As more sensitive data is shared and stored by third-party sites on the Internet, there will be a need to encrypt data stored at these sites. One drawback of encrypting data, is that it can be selectively shared only at a coarse-grained level (i.e., giving another party your private key). We develop a new cryptosystem for fine-grained sharing of encrypted data that we call Key-Policy Attribute-Based Encryption (KP-ABE). In our cryptosystem, ciphertexts are labeled with sets of attributes and private keys are associated with access structures that control which ciphertexts a user is able to decrypt. We demonstrate the applicability of our construction to sharing of audit-log information and broadcast encryption. Our construction supports delegation of private keys which subsumes Hierarchical Identity-Based Encryption (HIBE). E.3 [Data En-

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Exchange of attribute credentials is a means to establish mutual trust between strangers wishing to share resources or conduct business transactions. Automated Trust Negotiation (ATN) is an approach to regulate the exchange of sensitive information during this process. It treats credentials as potentially sensitive resources, access to which is under policy control. Negotiations that correctly enforce policies have been called “safe ” in the literature. Prior work on ATN lacks an adequate definition of this safety notion. In large part, this is because fundamental questions such as “what needs to be protected in ATN? ” and “what are the security requirements? ” are not adequately answered. As a result, many prior methods of ATN have serious security holes. We introduce a formal framework for ATN in which we give precise, usable, and intuitive definitions of correct enforcement of policies in ATN. We argue that our chief safety notion captures intuitive security goals. We give precise comparisons of this notion with two alternative safety notions that may seem intuitive, but that are seen to be inadequate under closer inspection. We prove that an approach to ATN from the literature meets the requirements set forth in the preferred safety definition, thus

Abstract—Cloud computing is an emerging computing paradigm in which resources of the computing infrastructure are provided as services over the Internet. As promising as it is, this paradigm also brings forth many new challenges for data security and access control when users outsource sensitive data for sharing on cloud servers, which are not within the same trusted domain as data owners. To keep sensitive user data confidential against untrusted servers, existing solutions usually apply cryptographic methods by disclosing data decryption keys only to authorized users. However, in doing so, these solutions inevitably introduce a heavy computation overhead on the data owner for key distribution and data management when fine-grained data access control is desired, and thus do not scale well. The problem of simultaneously achieving fine-grainedness, scalability, and data confidentiality of access control actually still remains unresolved. This paper addresses this challenging open issue by, on one hand, defining and enforcing access policies based on data attributes, and, on the other hand, allowing the data owner to delegate most of the computation tasks involved in fine-grained data access control to untrusted cloud servers without disclosing the underlying data contents. We achieve this goal by exploiting and uniquely combining techniques of attribute-based encryption (ABE), proxy re-encryption, and lazy re-encryption. Our proposed scheme also has salient properties of user access privilege confidentiality and user secret key accountability. Exten-sive analysis shows that our proposed scheme is highly efficient and provably secure under existing security models. I.

We introduce the provisional trust negotiation framework PROTUNE, for combining distributed trust management policies with provisional-style business rules and accesscontrol related actions. The framework features a powerful declarative metalanguage for driving some critical negotiation decisions, and integrity constraints for monitoring negotiations and credential disclosure. 1

Hidden credentials are useful in protecting sensitive resource requests, resources, policies, and credentials. We propose a significant performance improvement when implementing hidden credentials using Boneh/Franklin Identity Based Encryption. We also propose a substantially improved secret splitting scheme for enforcing complex policies, and show how it improves concealment of policies from nonsatisfying recipients. Categories and Subject Descriptors

In automated trust negotiation (ATN), two parties exchange digitally signed credentials that contain attribute information to establish trust and make access control decisions. Because the information in question is often sensitive, credentials are protected according to access control policies. In traditional ATN, credentials are transmitted either in their entirety or not at all. This approach can at times fail unnecessarily, either because a cyclic dependency makes neither negotiator willing to reveal her credential before her opponent, because the opponent must be authorized for all attributes packaged together in a credential to receive any of them, or because it is necessary to fully disclose the attributes, rather than merely proving they satisfy some predicate (such as being over 21 years of age). Recently, several cryptographic credential schemes and associated protocols have been developed to address these and other problems. However, they can be used only as fragments of an ATN process. This paper introduces a framework for ATN in which the diverse credential schemes and protocols can be combined, integrated, and used as needed. A policy language is introduced that enables negotiators to specify authorization requirements that must be met by an opponent to receive various amounts of information about certified attributes and the credentials that contain it. The language also supports the use of uncertified attributes, allowing them to be required as part of policy satisfaction, and to place their (automatic) disclosure under policy control.

Abstract—In an open environment such as the Internet, the decision to collaborate with a stranger (e.g., by granting access to a resource) is often based on the characteristics (rather than the identity) of the requester, via digital credentials: Access is granted if Alice’s credentials satisfy Bob’s access policy. The literature contains many scenarios in which it is desirable to carry out such trust negotiations in a privacy-preserving manner, i.e., so as minimize the disclosure of credentials and/or of access policies. Elegant solutions were proposed for achieving various degrees of privacy-preservation through minimal disclosure. In this paper, we present protocols that protect both sensitive credentials and sensitive policies. That is, Alice gets the resource only if she satisfies the policy, Bob does not learn anything about Alice’s credentials (not even whether Alice got access), and Alice learns neither Bob’s policy structure nor which credentials caused her to gain access. Our protocols are efficient in terms of communication and in rounds of interaction. Index Terms—Electronic commerce-security, management of computing and information systems, security and protection, authentication, access control, trust negotiation, hidden credentials, privacy. Ç 1

We examine the problem of providing useful feedback to users who are denied access to resources, while controlling the disclosure of the system security policies. High-quality feedback enhances the usability of a system, especially when permissions may depend on contextual information---time of day, temperature of a room and other factors that change unpredictably. However, providing too much information to the user may breach the confidentiality of the system policies.

There is a recent trend toward rule-based authorization systems to achieve flexible security policies. Also, new sensing technologies in pervasive computing make it possible to define context-sensitive rules, such as “allow database access only to staff who are currently located in the main office. ” However, these rules, or the facts that are needed to verify authority, often involve sensitive context information. This paper presents a secure context-sensitive authorization system that protects confidential information in facts or rules. Furthermore, our system allows multiple hosts in a distributed environment to perform the evaluation of an authorization query in a collaborative way; we do not need a universally trusted central host that maintains all the context information. The core of our approach is to decompose a proof for making an authorization decision into a set of sub-proofs produced on multiple different hosts, while preserving the integrity and confidentiality policies of the mutually untrusted principals operating these hosts. We prove the correctness of our algorithm. 1