Exclusion-intersection encryption

Identity-based encryption (IBE) has shown to be a useful cryptographic scheme enabling secure yet flexible role-based access control. We propose a new variant of IBE named as exclusion-intersection encryption: during encryption, the sender can specify the targeted groups that are legitimate and interested in reading the documents; there exists a trusted key generation centre generating the intersection private decryption keys on request. This special private key can only be used to decrypt the ciphertext which is of all the specified groups' interests, its holders are excluded from decrypting when the documents are not targeted to all these groups (e.g., the ciphertext of only a single group's interest). While recent advances in cryptographic techniques (e.g., attribute-based encryption or wicked IBE) can support a more general access control policy, the private key size may be as long as the number of attributes or identifiers that can be specified in a ciphertext, which is undesirable, especially when each user may receive a number of such keys for different decryption power. One of the applications of our notion is to support an ad-hoc joint project of two or more groups which needs extra helpers that are not from any particular group. © 2011 IEEE.published_or_final_versionThe 1st IEEE International Workshop on Security in Computers, Networking and Communications (SCNC 2011) in conjuntion with IEEE INFOCOM 2011, Shanghai, China, 10-15 April 2011. In Conference Proceedings of INFOCOM WKSHPS, 2011, p. 1048-1053The 1st IEEE International Workshop on Security in Computers, Networking and Communications (SCNC 2011) in conjuntion with IEEE INFOCOM 2011, Shanghai, China, 10-15 April 2011. In Conference Proceedings of INFOCOM WKSHPS, 2011, p. 1048-105

Towards Black-Box Accountable Authority IBE with Short Ciphertexts and Private Keys

At Crypto'07, Goyal introduced the concept of Accountable Authority Identity-Based Encryption as a convenient tool to reduce the amount of trust in authorities in Identity-Based Encryption. In this model, if the Private Key Generator (PKG) maliciously re-distributes users' decryption keys, it runs the risk of being caught and prosecuted. Goyal proposed two constructions: the first one is efficient but can only trace well-formed decryption keys to their source; the second one allows tracing obfuscated decryption boxes in a model (called weak black-box model) where cheating authorities have no decryption oracle. The latter scheme is unfortunately far less efficient in terms of decryption cost and ciphertext size. In this work, we propose a new construction that combines the efficiency of Goyal's first proposal with a very simple weak black-box tracing mechanism. Our scheme is described in the selective-ID model but readily extends to meet all security properties in the adaptive-ID sense, which is not known to be true for prior black-box schemes.Comment: 32 page

Decentralizing Attribute-Based Encryption

We propose a Multi-Authority Attribute-Based Encryption (ABE) system. In our system, any party can become an authority and there is no requirement for any global coordination other than the creation of an initial set of common reference parameters. A party can simply act as an ABE authority by creating a public key and issuing private keys to different users that reflect their attributes. A user can encrypt data in terms of any boolean formula over attributes issued from any chosen set of authorities. Finally, our system does not require any central authority. In constructing our system, our largest technical hurdle is to make it collusion resistant. Prior Attribute-Based Encryption systems achieved collusion resistance when the ABE system authority ``tied\u27\u27 together different components (representing different attributes) of a user\u27s private key by randomizing the key. However, in our system each component will come from a potentially different authority, where we assume no coordination between such authorities. We create new techniques to tie key components together and prevent collusion attacks between users with different global identifiers. We prove our system secure using the recent dual system encryption methodology where the security proof works by first converting the challenge ciphertexts and private keys to a semi-functional form and then arguing security. We follow a recent variant of the dual system proof technique due to Lewko and Waters and build our system using bilinear groups of composite order. We prove security under similar static assumptions to the LW paper in the random oracle model

Generalized Key Delegation for Hierarchical Identity-Based Encryption

International audienceIn this paper, we introduce a new primitive called identitybased encryption with wildcard key derivation (WKD-IBE, or "wicked IBE") that enhances the concept of hierarchical identity-based encryption (HIBE) by allowing more general key delegation patterns. A secret key is derived for a vector of identity strings, where entries can be left blank using a wildcard. This key can then be used to derive keys for any pattern that replaces wildcards with concrete identity strings. For example, one may want to allow the university's head system administrator to derive secret keys (and hence the ability to decrypt) for all departmental sysadmin email addresses sysadmin@*.univ.edu, where * is a wildcard that can be replaced with any string. We provide appropriate security notions and provably secure instantiations with different tradeoffs in terms of ciphertext size and efficiency. We also present a generic construction of identity-based broadcast encryption (IBBE) from any WKD-IBE scheme. One of our instantiation yields an IBBE scheme with constant ciphertext size

Decentralized Authorization with Private Delegation

Authentication and authorization systems can be found in almost every software system, and consequently affects every aspect of our lives. Despite the variety in the software that relies on authorization, the authorization subsystem itself is almost universally architected following a common pattern with unfortunate characteristics.The first of these is that there usually exists a set of centralized servers that hosts the set of users and their permissions. This results in a number of security threats, such as permitting the operator of the authorization system to view or even change the permission data for all users. Secondly, these systems do not permit federation across administrative domains, as there is no safe choice of system operator: any operator would have visibility and control in all administrative domains, which is unacceptable. Thirdly, these systems do not offer transitive delegation: when a user grants permission to another user, the permissions of the recipient are not predicated upon the permissions of the granter. This makes it very difficult to reason about permissions as the complexity of the system grows, especially in the federation across domains case where no party can have absolute visibility into all permissions.Whilst several other systems, such as financial systems (e.g. blockchains) and communication systems (e.g. Signal / WhatsApp) have recently been reinvented to incorporate decentralization and privacy, there has been little attention paid to improving the authorization systems. This work aims to address that by asking the question ``How can we construct an authorization system that supports first-class transitive delegation across administrative domains without trusting a central authority or compromising on privacy?''We survey several models for authorization and find that Graph Based Authorization, where principals are vertices in a graph and delegation between principals are edges in the graph, is capable of capturing transitive delegation as a first class primitive, whilst also retaining compatibility with existing techniques such as Discretionary Access Control or Role Based Access Control. A proof of permission in the Graph Based Authorization model is represented by a path through the graph formed from the concatenation of individual edges. Whilst prior implementations of Graph Based Authorization do not meet the decentralization or privacy-preserving goals, we find that this is not intrinsic, and can be remedied by introducing two new techniques. The first is the construction of a global storage tier that cryptographically proves its integrity, and the second is an encryption technique that preserves the privacy of attestations in global storage.The horizontally-scalable storage tier is based on a new data structure, the Unequivocable Log Derived Map, which is composed of three Merkle trees. Consistency proofs over these trees allow a server to prove that objects exist or do not exist within storage, as well as proving that the storage is append-only (no previously inserted objects have been removed). Our scheme advances prior work in this field by permitting efficient auditing that scales with the number of additions to the storage rather than scaling with the total number of stored objects. By utilizing cryptographic proofs of integrity, we force storage servers to either behave honestly, or become detected as compromised. Thus, even though the architecture is centralized for availability and performance, it is does not introduce any central authorities.The design of the storage does not ensure the privacy of the permission data stored within it. We address this through the introduction of Reverse Discoverable Encryption. This technique uses the objects representing grants of permission as a key dissemination channel, thus operating without communication between participants. By using Wildcard Key Derivation Identity Based Encryption in a non-standard way (with no central Private Key Generator) we allow for permission objects to be encrypted using the authorization policy as a key. Thus, RDE permits the recipient of some permissions to decrypt other compatible permissions granted to the grantee that could be concatenated together to form a valid proof. RDE therefore protects the privacy of permission objects in storage whilst still permitting decryption of those objects by authorized parties.We construct an implementation of these techniques, named WAVE, and evaluate its performance. We find that WAVE has similar performance to the widely used OAuth system and performs better than the equally widely used LDAP system, despite offering significantly better security properties. We present an advancement to Graph Based Authorization which efficiently represents complex authorization proofs as a compact subgraph rather than a sequence of linear paths, and present a technique for efficient discovery of such proofs.To validate our techniques and ensure their efficacy in practice, we pose an additional question: ``How can we leverage WAVE to improve the security of IoT communications?'' We present a microservice architecture that abstracts the interfaces of IoT devices to permit a uniform security policy to be applied to heterogeneous devices of similar function. This is achieved by enforcing security policy at the communication bus and using hardware abstraction microservices to adapt the interfaces that devices expose on this communication bus. We construct and evaluate an instance of this communication bus, WAVEMQ and find that, with appropriate caching, its performance is comparable to that of prior publish/subscribe information busses. We discover that by enforcing WAVE's security model in the core of the network, we gain a resistance to denial of service attacks. This is particularly valuable in the IoT context where devices are typically resource constrained or connected by a bandwidth-limited link