During large-scale protests, a repressive government will often disable the Internet to thwart communication between protesters. Smartphone mesh networks, which route messages over short-range, possibly ephemeral, radio connections between nearby phones, allow protesters to communicate without relying on centralized Internet infrastructure. Unfortunately, prior work on providing secure communication in Internet shutdown settings fails to adequately consider protester needs. Previous attempts fail to support efficient private group communication (a crucial requirement for protests), and evaluate their solutions in network environments which fail to accurately capture link churn, physical spectrum contention, and the mobility models found in realistic protest settings. In this paper, we introduce Amigo, a novel mesh messaging system which supports group communication through a decentralized approach to continuous key agreement, and forwards messages using a novel routing protocol. Amigo is uniquely designed to handle the challenges of ad-hoc routing scenarios, where dynamic network topologies and node mobility make achieving key agreement nontrivial. Our extensive simulations reveal the poor scalability of prior approaches, the benefits of Amigo's protest-specific optimizations, and the challenges that still must be solved to scale secure mesh networks to protests with thousands of participants.

The IEEE 802.11 protocols are used by millions of smartphone and tablet devices to access the Internet via Wi-Fi wireless networks or communicate with one another directly in a peer-to-peer mode. Insider attacks are those originating from a trusted node that had initially passed all the authentication steps to access the network and then got compromised. A trusted node that has turned rogue can easily perform Denial-of-Service (DoS) attacks on the Media Access Control (MAC) layer by illegally capturing the channel and preventing other legitimate nodes from communicating with one another. Insider attackers can alter the implementation of the IEEE 802.11 Distributed Coordination Function (DCF) protocol residing in the Network Interface Card (NIC) to illegally increase the probability of successful packet transmissions into the channel at the expenses of nodes that follow the protocol standards. The attacker fools the NIC to upgrade its firmware and forces in a version containing the malicious code. In this paper, we present a distributed solution to detect and isolate the attacker in order to minimize the impact of the DoS attacks on the network. Our detection algorithm enhances the DCF firmware to enable honest nodes to monitor each other's traffic and compare their observations against honest communication patterns derived from a two-dimensional Markov chain. A channel hopping scheme is then used on the physical layer (PHY) to evade the attacker. To facilitate communication among the honest member stations and minimize network downtime, we introduce two isolation algorithms, one based on identity-based encryption and another based on broadcast encryption. Our simulation results show that the latter enjoys quicker recovery time and faster network convergence.

Hierarchies arise in the context of access control whenever the user population can be modeled as a set of partially ordered classes (represented as a directed graph). A user with access privileges for a class obtains access to objects stored at that class and all descendant classes in the hierarchy. The problem of key management for such hierarchies then consists of assigning a key to each class in the hierarchy so that keys for descendant classes can be obtained via efficient key derivation. We propose a solution to this problem with the following properties: (1) the space complexity of the public information is the same as that of storing the hierarchy; (2) the private information at a class consists of a single key associated with that class; (3) updates (i.e., revocations and additions) are handled locally in the hierarchy; (4) the scheme is provably secure against collusion; and (5) each node can derive the key of any of its descendant with a number of symmetric-key operations bounded by the length of the path between the nodes. Whereas many previous schemes had some of these properties, ours is the first that satisfies all of them. The security of our scheme is based on pseudorandom functions, without reliance on the Random Oracle Model. Another substantial contribution of this work is that we are able to lower the key derivation time at the expense of modestly increasing the public storage associated with the hierarchy. Insertion of additional, so-called shortcut, edges, allows to lower the key derivation to a small constant number of steps for graphs that are total orders and trees by increasing the total number of edges by a small asymptotic factor such as O (log * n ) for an n -node hierarchy. For more general access hierarchies of dimension d , we use a technique that consists of adding dummy nodes and dimension reduction. The key derivation work for such graphs is then linear in d and the increase in the number of edges is by the factor O (log d − 1 n ) compared to the one-dimensional case. Finally, by making simple modifications to our scheme, we show how to handle extensions proposed by Crampton [2003] of the standard hierarchies to “limited depth” and reverse inheritance.

Devising effective Content Protection mechanisms and building satisfactory Digital Rights Management systems have been top priorities for the Publishing and Entertainment Industries in recent years. In this tutorial, we focus on protection tools and standards for entertainment content. We analyze the challenges of content protection systems, ranging from legal aspects and business models to cryptographic techniques. We discuss current technology licensing practices, describe state-of-the-art broadcast encryption mechanisms and forensic approaches, and present content protection standards such as 4C and AACS.

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.

Traitor Tracing Schemes constitute a very useful tool against piracy in the context of digital content broadcast. In such multi-recipient encryption schemes, each decryption key is fingerprinted and when a pirate decoder is discovered, the authorities can trace the identities of the users that contributed in its construction (called traitors). Public-key traitor tracing schemes allow for a multitude of non trusted content providers using the same set of keys, which makes the scheme "server-side scalable." To make such schemes also "client-side scalable," i.e. long lived and usable for a large population of subscribers that changes dynamically over time, it is crucial to implement efficient Add-user and Remove-user operations. Previous work on public-key traitor tracing did not address this dynamic scenario thoroughly, and there is no efficient scalable public key traitor tracing scheme that allows an increasing number of Add-user and Remove-user operations.To address these issues, we introduce the model of Scalable Public-Key Traitor Tracing, and present the first construction of such a scheme. Our model mandates for deterministic traitor tracing and an unlimited number of efficient Add-user operations and Remove-user operations. A scalable system achieves an unlimited number of revocations while retaining high level of efficiency by dividing the run-time of the system into periods. Each period has a saturation level for the number of revocations. When a period becomes saturated, an efficient new-period operation is issued by the system server that resets the saturation level. We present a formal adversarial model for our system taking into account its periodic structure, and we prove our construction secure, both against adversaries that attempt to cheat the revocation mechanism as well as against adversaries that attempt to cheat the traitor tracing mechanism.