that can affect the confidentiality of the user’s DepSky uses a set of Byzantine quorum system data in the cloud. These four types of attacks couldprotocols in order to implement the read and write occur when the malignant insider can determine textoperations in the system, so it needs only two passwords in the memory of a VM, cryptographic keyscommunication round trips for each operation to deal in the memory of VM files, and other confidential data.with several clouds. The use of several clouds needs a In addition, they argue that the recent researchvariety of locations, administration, design and mechanisms are not good enough to consider the issueimplementation, which are the requirements of the of data confidentiality and to protect data from theseByzantine quorum systems protocols [54]. Executing attacks. This does not mean that these mechanisms arecodes in servers is not required in the DepSky system not useful; rather that they do not focus on solving the(storage clouds) in contrast to other Byzantine problems that Rocha and Correia [42] address in theirprotocols that need some code execution [13],[21], research. Some of the solutions [39] are mechanisms[32],[33]. After using these protocols, the DepSky and are used as part of cloud computing solutions,system aims to deal with data confidentiality by while different types of solutions focus on solving thedecreasing the stored amount of data in each cloud [8]. whole data confidentiality issue intrinsic to cloud computing [8],[43]. Rocha and Correia [42] suggest4.6 Limitation of Current Solutions trusted computing and distributing trust among several cloud providers as a novel solution to solving security The problem of the malicious insider in the cloud problems and challenges in cloud computing. The ideainfrastructure which is the base of cloud computing is of replicating data among different clouds has beenconsidered by Rocha and Correia [42]. IaaS cloud applied in the single system DepSky [8]. Rocha andproviders provide the users with a set of virtual Correia [42] present the limitations of this work whichmachines from which the user can benefit by running occurs due to the fact that DepSky is only a storagesoftware on them. The traditional solution to ensure service like Amazon S3, and does not offer the IaaSdata confidentiality by data encryption is not sufficient cloud model. On the other hand, this system provides adue to the fact that the user’s data needs to be secure storage cloud, but does not provide security ofmanipulated in the virtual machines of cloud providers data in the IaaS cloud model. This is because it useswhich cannot happen if the data has been encrypted data encryption and stores the encrypted key in the[42]. Administrators manage the infrastructure and as clouds by using a secret sharing technique, which isthey have remote access to servers, if the administrator inappropriate for the IaaS cloud model [42].isa malicious insider, then he can gain access to the Table 1 details the security risks addressed in theuser’s data [29]. Van Dijk and Juels [52] present some previous research and the mechanisms that have beennegative aspects of data encryption in cloud proposed as a solution for these security risks in thecomputing. In addition, they assume that if the data is cloud computing environment. Security risk issues inprocessed from different clients, data encryption cloud computing have attracted much research interestcannot ensure privacy in the cloud. in recent years. Although cloud providers are aware of the It is clear from the table that in the past moremalicious insider danger, they assume that they have research has been conducted into single clouds thancritical solutions to alleviate the problem [22]. Rocha into multi-clouds. Multi–clouds can address theand Correia [42] determine possible attackers for IaaS security issues that relate to data integrity, datacloud providers. For example, Grosse et al. [22] intrusion, and service availability in multi-clouds. Inpropose one solution is to prevent any physical access addition, most of the research has focused on providingto the servers. However, Rocha and Correia [42] argue secure “cloud storage” such as in DepSky. Therefore,that the attackers outlined in their work have remote providing a cloud database system, instead of normalaccess and do not need any physical access to the cloud storage, is a significant goal in order to runservers. Grosse et al. [22] propose another solution is queries and deal with databases; in other words, toto monitor all access to the servers in a cloud where the profit from a database-as-a-service facility in a clouduser’s data is stored. However, Rocha and Correia [42] computing-environment.claim that this mechanism is beneficial for monitoring Table 1 illustrates that in 2009, 67% of the researchemployee’s behavior in terms of whether they are on security in cloud computing addressed the issue of afollowing the privacy policy of the company or not, but single cloud, whereas 33% of the research in the sameit is not effective because it detects the problem after it year addressed the issue of multi-clouds. In 2010, 80%has happened. of research focused on single clouds while only 20% or research was directed in the area of multi-clouds. 5496
  8. 8. Addressed Security Risks Type of cloud Type of service Mechanism availability intrusion integrity Security Security Privacy/ Service Cloud Year Data Data Ref Single Multi Cloud Cloud cloud clouds storage database Multi shares+ secret [5] 2011 √ √ sharing algorithm DepSky,(Byzantine + √ √ [8] 2011 √ √ √ √ secret sharing + cryptography) √ √ √ [42] 2011 √ survey RAID-like √ √ [3] 2010 √ techniques+ introduced RACS ICStore ,(client- √ √ [11] 2010 √ √ centric distributed protocols) [17] 2010 √ √ SPORC, (fork) √ [22] 2010 √ [25] 2010 √ cryptography √ √ [30] 2010 Depot, (FJC) √ √ [48] 2010 √ √ Venus √ √ √ √ √ [49] 2010 √ √ survey [51] 2010 √ √ √ [52] 2010 √ √ √ √ HAIL (Proofs + √ √ [10] 2009 √ √ √ cryptography) √ √ √ [12] 2009 √ survey [16] 2009 √ √ encrypted cloud VPN √ √ [41] 2009 √ √ √ [43] 2009 √ √ √ TCCP √ √ homomorphic token + √ √ [55] 2009 √ √ erasure-coded [7] 2007 √ PDP schemes [19] 2007 √ √ √ Table 1. Related Work on Cloud Computing Security.5. Future Work secret sharing algorithm with a polynomial function technique which claims that even with full knowledge For future work, we aim to provide a framework to of (k – 1) clouds, the service provider will not have anysupply a secure cloud database that will guarantee to knowledge of vs (vs is the secret value) [47]. We haveprevent security risks facing the cloud computing used this technique in previous databases-as-a-servescommunity. This framework will apply multi-clouds research [5]. In other words, hackers need to retrieveand the secret sharing algorithm to reduce the risk of all the information from the cloud providers to knowdata intrusion and the loss of service availability in the the real value of the data in the cloud. Therefore, if thecloud and ensure data integrity. attacker hacked one cloud provider’s password or even In relation to data intrusion and data integrity, two cloud provider’s passwords, they still need to hackassume we want to distribute the data into three the third cloud provider (in the case where k = 3) todifferent cloud providers, and we apply the secret know the secret which is the worst case scenario.sharing algorithm on the stored data in the cloud Hence, replicating data into multi-clouds by using aprovider. An intruder needs to retrieve at least three multi-share technique [5] may reduce the risk of datavalues to be able to find out the real value that we want intrusion and increase data integrity. In other words, itto hide from the intruder. This depends on Shamir’s will decrease the risk of the Hyper-Visor being hacked 5497
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