Cryptographic Role-Based Access Control, Reconsidered

A significant shortcoming of traditional access control mechanisms is their heavy reliance on reference monitors. Being single points of failure, monitors need to run in protected mode and have permanent online presence in order to handle all access requests. Cryptographic access control offers an alternative solution that provides better scalability and deployability. It relies on security guarantees of the underlying cryptographic primitives and the appropriate key distribution/management in the system. In order to rigorously study security guarantees that a cryptographic access control system can achieve, providing formal security definitions for the system is of great importance, since the security guarantee of the underlying cryptographic primitives cannot be directly translated into those of the system. In this paper, we follow the line of the existing studies on the cryptographic enforcement of Role-Based Access Control (RBAC). Inspired by the study focusing on the relation between the existing security definitions for such systems, we identify two types of attacks not described in the existing works. Therefore, we propose two new security definitions with the goal of appropriately modeling cryptographic enforcement of Role-Based Access Control policies and studying the relation between our new definitions and the existing ones. In addition, we show that the cost of supporting dynamic policy updates is inherently expensive by presenting two lower bounds for such systems that guarantee correctness and secure access

Cryptographic Role-Based Access Control, Reconsidered

A significant shortcoming of traditional access control mechanisms is their heavy reliance on reference monitors. Being single points of failure, monitors need to run in protected mode and have permanent online presence in order to handle all access requests. Cryptographic access control offers an alternative solution that provides better scalability and deployability. It relies on security guarantees of the underlying cryptographic primitives and the appropriate key distribution/management in the system. In order to rigorously study security guarantees that a cryptographic access control system can achieve, providing formal security definitions for the system is of great importance, since the security guarantee of the underlying cryptographic primitives cannot be directly translated into those of the system. In this paper, we follow the line of the existing studies on the cryptographic enforcement of Role-Based Access Control (RBAC). Inspired by the study focusing on the relation between the existing security definitions for such systems, we identify two types of attacks not described in the existing works. Therefore, we propose two new security definitions with the goal of appropriately modeling cryptographic enforcement of Role-Based Access Control policies and studying the relation between our new definitions and the existing ones. In addition, we show that the cost of supporting dynamic policy updates is inherently expensive by presenting two lower bounds for such systems that guarantee correctness and secure access

Access Control in Publicly Verifiable Outsourced Computation

Publicly Verifiable Outsourced Computation (PVC) allows devices with restricted re-sources to delegate expensive computations to more powerful external servers, and to verify the correctness of results. Whilst highlybeneficial in many situations, this increases the visi-bility and availability of potentially sensitive data, so we may wish to limit the sets of entities that can view input data and results. Additionally, it is highly unlikely that all users have identical and uncontrolled access to all functionality within an organization. Thus there is a need for access control mechanisms in PVC environments. In this work, we define a new framework for Publicly Verifiable Outsourced Computation with Access Control (PVC-AC). We formally define algorithms to provide different PVC functionality for each entity within a large outsourced computation environment, and discuss the forms of access control policies that are applicable, and necessary, in such environments, as well as formally modelling the resulting security properties. Finally, we give an example instantiation that (in a black-box and generic fashion) combines existing PVC schemes with symmetric Key Assignment Schemes to cryptographically enforce the policies of interest.

Preventing unauthorized data flows

Trojan Horse attacks can lead to unauthorized data flows and can cause either a confidentiality violation or an integrity violation. Existing solutions to address this problem employ analysis techniques that keep track of all subject accesses to objects, and hence can be expensive. In this paper we show that for an unauthorized flow to exist in an access control matrix, a flow of length one must exist. Thus, to eliminate unauthorized flows, it is sufficient to remove all one-step flows, thereby avoiding the need for expensive transitive closure computations. This new insight allows us to develop an efficient methodology to identify and prevent all unauthorized flows leading to confidentiality and integrity violations. We develop separate solutions for two different environments that occur in real life, and experimentally validate the efficiency and restrictiveness of the proposed approaches using real data sets. © IFIP International Federation for Information Processing 2017

Towards Practical Access Control and Usage Control on the Cloud using Trusted Hardware

Cloud-based platforms have become the principle way to store, share, and synchronize files online. For individuals and organizations alike, cloud storage not only provides resource scalability and on-demand access at a low cost, but also eliminates the necessity of provisioning and maintaining complex hardware installations. Unfortunately, because cloud-based platforms are frequent victims of data breaches and unauthorized disclosures, data protection obliges both access control and usage control to manage user authorization and regulate future data use. Encryption can ensure data security against unauthorized parties, but complicates file sharing which now requires distributing keys to authorized users, and a mechanism that prevents revoked users from accessing or modifying sensitive content. Further, as user data is stored and processed on remote ma- chines, usage control in a distributed setting requires incorporating the local environmental context at policy evaluation, as well as tamper-proof and non-bypassable enforcement. Existing cryptographic solutions either require server-side coordination, offer limited flexibility in data sharing, or incur significant re-encryption overheads on user revocation. This combination of issues are ill-suited within large-scale distributed environments where there are a large number of users, dynamic changes in user membership and access privileges, and resources are shared across organizational domains. Thus, developing a robust security and privacy solution for the cloud requires: fine-grained access control to associate the largest set of users and resources with variable granularity, scalable administration costs when managing policies and access rights, and cross-domain policy enforcement. To address the above challenges, this dissertation proposes a practical security solution that relies solely on commodity trusted hardware to ensure confidentiality and integrity throughout the data lifecycle. The aim is to maintain complete user ownership against external hackers and malicious service providers, without losing the scalability or availability benefits of cloud storage. Furthermore, we develop a principled approach that is: (i) portable across storage platforms without requiring any server-side support or modifications, (ii) flexible in allowing users to selectively share their data using fine-grained access control, and (iii) performant by imposing modest overheads on standard user workloads. Essentially, our system must be client-side, provide end-to-end data protection and secure sharing, without significant degradation in performance or user experience. We introduce NeXUS, a privacy-preserving filesystem that enables cryptographic protection and secure file sharing on existing network-based storage services. NeXUS protects the confidentiality and integrity of file content, as well as file and directory names, while mitigating against rollback attacks of the filesystem hierarchy. We also introduce Joplin, a secure access control and usage control system that provides practical attribute-based sharing with decentralized policy administration, including efficient revocation, multi-domain policies, secure user delegation, and mandatory audit logging. Both systems leverage trusted hardware to prevent the leakage of sensitive material such as encryption keys and access control policies; they are completely client-side, easy to install and use, and can be readily deployed across remote storage platforms without requiring any server-side changes or trusted intermediary. We developed prototypes for NeXUS and Joplin, and evaluated their respective overheads in isolation and within a real-world environment. Results show that both prototypes introduce modest overheads on interactive workloads, and achieve portability across storage platforms, including Dropbox and AFS. Together, NeXUS and Joplin demonstrate that a client-side solution employing trusted hardware such as Intel SGX can effectively protect remotely stored data on existing file sharing services

A Fast Attribute Based Encryption

Our new Access Control Encryption is an implementation of CP-ABE, when used as part of a key delivery mechanism for an encrypted Data Base. It focuses on improving performance. In ACE the access policies are any predicates over the set of object attributes. Efficiency gains are most pronounced when the DNF representations of policies are compact. In ACE, within the life span of the keys, each user has to perform very few ABE decryptions, regardless of the number of policies accessible to her. Keys to most objects are then computed using only symmetric key decryptions. ACE is not the first to utilize symmetric key cryptography to reduce the number of CP-ABE operations, when access policies form a multi-level partially ordered set. However, in addition to this significant saving, ACE also takes advantage of overlaps among policies on clauses of the policies, thus further reducing computational complexity. Let R denote the number of user roles, N be the number of object access policies, k the ratio between the cost of CP-ABE encryption and symmetric key encryption complexities (for 10 attributes k is about a million), and N=cR. The gain factor of ACE vs. a competing hybrid system is kc/(k+c). Usually c>>1, but in some systems it may happen that c<1. ACE is composed of two sub systems encrypting the same messages: A CP-ABE and a symmetric key encryption system. We prove that ACE is secure under a new Uniform Security Game that we propose and justify, assuming that its building blocks, namely CP-ABE and block ciphers are secure. We require that CP-ABE be secure under the Selective Set Model, and that the block cipher be secure under Multi-User CPA, which we define. We present Policy Encryption (PE) that can replace CP-ABE as a component of ACE. In many cases, PE is more efficient than CP-ABE. However PE does not prevent collusions. Instead it limits collusions. PE is useful in those cases where owners can compartmentalize objects and subjects, so that within each compartment the owners can tolerate collusions. PE prevents inter compartmental collusions. PE has also the following appealing properties: It relies on older hence more reliable intractability assumption, the Computational Diffie-Hellman assumption, whereas CP-ABE relies on the newer Bilinear Diffie-Hellman assumption. PE uses off-the shelf standard crypto building blocks with one small modification, with proven security. For a small number of compartments PE is much faster than CP-ABE. PE and CP-ABE can coexist in the same system, where ABE is used in high security compartments. We apply ACE to a practical financial example, the Consolidate Audit Trail (CAT), which is expected to become the largest repository of financial data in the world