DB-SECaaS: a cloud-based protection system for document-oriented NoSQL databases
EURASIP Journal on Information Security volume 2016, Article number: 16 (2016)
The trend of cloud databases is leaning towards Not Only SQL (NoSQL) databases as they provide better support for scalable storage and quick retrieval of exponentially voluminous data. One of the more prominent types of NoSQL databases is document-based storage, which is being increasingly used in the dynamic cloud paradigm. However, there are inherent security issues in cloud, including remote data residency along with the non-existent control of owners over their own data. In addition to that, the inherent security features of most document-based NoSQL databases lack granular access control and robust confidentiality mechanisms. There is also a distinct lack of a comprehensive solution that effectively caters to all the security requirements of a document-oriented database in cloud. In order to overcome these issues, we propose a database security-as-a-service (DB-SECaaS) system over document-oriented database hosted in cloud, which provides authentication, fine-grained authorization, and encryption of the database objects, while ensuring that access to the data is granted only to authorized users on a need-to-know basis. The paper shows that the DB-SECaaS system strongly enhances the security of document-oriented databases on cloud, and it is thus expected to facilitate the industry to reap the benefits of NoSQL without worrying over security issues. In order to certify the abovementioned security enhancements, provided by DB-SECaaS, the paper also provides a formal analysis of DB-SECaaS using the Scyther model checker. As a proof of concept, the core functionalities of the protocol, i.e., authorization, authentication, and encryption, are formally modeled in Scyther to formally verify that the proposed framework mitigates privacy and security concerns.
Not Only SQL (NoSQL) databases provide a comprehensive solution for a wide range of database issues, characterized by basically available, soft state, eventually consistent (BASE) in lieu of relational models of atomicity, consistency, isolation, and durability (ACID) . Compared to relational database management system (RDBMS), NoSQL is found to have an accelerated rate of data processing. It also provides a relatively inexpensive way for enterprises to efficiently manage large volumes of data . Among the many types of NoSQL databases, an important and widely used type is document-oriented storage, wherein an object model is stored as a document. Being schema-free, it allows great flexibility for updating data, without the need for any significant restructuring. The complexity of the documents varies based on the user’s preference. Also, the independence of documents from one another improves performance and decreases concurrency side effects . CouchDB , RavenDB , and MongoDB  are some of the most popular document-based NoSQL databases.
With the evolution of distributed computing into the popular cloud paradigm, the aforementioned benefits of document storage have come to the forefront, considering its support for distributed storage and scalability at will. Cloud exploits this advantage by providing a more cost-efficient, outsourced database management solution that allows enterprises to easily store, manage, and process large volumes of data. Recently, many solutions, such as Mongolab , RavenHQ , and Cloudant  have been developed that commercially provide document-oriented storage in cloud. However, while considerable efforts are being made to enhance the performance and provide flexible scalability options, the more pressing issues of security in document-oriented databases seems to be largely absent from the research landscape (see Section 5). At best, most document databases provide role-based access control, which is coarse-grained, where users having the permission to read from or write to a database can apply the operation on the entire database. Even though this model of access control ensures authorized access, it leaves the database vulnerable to attacks from malicious insiders, which, according to a report by McAfee , poses a considerable threat. In addition, encryption of data, both in transit and at rest, is provided, using SSL for the former and third-party services for the latter (see Table 4). Moreover, there are no holistic solutions specifically tailored for document-oriented databases that fulfill all the security requirements. The recent breaches in cloud databases are a testament to their vulnerabilities [11–13].
To overcome these limitations, we have developed a comprehensive database security-as-a-service (DB-SECaaS) system, on top of a document-oriented database in cloud, which would provide strong authentication, fine-grained authorization, and data encryption to ensure maximum security for the document-oriented database layer lying underneath. The system has a service-oriented architecture, which allows deploying individual components for performing specific functions as separate services. This choice of architecture introduces abstraction to the services; therefore, making it easy for different document-oriented databases to seamlessly integrate the DB-SECaaS without having to customize and embed it into their source code. The “as-a-service” concept provides an economic advantage for the businesses, since they would be able to outsource the security of their document-oriented database without having to buy the requisite hardware or hire experts to operate them. Therefore, our DB-SECaaS system would be equally effective for any document-oriented database because of its utilization regardless of the individual underlying architectures.
Our system provides strong authentication using the Federal Information Processing Standards 196 (FIPS 196)  challenge-response protocol, which is designed to mitigate replay attacks. We also add an additional layer of security using security assertion markup language (SAML) authentication assertions to validate the identity of the user and the various services participating in the system . We offer a fine-grained authorization at the granularity of attribute and document level of the database, based on eXtensible access control markup language (XACML) 3.0 , which is an access control policy creation language. The usage of SAML and XACML enhances the interoperability of the system, since they are both well-known standards. The fine-grained XACML policies restrict the access level of the users to ensure that a user can only perform certain operations on a certain data, if and only if she is allowed to do so. In addition, we also offer encryption to strengthen the protection detail for the stored data.
The paper also presents a formal analysis and verification on handover procedures of DB-SECaaS using the Scyther model checker  to extract and debug the main security flaws and threats that might exist in such procedures. Scyther allows the analysis of many potential attacks, such as man-in-the-middle attack, replay attack, message tampering, and information leakage (identity), which can be launched on protocols. We have analyzed these attacks using various Scyther attributes like Alive, Niagree, Nisynch, and secret.
The rest of the paper is organized as follows: Section 2 explains the detailed architecture of the proposed DB-SECaaS system. Section 3 describes the workflow and the intercommunication of all the services that make up the DB-SECaaS system. We evaluate the security of the DB-SECaaS system in Section 4. In Section 5, we provide an overview of the related work while highlighting our contributions and finally, Section 6 concludes the paper.
2 The DB-SECaaS system
As mentioned previously, the proposed system is a comprehensive DB-SECaaS system for hosting document-based NoSQL in cloud. It offers all the primarily required security services for the underlying database, specifically strong authentication, fine-grained authorization, data encryption, and data integrity. The service-oriented architecture ensures that the system provides effective security mechanisms to any document-based database. Our system constitutes multiple independent services that handle important security features. Each of them handles an important security feature and communicates with other services, working together to provide holistic security to the document-oriented database.
For the sake of simplicity, we do not attach the requisite aaS for as-a-service every time they are mentioned. In order to provide these services, we utilize renowned standards, namely SAML , XACML , and FIPS 196 . The three main services at play are authentication service, fine-grained authorization service, and encryption service, which further depend on services that complete their functionality. In the following subsections, we describe these services that make up the DB-SECaaS. Figure 1 illustrates the architecture for building the system in the context of cloud services.
2.1 Authentication service
The authentication service is responsible for verifying the identities of the database users as well as the inter-system requesting parties. It is composed of three services: strong authentication as a service, identity management as a service, and certificate authority as a service.
Strong authentication (SA) as a service: The SA service performs authentication using the FIPS 196 mutual authentication protocol. Its extended security functions include verification of certificates by the CA service and verification of identities by the IDM service. SA service is also used to issue and store SAML authentication tickets to other services and users.
Identity management (IDM) as a service: The IDM service manages the identity credentials of the database users, and once their identity is verified, it issues SAML authentication assertions to confirm their authenticity.
Certificate authority (CA) as a service: The CA1 service, with the support of XML key management specification (XKMS), provides digital signature services and issues certificates, which consist of a public key and the necessary credentials required to verify the database users and the services, for authentication purposes. The CA service checks the authenticity and also manages the keys and certificates thus, establishing a reliable and safe networking environment. The XKMS is used to integrate and allow the easy management of the CA service, thereby reducing the complexity of managing CA at the end users and other DB-SEC system services. The integration of XKMS with CA provides a simple XML-based protocol for processing key/certificate information by eliminating the need for user applications and other services to understand the CA syntax and semantics.
The aforementioned authentication services are used not only for verifying the users but also for other DB-SECaaS system services. The entities can mutually or unilaterally authenticate each other before exchanging the messages or share the data.
2.2 Fine-grained authorization service
The fine-grained authorization service is responsible for protecting the data from unauthorized disclosure. It is primarily composed of policy administration point as a service, policy decision point as a service, and policy enforcement point as a service.
Policy administration point (PAP) as a service: The PAP service allows users to create XACML policies that are compliant with the fine-grained access control (FGAC). System administrators can create fine-grained policies through the PAP service and modify them any time, as per their requirements. These policies are stored in a policy repository on cloud.
Policy enforcement point (PEP) as a service: The PEP service acts as an intermediary between the policy decision point (PDP) service and the client application by capturing the client’s request for accessing documents or collections in the document-oriented database and converting it into an SAML-wrapped XACML authorization decision query and sending it over to the PDP service. It is also responsible for interpreting the authorization decision that it receives from the PDP service and converting it into the native form that the client application can understand.
Policy decision point as a service: The PDP service is responsible for evaluating the authorization decision request sent by the PEP service. Once it receives the request from the PEP service, it fetches policies from the policy repository and finds the policies applicable to the request. It then determines whether to permit or deny the user access to the database. If the permission is granted, it puts forth a SAML attribute request to the key distribution (KD) service to retrieve the requested resource. If the permission is denied, the PDP service returns a response to the PEP service, which is then forwarded to the client.
2.3 Collection confidentiality service
This encryption service is responsible for performing the encryption and decryption of the data stored in the collection on the request of a privileged user. When the data owner initiates the request to store the data in collection, it first passes through the encryption service where the data is encrypted through the collection key. By default, every collection is encrypted with a unique key, i.e., all the data which is to be stored in the same collection is encrypted by using the same key. However, if security is an utmost concern, then fine-grained encryption can also be provided where each column of the table has a unique key. This kind of encryption increases the security at the cost of performance of the system. After encryption, the service stores the encrypted data in the document-oriented NoSQL database. There are two sub-modules, i.e., key distribution service and encryption service, at the core of the encryption service.
Key distribution as a service: The KD service operates with symmetric encryption and shares secret symmetric keys for the encryption and decryption of the data stored in document-based NoSQL. KD service separately manages the keys for each user and for each resource. A complete mapping between keys, users, and document-oriented database resources is provided to avoid conflicts and searching overhead. This service is also responsible for creating, managing, and distributing the keys across different services within the proposed system. PDP service sends an attribute query request (discussed in the next section) to the KD service for the retrieval of keys from the KD database. KD service validates the request through CA (FIPS 196 protocol) and checks for keys corresponding to the attributes mentioned in the request. The keys are passed to the encryption service for further processing of encryption/decryption operation.
Encryption as a service: This service handles the encryption and decryption of the data using the advanced encryption standard (AES) algorithm, after getting keys from the KD service. Its working process is the same as traditional encryption-decryption models; it is responsible for handling the designated operations based on user queries to document-oriented database.
2.4 Document-oriented NoSQL database
This can be any document-oriented NoSQL database in cloud that will make use of all the security services that our system provides, for example, MongoDB, CouchDB, and RavenDB.
3 Execution flow of the DB-SECaaS
In this section, we will provide a detailed description of the workflow of our system, which includes the three basic security features that our system provides, i.e., authentication, authorization, and encryption. The system works in four phases, namely policy creation phase, authentication phase, fine-grained authorization phase, and encryption phase.
3.1 Policy creation phase
In this phase, the system administrator creates fine-grained policies that are meant to effectively filter out unauthorized users from getting access to the document-oriented database. The steps comprising this phase are described below:
Administrators define fine-grained policies through the PAP service and can modify and create new policies at any time.
Policies are pushed into a policy repository, which the PDP service uses to evaluate authorization decisions.
3.2 Authentication phase
This phase deals with the authentication of the user and DB-SEC system services, which is handled by the SA service, the IDM, and the CA service, compliant with the FIPS 196 mutual authentication protocol. Figure 2 shows the workflow of the user authentication phase. The same procedure has also been followed for services authentication with each other.
The user of the DB-SECaaS system sends a login request to the SA service as the first message in the FIPS 196 strong authentication protocol.
The SA service checks the validity of the user with the IDM service and
Sends the response back to the user as per the FIPS 196 message.
The user submits the certificate to the SA service, which then verifies it using certificate chain validation and the OCSP protocol.
The certificate request and response mechanism follows the XKMS protocol to perform the basic public key infrastructure (PKI) operations at the CA service. In other words, user application (web browser) sends XKMS messages over simple object access protocol (SOAP) to perform the operations.
If both verifications are successful, SA service will execute the FIPS 196 compliant challenge/response protocol with the user.2
After the successful execution of challenge-response protocol and user verification, the SA service issues the SAML ticket, which is sent back to the user (client application) along with the SAML authentication response <saml: Response >, consequently asserting the user’s identification. The <saml: Response > contains, among other tags, a SAML assertion <saml: Assertion > and authentication statement <saml:AuthnStatement >, which in turn contains the evidence of authentication and the user and issuer’s attributes.
The user stores the SAML ticket locally in a file on a disk. This SAML ticket is also stored by the SA service for verification at the next steps. The final results of the authentication procedure are that the user has the SAML ticket on his/her disk and the SA service has the copy of the ticket issued to the user.
The authentication workflow mentioned above, is also used by other DB-SEC services of the system to authenticate one another. For instance, in order to send the authorization request and response, the PEP and PDP services should first mutually authenticate each other using the authentication mechanism defined above. Similarly, the KD and encryption services should first authenticate before sending the database keys. In other words, all the services of DB-SECaaS system must be authenticated and a SAML ticket is assigned to each service for a specific lifetime (more than the lifetime of the user SAML ticket), during which the services can communicate with each other for operations execution.
3.3 Authorization phase
Once the user has been authenticated, she can make a request to access the collection, document, or an attribute of a document, where the fine-grained authorization service steps are in. Figure 3 depicts the internal working of the fine-grained authorization service.
User requests data from the document-oriented database and, along with the request, sends the SAML authentication ticket, which is intercepted by the PEP service.
PEP server passes the SAML ticket to the SA service, which checks the ticket and confirms its validity.
The SA service returns its decision back to the PEP.
The PEP service passes a XACML-SAML authorization decision query <xacml-samlp:XACMLAuthzDecisionQuery > to the PDP service for approval. The request contains the user ID, action required, and indication of all document-oriented database resources needed to fulfill the request. The PDP makes the decision of whether the user should be allowed access or not.
In order to evaluate the user’s request, the PDP service fetches the XACML policies from the policy repository. The PDP service will use the XACML subject, resource, and action attributes within the authorization query to decide which policy is applicable and whether the subject can perform the specified action on the requested resource (see Fig. 4).
Information and policy in hand, the PDP service is able to render a decision.
If the request is denied, the PDP service sends the “Deny” decision to the PEP service. In case the access is granted, the PDP service generates and sends the SAML authorization ticket and response in the form of <samlp: Response > (see Fig. 5) to the PEP, which then sends it to the user application.
In addition, the PDP service also simultaneously sends a SAML attribute query <samlp:AttributeQuery > to the document database to retrieve the data (attributes values, documents, collection) for the requested data, but since the database is encrypted, the request will be intercepted by the KD service.
The SAML authorization ticket received from PEP is stored at the user’s local disk as well as at PDP service to allow the user to access the same resource again within the interval specified in the ticket.
These SAML tickets, generated at steps 6 and 8 of authentication and authorization phases, represent single sign-on (SSO) into the network or cloud. The ticket has a default lifetime of 8 hrs. The advantage of SSO is that if a user tries to access the same data from the document-oriented database within a specific interval, mentioned in the ticket, then she does not require to follow the same authentication and authorization steps again. Rather, the user application only needs to represent the SAML authentication and authorization ticket to the PEP, which is forwarded to the PDP and SA services for onward decision. This procedure of SSO and re-usability of SAML ticket saves the time and decreases the overhead of calling same operations repeatedly.
3.4 Encryption phase
The encryption phase is shown in Fig. 6. Once the PDP sends the attribute query to the KD service to retrieve the key, the following steps take place inside the encryption service:
The KD service receives the SAML Attribute Query <samlp:AttributeQuery > (see Fig. 7) from the PDP, which requests the data object(s) that the user requested.
The KD service verifies with the CA if the request is actually from the PDP service so that it can mitigate the risk of repudiation. The CA service would extract the signature from the attribute query to verify the sender of the request.
Once the attribute query has been verified, the KD service communicates with the database to retrieve the desired keys.
KD service sends the key to the encryption service for decryption process.
Encryption service retrieves the encrypted data from the document-oriented database and decrypts it using keys.
Data is finally sent to the user, enveloped in the SAML attribute response <samlp: Response > (see Fig. 8).
Figure 9 shows the overview of the workflow of the system, a holistic view of the interaction of the services with each other.
4 Security evaluation
The main objective of any security feature or parameter within a system is to defend against the attacks and to patch the vulnerabilities from being exploited by the attacker. Thus, every newly developed system must be verified to ensure that it is free of security loopholes or weaknesses. We have separately evaluated the effectiveness of the major services the DB-SECaaS has to offer, i.e., fine-grained authorization service and the encryption service via qualitative measures as per the National Institute of Standards and Technology (NIST) recommendations. Moreover, we have used a well-known formal verification tool, known as Scyther , to verify secure communication within our system.
4.1 Evaluation of the authorization service
The evaluation of the authorization service has been done with respect to the NIST-identified security metrics for access control models [19, 20]. NIST guidelines for access control evaluation metrics provide a complete ontology for access control policy. This ontology determines the relationships between access control primitives. For fine-grained authorization service, we have also designed an ontology that explains its core features and functions. The fine-grained authorization service is then evaluated using the metrics, defined by NIST, which are further divided into two categories: administration and enforcement. Table 1 explains the access control metrics supported by the system.
The NIST guidelines for access control evaluation metrics provide a complete ontology for access control policy. This ontology determines the relationships between access control primitives. For fine-grained authorization service, we have also designed an ontology that explains its core features and functions. The fine-grained authorization service is then evaluated using the metrics, defined by NIST, which are further divided into two categories: administration and enforcement. Table 1 explains the access control metrics supported by the system.
4.2 Evaluation of encryption service
Evaluation of encryption service is carried out through the given guidelines and recommendations of NIST on encryption and key management . For the effective use of cryptography in databases, it is essential to properly create and manage the keys. We have used the KD service for secure key generation, storage, and distribution via strong algorithms. These keys are used in symmetric key algorithms for protecting data against unauthorized attempts and modifications. Table 2 presents the evaluation of our encryption service according to the guidelines and recommendations given by NIST. The evaluation of encryption service helps to approve its correct functioning for document-oriented databases.
4.3 Formal analysis
In this subsection, we provide a detailed description of our formal analysis of DB-SECaaS using Scyther, which has been shown to be a quite effective model checking tool for verification, falsification, and analysis of security protocols. Scyther can verify protocols with unbounded number of sessions, with guaranteed termination. Scyther formally analyzes security protocols under the assumption of perfect cryptography; for example, it may validate that an attacker can learn nothing from an encrypted message unless it has a key.
Model checking is a state-space-based formal technique for modeling and analyzing hardware and software systems. It performs exhaustive verification of a system modeled as a finite state machine (FSM) in an automatic manner. The verification process consists of the following steps:
The system to be verified is described as a state-space model in a formal notation specified by the model checker.
The properties of the system that need to be verified are extracted from the working of the system and formally specified using temporal logic, like linear temporal logic (LTL) or computational tree logic (CTL).
The formal model of the system along with its properties are fed to the model checker to automatically verify if the system meets the required specifications.
One of the most powerful features of model checking is the provision of generating a counter-trace in case of a failing property. This counter-trace can be used to determine the cause of the error, which could be a modeling issue or a bug in the system under consideration. However, the state-space model of a complex system may become very large making the verification task quite complex computationally, i.e., a problem that is usually referred to use the state-space explosion problem. Generating a more abstract model of the system or using bounded model checking (BMC)  are commonly used to overcome the state-space explosion problem. BMC reduces the state-space exploration effort by restricting the search for a counterexample for a given property within k-bound levels.
In order to verify the security of DB-SECaaS, we propose to focus on three of its main security features, i.e., authentication, authorization, and encryption. The specification of the roles in all three phases is done individually for the verification of corresponding properties. These properties are formally specified in the form of Scyther claims.
We use five claims that Scyther supports namely, Secret, Alive, Niagree, Weakagree, and Nisynch. Secret makes sure that the confidentiality of our messages being sent/received is not compromised. For the DB-SECaaS system, it ascertains secure communication between the database application, SA service, and IDM service. Alive validates that the communication partner has sent a response to the requesting entity and is available. It ensures that the responding entity is available when a request is sent to it, e.g., it can be used to check if the CA is alive. Nisynch is a synchronization claim that ensures that the protocol is in synch and that it is secure against replay attacks. It can be used to validate that the PEP and PDP services are synchronized. Niagree is the non-injective agreement claim that verifies source authentication, for instance, it can be used to ascertain that the CA authenticates the source effectively.
4.3.1 Model description
The Scyther model for DB-SECaaS mainly describes the behavior of the protocol in terms of its roles, i.e., either an initiator or a responder. Our system consists of multiple communicating agents, including the client, and the various component service agents of DB-SECaaS, e.g., KDS, PEPaaS, and PDPaaS. Agents execute their runs to achieve their security goals. Every role specification consists of a sequence of events describing the messages, which the agent shall send and receive, as well as certain security claims. An intruder may try to oppose these security goals. The capabilities of the intruder determine its strength in attacking a protocol run. In order to resist attacks, an agent can make use of cryptographic primitives when constructing messages. More details about the message elements that are used in our formal model can be found in .
The actual behavior of the entire system, consisting of the intruder and a set of agents executing a number of runs, is encoded in the traces of the system. Every claim event in a trace results in a declaration about the trace that may or may not be true. A secrecy claim event is essentially the statement that something is never known to the adversary. Authorization is the process of determining which permissions a person or system are supposed to have. Authentication is captured by the notion of synchronization, which in turn requires that the corresponding send and receive messages are executed in the expected order. Effective security must employ both strong authentication and strict authorization.
The following claims are used, where the x denotes the role for which the claim is tested and y is the message:
Claim (x, Secret, y): The agent performing the role x knows that the intruder will never have knowledge of y.
Claim (x, Nisynch): The agent performing the role x knows that the message it received is from an authenticated sender .
Protocol specification: Describes the behavior of each of the roles in the protocol. Most often, a role in a security protocol is specified as a sequential list of events.
Agent model: The agents execute the roles of the protocol. The agent model is based on a closed world assumption. This means that honest agents show only the behavior described in the protocol specification.
Communication model: Describes how the messages are exchanged between the agents. We assume asynchronous communication with a single network buffer because this is the most general approach.
Threat model: It is based on a parameter in the semantics of the model based on the Dolev and Yao’s network threat model , where the intruder has complete control over the communication network.
Cryptographic primitives: They are idealized mathematical constructs, such as encryption, using the black box approach. This means that an adversary cannot learn anything from an encrypted message except if he has the key.
Security requirements: They are expressed as safety properties (i.e., something bad will never happen).
4.3.2 Formal verification results
The strength of any security application is the effectiveness with which it prevents attacks. Our evaluation through Scyther verifies the strength and validity of the DB-SECaaS for document-based databases on cloud. We have designed the DB-SECaaS system to provide protection to the underlying document-oriented NoSQL database by providing certain security features. Whether these features are effective or not depends on how successfully the system circumvents attacks, and the claims in Scyther help validating the mitigation of attacks. Table 3 provides the specific attacks our solution mitigates and the mechanisms it employs to do so. The results show that the provided security feature are quite effective, and given the fact that the system employs a secure communication protocol, like SAML, all the interactions between the system entities are also protected from external interferences. Therefore, apart from providing security services, our findings show that the DB-SECaaS system itself mitigates the security concerns which are raised in the previous sections.
More details about the results and specifications can be found in . All phases of the system have been analyzed separately in order to ascertain the mitigation of all possible cryptographic attacks. Our analysis results conclude that there are no attacks that can be identified within the given bounds. There can be attacks outside the bounded state-space, but the state-space in the case of our protocol is very huge; thus, the model cannot be checked rigorously due to the infamous problem of state-space explosion in model checking. We overcame this problem by using a modular approach and thus analyzing sub-modules of the complete system at a time. Authentication, authorization, and encryption phases are verified individually with the properties, as shown in the analysis exhibits.
5 Related work
A lot of research has been carried out in the last few years to mitigate the security challenges in cloud databases, but contributions specifically dealing with NoSQL databases are few and far between. Moreover, to the best of our knowledge, none of the existing works propose a comprehensive security solution for NoSQL databases in cloud. There are a few works available regarding document-oriented NoSQL security, which serves to emphasize the immaturity of security techniques in this particular domain. Due to the lack of research in our area of focus, we have carried out an extensive survey with a three-dimensional approach: we begin with solutions that provide a particular security service for cloud databases; subsequently, we move on to solutions that increasingly incorporate more security features to protect cloud databases, like security-as-a-service (DB-SECaaS). Finally, we narrow it down by enlisting the security mechanisms that some of the more popular document-oriented databases have to offer.
5.1 Focused security solutions for cloud databases
Delettre et al.  have proposed a data concealment security component in order to ensure the confidentiality of the data stored in cloud. Tao et al.  also provide confidentiality through fully homomorphic encryption, which performs functions on encrypted data stored in the cloud via an encryption proxy. In a distinctive approach, presented in , Ferreti et al. get rid of the encryption proxy, making the operations on encrypted data much faster. Bracci et al.  propose to provide adequate encryption and key management support by using a real use case of Vitaever, i.e., a home health-care SaaS application deployed on Amazon AWS. Sanka et al.  propose symmetric key sharing between CSP and the user by using the Diffie-Hellman key exchange protocol. The authors make a lot of assumptions, which can be potentially harmful in the real-world scenario.
Wang et al.  and Tribhuwan et al.  make use of homomorphic tokens to achieve data correctness. The only essential security service this covers is the integrity of the data in cloud. NETDB2 Multi-Shares (NETDB2 MS)  ensures privacy in database-as-a-service and is based on multi-service providers and the secret sharing algorithm instead of encryption used by the existing NetDB2 service. This new model circumvents the high cost of data encryption and decryption.
All the aforementioned works are commendable in their own right; however, all of them offer focused solutions for data security in cloud environment, mainly providing one security service at a time, for instance, confidentiality. We, however, are looking for more holistic solutions that incorporate all the basic security requirements.
5.2 Security-as-a-service solutions for cloud databases
A few fairly recent attempts have been made to present a more inclusive solution for data security in cloud. A three-dimensional approach to securing data in cloud is presented in , wherein data is categorized on the basis of a priority rating, which incorporates confidentiality, integrity, and availability. Sood  extends this system by adding data protection via the 128-bit SSL encryption and strengthening the authentication mechanism. Mohamed et al.  propose a similar data security model for cloud environments, having three layers, where each layer handles specific security tasks: authentication, data encryption, data integrity and user privacy, and fast data recovery. Islam et al.  present an agent-based framework for providing confidentiality, integrity, and authenticity to data storage in cloud.
Akin to what we are proposing for cloud, Yamany et al.  presented an intelligent service-oriented architecture security framework, which includes authentication and security service (NSS), as well as the authorization service (AS), based on the WS-* security standards. Similarly, Song et al.  proposed a generic framework of data-protection-as-service (DPaaS), which integrates various protection modules including access control, key management, and logging, to provide a combined multi-tier protection mechanisms for the current cloud. This, like , is just a concept that aims to provide integrity, privacy, confidentiality, and fine-grained access control in cloud. In particular, these methods do not target NoSQL databases. Hussain et al.  introduce security-as-a-service (DB-SECaaS), a user-centric architecture that provides security services for cloud computing on its different levels (SaaS, PaaS, and IaaS).
The papers, mentioned in this subsection, aim to provide multiple security services through one solution, which is essentially our goal as well. However, most of them are merely concepts and do not discuss implementation details. Also, they are not sensitive to the security requirements of document-oriented databases in cloud, which is our domain of interest.
5.3 Document-oriented database security
Based on the research we have conducted, we have found that document-oriented databases are increasingly incorporating more and more security mechanisms to ensure data security. Table 4 provides a comparison of the basic security features provided by some of the more popular document-oriented NoSQL databases, i.e., CouchDB , MongoDB , and RavenDB , with DB-SECaaS.
From Table 4, we can conclude that even the most popular of the document-oriented databases cannot provide comprehensive security in the most efficient manner. Authorization mechanisms need to be more fine-grained, not only based on roles but also on resources. But on the whole, they are all lacking in one or more aspects of security and this needs to be addressed if document-based databases in cloud are to be widely adopted.
We have reviewed the above mentioned papers, most of which provide specific solutions for cloud security. Some focus on specific security services, like confidentiality or availability, while others aim to mitigate specific risks or threats that exist in the environment. None of them provide a holistic security solutions for NoSQL databases, and those that tackle this problem have certain weaknesses or have not been implemented and just propose a concept. Not to mention, none of them aims to provide security for NoSQL databases in the cloud. Another notable attribute of most solutions is the tight coupling of security with the underlying database, which restricts the solution from being applied to any other database.
In order to overcome the identified weaknesses, we have implemented a security-as-a-service system over document-oriented NoSQL in cloud, which provides authentication, fine-grained authorization, as well as encryption, using well-established industry standards. In contrast to most solutions, ours is generic; and its layered architecture support any document-oriented database.
Document-oriented NoSQL in cloud adds the advantages of cloud computing—like scalability, inexpensiveness, and pay-per-use—to the benefits of document-based databases storage capacity for large volumes of data. On the other hand, however, the security weaknesses of cloud and document-based NoSQL also add up, outweighing the advantages. The proposed solution tends to enhance the security of document-based NoSQL databases in cloud and to, consequently, increase its adoption in enterprises. With these motivations, we have developed the DB-SECaaS system that protects the underlying document-oriented database in cloud by providing authentication, fine-grained authorization, and data encryption services. In order to implement the DB-SECaaS, we used the Javax cryptographic libraries, and well-known industry standards SAML 2.0 and XACML 3.0. We separately analyzed the major functional modules, i.e., the fine-grained authorization service and the encryption service, according to the NIST-defined standards. We used Scyther to further evaluate the security of our DB-SECaaS system. We also included a list of the potential security attacks that our system helps in mitigating. In addition, we have also defined classes of data security for document-based databases and have found that our system provides data security of class 3. After a thorough evaluation of our DB-SECaaS system, we conclude that the mechanisms for providing major security features are effective and afford optimal protection to document-oriented NoSQL databases. Using the Scyther security verification tool, the authentication, authorization, and encryption components of the proposed framework are tested against possible attacks with promising results. The Scyther tool introduces adversaries that can compromise agents during the run of the protocol learning their keys or the random numbers generated. It can verify a protocol against adversaries that range from the usual model in symbolic analysis, the Dolev-Yao, to much more powerful adversaries. Thus, the successful verification of DB-SECaaS using Scyther definitely raises the level of trust in its security aspects.
In the future, we plan focus on the performance of DB-SECaas so that it can provide protection while minimizing its latency. We also aim to incorporate new security features into the system, including auditing, which involves the tracking of a database user’s actions. This would elevate the security level of the database to class 4 (as per Table 1). Using the DB-SECaaS system, enterprises can host their critical data in document-based NoSQL databases on the cloud securely, where the owner of the data would be able to control access to their data.
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YG has penned the majority of this manuscript and has been involved in the process of the initial system design and its evolution to its current stage. RM has made key contributions in this research article, initially, by proposing the concept and then providing continuous support to the first author during all stages of the paper drafting. She was involved in the paper conception and provided major feedbacks till the final version of the paper. MAS has provided constant guidance and supervision through every iteration of this article. AR developed a formal model of the proposed protection system and analyzed it using the Scyther tool. OH supervised the work of Abid and ascertained the correctness of his formal models. Moreover, he also reviewed the text in the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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Ghazi, Y., Masood, R., Rauf, A. et al. DB-SECaaS: a cloud-based protection system for document-oriented NoSQL databases. EURASIP J. on Info. Security 2016, 16 (2016). https://doi.org/10.1186/s13635-016-0040-5