We consider the problem of efficient key management and user revocation in cryptographic file systems that allow shared access to files. A performance-efficient solution to user revocation in such systems is lazy revocation, a method that delays the re-encryption of a file until the next write to that file. We formalize the notion of key-updating schemes for lazy revocation, an abstraction to manage cryptographic keys in file systems with lazy revocation, and give a security definition for such schemes. We give two composition methods that combine two secure key-updating schemes into a new secure scheme that permits a larger number of user revocations. We prove the security of two slightly modified existing constructions and propose a novel binary tree construction that is also provable secure in our model.

A forward-secure encryption scheme protects secret keys from exposure by evolving the keys with time. Forward security has several unique requirements in hierarchical identity-based encryption (HIBE) scheme: (1) users join dynamically; (2) encryption is joining-time-oblivious; (3) users evolve secret keys autonomously. We present a scalable forward-secure HIBE (fs-HIBE) scheme satisfying the above properties. We also show how our fs-HIBE scheme can be used to construct a forward-secure public-key broadcast encryption scheme, which protects the secrecy of prior transmissions in the broadcast encryption setting. We further generalize fs-HIBE into a collusion-resistant multiple hierarchical ID-based encryption scheme, which can be used for secure communications with entities having multiple roles in role-based access control. The security of our schemes is based on the bilinear Diffie-Hellman assumption in the random oracle model. 1

Cryptographic computations are often carried out on insecure devices for which the threat of key exposure represents a serious and realistic concern. In an effort to mitigate the damage caused by exposure of secret data stored on such devices, the paradigm of forward security was introduced. In this model, secret keys are updated at regular intervals throughout the lifetime of the system; furthermore, exposure of a secret key corresponding to a given interval does not enable an adversary to "break" the system (in the appropriate sense) for any prior time period. A number of constructions of forward-secure digital signature schemes and symmetric-key schemes are known. We present the first construction of a forward-secure public-key whose security is based on the bilinear Diffie-Hellman assumption in the random oracle model. Our scheme can be extended to achieve chosen-ciphertext security at minimal additional cost. The construction we give is quite efficient: all parameters of the scheme grow (at most) poly-logarithmically with the total number of time periods.

. Rather than use a shared key directly to cryptographically process (e.g. encrypt or authenticate) data one can use it as a master key to derive subkeys, and use the subkeys for the actual cryptographic processing. This popular paradigm is called re-keying, and the expectation is that it is good for security. In this paper we provide concrete security analyses of various re-keying mechanisms and their usage. We show that re-keying does indeed \increase" security, eectively extending the lifetime of the master key and bringing signicant, provable security gains in practical situations. We quantify the security provided by dierent rekeying processes as a function of the security of the primitives they use, thereby enabling a user to choose between dierent re-keying processes given the constraints of some application. 1 Introduction Re-keying (also called key-derivation) is a commonly employed paradigm in computer security systems, about whose security benets users appe...

In this paper, we present a mechanism for securing group communications in Wireless Sensor Networks (WSN). First, we derive an extension of Logical Key Hierarchy (LKH). Then we merge the extension with directed diffusion. The resulting protocol, LKHW, combines the advantages of both LKH and directed diffusion: robustness in routing, and security from the tried and tested concepts of secure multicast. In particular, LKHW enforces both backward and forward secrecy, while incurring an energy cost that scales roughly logarithmically with the group size. This is the first security protocol that leverages directed diffusion, and we show how directed diffusion can be extended to incorporate security in an efficient manner.

We present a formal model and a simple architecture for robust pseudorandom generation that ensures resilience in the face of an observer with partial knowledge/control of the generator’s entropy source. Our model and architecture have the following properties: • Resilience. The generator’s output looks random to an observer with no knowledge of the internal state. This holds even if that observer has complete control over data that is used to refresh the internal state. • Forward security. Past output of the generator looks random to an observer, even if the observer learns the internal state at a later time. • Backward security/Break-in recovery. Future output of the generator looks random, even to an observer with knowledge of the current state, provided that the generator is refreshed with data of sufficient entropy. Architectures such as above were suggested before. This work differs from previous attempts in that we present a formal model for robust pseudo-random generation, and provide a formal proof within this model for the security of our architecture. To our knowledge, this is the first attempt at a rigorous model for this problem. Our formal modeling advocates the separation of the entropy extraction phase from the output generation phase. We argue that the former is information-theoretic in nature, and could therefore rely on combinatorial and statistical tools rather than on cryptography. On the other hand, we show that the latter can be implemented using any standard (non-robust) cryptographic PRG. We also discuss the applicability of our architecture for applications such as /dev/(u)random in Linux and pseudorandom generation on smartcards.

Forward-secure signatures (FSSs) have recently received much attention from the cryptographic theory community as a potentially realistic way to mitigate many of the difficulties digital signatures face with key exposure. However, no previous works have explored the practical performance of these proposed constructions in realworld applications, nor have they compared FSS to traditional, nonforward -secure, signatures in a non-asymptotic way.

We study Pseudorandom Number Generators (PRNGs) as used in practice. We first give a general security framework for PRNGs, incorporating the attacks that users are typically concerned about. We then analyze the most popular ones, including the ANSI X9.17 PRNG and the FIPS 186 PRNG. Our results also suggest ways in which these PRNGs can be made more efficient and more secure.

Abstract. Wireless sensors are employed in a wide range of applications. One common feature of most sensor settings is the need to communicate sensed data to some collection point or sink. This communication can be direct (to a mobile collector) or indirect – via other sensors towards a remote sink. In either case, a sensor might not be able to communicate to a sink at will. Instead it collects data and waits (for a potentially long time) for a signal to upload accumulated data directly. In a hostile setting, a sensor may be compromised and its post-compromise data can be manipulated. One important issue is forward security – how to ensure that pre-compromise data cannot be manipulated? Since a typical sensor is limited in storage and communication facilities, another issue is how to minimize resource consumption due to accumulated data. It turns out that current techniques are insufficient to address both challenges. To this end, we explore the notion of Forward-Secure Sequential Aggregate (FssAgg) authentication Schemes. We consider FssAgg authentication schemes in the contexts of both conventional and public key cryptography and construct a FssAgg MAC scheme and a FssAgg signature scheme, each suitable under different assumptions. This work represents the initial investigation of Forward-Secure Aggregation and, although the proposed schemes are not optimal, it opens a new direction for follow-on research.

Abstract. We construct an intrusion-resilient symmetric-key authenticated key exchange (AKE) protocol in the bounded retrieval model. The model employs a long shared private key to cope with an active adversary who can repeatedly compromise the user’s machine and perform any efficient computation on the entire shared key. However, we assume that the attacker is communication bounded and unable to retrieve too much information during each successive break-in. In contrast, the users read only a small portion of the shared key, making the model quite realistic in situations where storage is much cheaper than bandwidth. The problem was first studied by Dziembowski [Dzi06a], who constructed a secure AKE protocol using random oracles. We present a general paradigm for constructing intrusion-resilient AKE protocols in this model, and show how to instantiate it without random oracles. The main ingredients of our construction are UC-secure password authenticated key exchange and tools from the bounded storage model. 1