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## Über dieses Buch

This book constitutes the refereed proceedings of the 27th IFIP WG 11.3 International Conference on Data and Applications Security and Privacy, DBSec 2013, held in Newark, NJ, USA in July 2013. The 16 revised full and 6 short papers presented were carefully reviewed and selected from 45 submissions. The papers are organized in topical sections on privacy, access control, cloud computing, data outsourcing, and mobile computing.

## Inhaltsverzeichnis

### Extending Loose Associations to Multiple Fragments

Data fragmentation has been proposed as a solution for protecting the confidentiality of sensitive associations when publishing data at external servers. To enrich the utility of the published fragments, a recent approach has put forward the idea of complementing them with loose associations, a sanitized form of the sensitive associations broken by fragmentation. The original proposal considers fragmentations composed of two fragments only, and supports the definition of a loose association between this pair of fragments. In this paper, we extend loose associations to multiple fragments. We first illustrate how the publication of multiple loose associations between pairs of fragments of a generic fragmentation can potentially expose sensitive associations. We then describe an approach for supporting the more general case of publishing a loose association among an arbitrary set of fragments.
Sabrina De Capitani di Vimercati, Sara Foresti, Sushil Jajodia, Giovanni Livraga, Stefano Paraboschi, Pierangela Samarati

### Database Fragmentation with Encryption: Under Which Semantic Constraints and A Priori Knowledge Can Two Keep a Secret?

Database outsourcing to semi-honest servers raises concerns against the confidentiality of sensitive information. To hide such information, an existing approach splits data among two supposedly mutually isolated servers by means of fragmentation and encryption. This approach is modelled logic-orientedly and then proved to be confidentiality preserving, even if an attacker employs some restricted but nevertheless versatile class of a priori knowledge to draw inferences. Finally, a method to compute a secure fragmentation schema is developed.
Joachim Biskup, Marcel Preuß

### Differentially Private Multi-dimensional Time Series Release for Traffic Monitoring

Sharing real-time traffic data can be of great value to understanding many important phenomena, such as congestion patterns or popular places. To this end, private user data must be aggregated and shared continuously over time with data privacy guarantee. However, releasing time series data with standard differential privacy mechanism can lead to high perturbation error due to the correlation between time stamps. In addition, data sparsity in the spatial domain imposes another challenge to user privacy as well as utility. To address the challenges, we propose a real-time framework that guarantees differential privacy for individual users and releases accurate data for research purposes. We present two estimation algorithms designed to utilize domain knowledge in order to mitigate the effect of perturbation error. Evaluations with simulated traffic data show our solutions outperform existing methods in both utility and computation efficiency, enabling real-time data sharing with strong privacy guarantee.
Liyue Fan, Li Xiong, Vaidy Sunderam

### Policy Analysis for Administrative Role Based Access Control without Separate Administration

Access control is widely used in large systems for restricting resource access to authorized users. In particular, role based access control (RBAC) is a generalized approach to access control and is well recognized for its many advantages in managing authorization policies.
This paper considers user-role reachability analysis of administrative role based access control (ARBAC), which defines administrative roles and specifies how members of each administrative role can change the RBAC policy. Most existing works on user-role reachability analysis assume the separate administration restriction in ARBAC policies. While this restriction greatly simplifies the user-role reachability analysis, it also limits the expressiveness and applicability of ARBAC. In this paper, we consider analysis of ARBAC without the separate administration restriction and present new techniques to reduce the number of ARBAC rules and users considered during analysis. We also present a number of parallel algorithms that speed up the analysis on multi-core systems. The experimental results show that our techniques significantly reduce the analysis time, making it practical to analyze ARBAC without separate administration.
Ping Yang, Mikhail Gofman, Zijiang Yang

### Toward Mining of Temporal Roles

In Role-Based Access Control (RBAC), users acquire permissions through their assigned roles. Role mining, the process of finding a set of roles from direct user-permission assignments, is essential for successful implementation of RBAC. In many organizations it is often required that users are given permissions that can vary with time. To handle such requirements, temporal extensions of RBAC like Temporal-RBAC (TRBAC) and Generalized Temporal Role-Based Access Control (GTRBAC) have been proposed. Existing role mining techniques, however, cannot be used to process the temporal element associated with roles in these models. In this paper, we propose a method for mining roles in the context of TRBAC. First we formally define the Temporal Role Mining Problem (TRMP), and then show that the TRMP problem is NP-complete and present a heuristic approach for solving it.
Barsha Mitra, Shamik Sural, Vijayalakshmi Atluri, Jaideep Vaidya

### Towards User-Oriented RBAC Model

Role mining recently has attracted much attention from the role-based access control (RBAC) research community as it provides a machine-operated means of discovering roles from existing permission assignments. While there is a rich body of literature on role mining, we find that user experience/perception - one ultimate goal for any information system - is surprisingly ignored by the existing works. This work is the first to study role mining from the end-user perspective. Specifically, based on the observation that end-users prefer simple role assignments, we propose to incorporate to the role mining process a user-role assignment sparseness constraint that mandates the maximum number of roles each user can have. Under this rationale, we formulate user-oriented role mining as two specific problems: one is user-oriented exact role mining problem (RMP), which is obliged to completely reconstruct the given permission assignments, and the other is user-oriented approximate RMP, which tolerates a certain amount of deviation from the complete reconstruction. The extra sparseness constraint poses a great challenge to role mining, which in general is already a hard problem. We examine some typical existing role mining methods to see their applicability to our problems. In light of their insufficiency, we present a new algorithm, which is based on a novel dynamic candidate role generation strategy, tailored to our problems. Experiments on benchmark datasets demonstrate the effectiveness of our proposed algorithm.
Haibing Lu, Yuan Hong, Yanjiang Yang, Lian Duan, Nazia Badar

### Hypervisor Event Logs as a Source of Consistent Virtual Machine Evidence for Forensic Cloud Investigations

Cloud Computing is an emerging model of computing where users can leverage the computing infrastructure as a service stack or commodity. The security and privacy concerns of this infrastructure arising from the large co-location of tenants are, however, significant and pose considerable challenges in its widespread deployment. The current work addresses one aspect of the security problem by facilitating forensic investigations to determine if these virtual tenant spaces were maliciously violated by other tenants. It presents the design, application and limitations of a software prototype called the Virtual Machine (VM) Log Auditor that helps in detecting inconsistencies within the activity timelines for a VM history. A discussion on modeling a consistent approach is also provided.
Sean Thorpe, Indrajit Ray, Tyrone Grandison, Abbie Barbir, Robert France

### TerraCheck: Verification of Dedicated Cloud Storage

When hardware resources are shared between mutually distrustful tenants in the cloud, it may cause information leakage and bring difficulties to regulatory control. To address these concerns, cloud providers are starting to offer hardware resources dedicated to a single user. Cloud users have to pay more for such dedicated tenancy; however, they may not be able to detect the unexpected misuse of their dedicated storage due to the abstraction layer of the cloud. In this paper, we propose TerraCheck to help cloud users verify if their dedicated storage devices have been misused to store other users’ data. TerraCheck detects the malicious occupation of the dedicated device by monitoring the change of the shadow data that are residual bits intentionally left on the disk and are invisible by the file system. When the cloud providers share the dedicated disk with other users, such misuses can be detected since the shadow data will be overwritten and become irretrievable. We describe the theoretical framework of TerraCheck and show experimentally that TerraCheck works well in practice.
Zhan Wang, Kun Sun, Sushil Jajodia, Jiwu Jing

### Fair Private Set Intersection with a Semi-trusted Arbiter

A private set intersection (PSI) protocol allows two parties to compute the intersection of their input sets privately. Most of the previous PSI protocols only output the result to one party and the other party gets nothing from running the protocols. However, a mutual PSI protocol in which both parties can get the output is highly desirable in many applications. A major obstacle in designing a mutual PSI protocol is how to ensure fairness. In this paper we present the first fair mutual PSI protocol which is efficient and secure. Fairness of the protocol is obtained in an optimistic fashion, i.e. by using an offline third party arbiter. In contrast to many optimistic protocols which require a fully trusted arbiter, in our protocol the arbiter is only required to be semi-trusted, in the sense that we consider it to be a potential threat to both parties’ privacy but believe it will follow the protocol. The arbiter can resolve disputes without knowing any private information belongs to the two parties. This feature is appealing for a PSI protocol in which privacy may be of ultimate importance.
Changyu Dong, Liqun Chen, Jan Camenisch, Giovanni Russello

### Bloom Filter Bootstrap: Privacy-Preserving Estimation of the Size of an Intersection

This paper proposes a new privacy-preserving scheme for estimating the size of the intersection of two given secret subsets. Given the inner product of two Bloom filters (BFs) of the given sets, the proposed scheme applies Bayesian estimation under assumption of beta distribution for an a priori probability of the size to be estimated. The BF retains the communication complexity and the Bayesian estimation improves the estimation accuracy.
An possible application of the proposed protocol is an epidemiological datasets regarding two attributes, Helicobactor pylori infection and stomach cancer. Assuming information related to Helicobactor Pylori infection and stomach cancer are separately collected, the protocol demonstrates that a χ 2-test can be performed without disclosing the contents of the two confidential databases.
Hiroaki Kikuchi, Jun Sakuma

### Using Safety Constraint for Transactional Dataset Anonymization

In this paper, we address privacy breaches in transactional data where individuals have multiple tuples in a dataset. We provide a safe grouping principle to ensure that correlated values are grouped together in unique partitions that enforce l-diversity at the level of individuals. We conduct a set of experiments to evaluate privacy breach and the anonymization cost of safe grouping.
Bechara Al Bouna, Chris Clifton, Qutaibah Malluhi

### Practical Immutable Signature Bouquets (PISB) for Authentication and Integrity in Outsourced Databases

Database outsourcing is a prominent trend that enables organizations to offload their data management overhead (e.g., query handling) to the external service providers. Immutable signatures are ideal tools to provide authentication and integrity for such applications with an important property called immutability. Signature immutability ensures that, no attacker can derive a valid signature for unposed queries from previous queries and their corresponding signatures. This prevents an attacker from creating his own de-facto services via such derived signatures. Unfortunately, existing immutable signatures are very computation and communication costly (e.g., highly interactive), which make them impractical for task-intensive and heterogeneous applications.
In this paper, we developed two new schemes that we call Practical and Immutable Signature Bouquets (PISB ), which achieve efficient immutability for outsourced database systems. Both PISB  schemes are very simple, non-interactive, and computation/communication efficient. Our generic scheme can be constructed from any aggregate signature coupled with a standard signature. Hence, it can flexibly provide performance trade-offs for various types of applications. Our specific scheme is constructed from Condensed-RSA and Sequential Aggregate RSA. It has a very low verifier computational overhead and end-to-end delay with a small signature size. We showed that PISB  schemes are secure and also much more efficient than previous alternatives.
Attila A. Yavuz

### Optimal Re-encryption Strategy for Joins in Encrypted Databases

In order to perform a join in a deterministically, adjustably encrypted database one has to re-encrypt at least one column. The problem is to select that column that will result in the minimum number of re-encryptions even under an unknown schedule of joins. Naive strategies may perform too many or even infinitely many re-encryptions. We provide two strategies that allow for a much better performance. In particular the asymptotic behavior is O(n 3/2) resp. O(n logn) re-encryptions for n columns. We show that there can be no algorithm better than O(n logn). We further extend our result to element-wise re-encryptions and show experimentally that our algorithm results in the optimal cost in 41% of the cases.
Florian Kerschbaum, Martin Härterich, Patrick Grofig, Mathias Kohler, Andreas Schaad, Axel Schröpfer, Walter Tighzert

### Access Control and Query Verification for Untrusted Databases

With the advent of Cloud Computing, data are increasingly being stored and processed by untrusted third-party servers on the Internet. Since the data owner lacks direct control over the hardware and the software running at the server, there is a need to ensure that the data are not read or modified by unauthorized entities. Even though a simple encryption of the data before transferring it to the server ensures that only authorized entities who have the private key can access the data, it has many drawbacks. Encryption alone does not ensure that the retrieved query results are trustworthy (e.g., retrieved values are the latest values and not stale). A simple encryption can not enforce access control policies where each entity has access rights to only a certain part of the database. In this paper, we provide a solution to enforce access control policies while ensuring the trustworthiness of the data. Our solution ensures that a particular data item is read and modified by only those entities who have been authorized by the data owner to access that data item. It provides privacy against malicious entities that somehow get access to the data stored at the server. Our solutions allow easy change in access control policies under the lazy revocation model under which a user’s access to a subset of the data can be revoked so that the user can not read any new values in that subset of the data. Our solution also provides correctness and completeness verification of query results in the presence of access control policies. We implement our solution in a prototype system built on top of Oracle with no modifications to the database internals. We also provide an empirical evaluation of the proposed solutions and establish their feasibility.
Rohit Jain, Sunil Prabhakar

### Quantitative Security Risk Assessment of Android Permissions and Applications

The booming of the Android platform in recent years has attracted the attention of malware developers. However, the permissions-based model used in Android system to prevent the spread of malware, has shown to be ineffective. In this paper, we propose DroidRisk, a framework for quantitative security risk assessment of both Android permissions and applications (apps) based on permission request patterns from benign apps and malware, which aims to improve the efficiency of Android permission system. Two data sets with 27,274 benign apps from Google Play and 1,260 Android malware samples were used to evaluate the effectiveness of DroidRisk. The results demonstrate that DroidRisk can generate more reliable risk signal for warning the potential malicious activities compared with existing methods. We show that DroidRisk can also be used to alleviate the overprivilege problem and improve the user attention to the risks of Android permissions and apps.
Yang Wang, Jun Zheng, Chen Sun, Srinivas Mukkamala

### A Model for Trust-Based Access Control and Delegation in Mobile Clouds

Multi-tenancy, elasticity and dynamicity pose several novel challenges for access control in mobile smartphone clouds such as the Android$$\textsuperscript\texttrademark$$ cloud. Accessing subjects may dynamically change, resources requiring protection may be created or modified, and a subject’s access requirements to resources may change during the course of the application execution. Cloud tenants may need to acquire permissions from different administrative domains based on the services they require. Moreover, all the entities participating in a cloud may not be trusted to the same degree. Traditional access control models are not adequate for mobile clouds. In this work, we propose a new access control framework for mobile smartphone clouds. We formalize a trust-based access control model with delegation for providing fine-grained access control. Our model incorporates the notion of trust in the Role-Based Access Control (RBAC) model and also formalizes the concept of trustworthy delegation.
Indrajit Ray, Dieudonne Mulamba, Indrakshi Ray, Keesook J. Han

### Result Integrity Verification of Outsourced Frequent Itemset Mining

The data-mining-as-a-service (DMaS) paradigm enables the data owner (client) that lacks expertise or computational resources to outsource its mining tasks to a third-party service provider (server). Outsourcing, however, raises a serious security issue: how can the client of weak computational power verify that the server returned correct mining result? In this paper, we focus on the problem of frequent itemset mining, and propose efficient and practical probabilistic verification approaches to check whether the server has returned correct and complete frequent itemsets.
Boxiang Dong, Ruilin Liu, Hui (Wendy) Wang

### An Approach to Select Cost-Effective Risk Countermeasures

Security risk analysis should be conducted regularly to maintain an acceptable level of security. In principle, all risks that are unacceptable according to the predefined criteria should be mitigated. However, risk mitigation comes at a cost, and only the countermeasures that cost-efficiently mitigate risks should be implemented. This paper presents an approach to integrate the countermeasure cost-benefit assessment into the risk analysis and to provide decision makers with the necessary decision support. The approach comes with the necessary modeling support, a calculus for reasoning about the countermeasure cost and effect, as well as means for visualization of the results to aid decision makers.
Le Minh Sang Tran, Bjørnar Solhaug, Ketil Stølen

### Enhance Biometric Database Privacy: Defining Privacy-Preserving Drawer Size Standard for the Setbase

Shamir proposed the setbase approach as a means of improving security and privacy of the traditional biometric system. In this paper, we propose privacy-preserving drawer size standards for the biometric setbase. The proposal incorporates database privacy metrics such as k-anonymity and l-diversity into the definition of privacy-preserving drawer size standard for the biometric setbase. We also empirically evaluate the system reliability of the prototype setbase for the purpose of studying the trade-off values between the level of privacy protection and the level of system security.
Benjamin Justus, Frédéric Cuppens, Nora Cuppens-Boulahia, Julien Bringer, Hervé Chabanne, Olivier Cipiere

### Rule Enforcement with Third Parties in Secure Cooperative Data Access

In this paper, we consider the scenario where a set of parties need to cooperate with one another. To safely exchange the information, a set of authorization rules is given to the parties. In some cases, a trusted third party is required to perform the expected operations. Since interactions with the third party can be expensive and there maybe risk of data exposure/misuse, it is important to minimize their use. We formulate the minimization problem and show the problem is in NP-hard. We then propose a greedy algorithm to find close to optimal solutions.
Meixing Le, Krishna Kant, Sushil Jajodia

### Unlinkable Content Playbacks in a Multiparty DRM System

We present a solution to the problem of privacy invasion in a multiparty digital rights management scheme. (Roaming) users buy content licenses from a content provider and execute it at any nearby content distributor. Our approach, which does not need any trusted third party—in contrast to most related work on privacy-preserving DRM—is based on a re-encryption scheme that runs on any mobile Android device. Only a minor security-critical part needs to be performed on the device’s smartcard which could, for instance, be a SIM card.
Ronald Petrlic, Stephan Sekula

### Analysis of TRBAC with Dynamic Temporal Role Hierarchies

The temporal role based access control (TRBAC) models support the notion of temporal roles, user-to-role and permission-to-role assignment, as well as allow role enabling. In this paper, we argue that role hierarchies can be temporal in nature with a dynamism that allows it to have a different structure in different time intervals; and safety analysis of such extensions is crucial. Towards this end, we propose the temporal role based access control model extended with dynamic temporal role hierarchies, denoted as TRBACRH, and offer an approach to perform its safety analysis. We also present an administrative model to govern changes to the proposed role hierarchy.
Emre Uzun, Vijayalakshmi Atluri, Jaideep Vaidya, Shamik Sural

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