Gabriele Oligeri

Intrusion-Resilience in Mobile Unattended WSNs

Wireless Sensor Networks (WSNs) are susceptible to a wide range of attacks due to their distributed nature, limited sensor resources and lack of tamper-resistance. Once a sensor is corrupted, the adversary learns all secrets and (even if the sensor is later released) it is very difficult for the sensor to regain security, i.e., to obtain intrusion-resilience. Existing solutions rely on the presence of an on-line trusted third party, such as a sink, or on the availability of secure hardware on sensors. Neither assumption is realistic in large-scale Unattended WSNs (UWSNs), characterized by long periods of disconnected operation and periodic visits by the sink. In such settings, a mobile adversary can gradually corrupt the entire network during the intervals between sink visits. As shown in some recent work, intrusionresilience in UWSNs can be attained (to a degree) via cooperative self-healing techniques. In this paper, we focus on intrusion-resilience in Mobile Unattended Wireless Sensor Networks (μUWSNs) where sensors move according to some mobility model. We argue that sensor mobility motivates a specific type of adversary and defending against it requires new security techniques. Concretely, we propose a cooperative protocol that - by leveraging sensor mobility - allows compromised sensors to recover secure state after compromise. This is obtained with very low overhead and in a fully distributed fashion. We provide a thorough analysis of the proposed protocol and support it by extensive simulation results.

Frame error model in rural Wi-Fi networks

Commonly used frame loss models for simulations over Wi-Fi channels assume a simple double regression model with threshold. This model is widely accepted, but few measurements are available in the literature that try to validate it. As far as we know, none of them is based on field trials at the frame level. We present a series of measurements for relating transmission distance and packet loss on a Wi-Fi network in rural areas and propose a model that relates distance with packet loss probability. We show that a simple double regression propagation model like the one used in the ns-2 simulator can miss important transmission impairments that are apparent even at short transmitter-receiver distances. Measurements also show that packet loss at the frame level is a Bernoullian process for time spans of few seconds. We relate the packet loss probability to the received signal level using standard models for additive white Gaussian noise channels. The resulting model is much more similar to the measured channels than the simple models where all packets are received when the distance is below a given threshold and all are lost when the threshold is exceeded.

Securing Mobile Unattended WSNs against a Mobile Adversary

One important factor complicating security in Wireless Sensor Networks (WSNs) is lack of inexpensive tamper-resistant hardware in commodity sensors. Once an adversary compromises a sensor, all memory and forms of storage become exposed, along with all secrets. Thereafter, any cryptographic remedy ceases to be effective. Regaining sensor security after compromise (i.e., intrusion-resilience) is a formidable challenge. Prior approaches rely on either (1) the presence of an on-line trusted third party (sink), or (2) the availability of a True Random Number Generator (TRNG) on each sensor. Neither assumption is realistic in large-scale Unattended Wireless Sensor Networks (UWSNs) composed of low-cost commodity sensors. periodic visits by the sink.) Previous work has demonstrated that sensor collaboration is an effective, yet expensive, means of attaining intrusion-resilience in UWSNs. In this paper, we explore intrusion resilience in Mobile UWSNs in the presence of a powerful mobile adversary. We show how the choice of the sensor mobility model influences intrusion resilience with respect to this adversary. We also explore self healing protocols that require only local communication. Results indicate that sensor density and neighborhood variability are the two key parameters affecting intrusion resilience. Our findings are supported by extensive analyses and simulations.

Measurement-based frame error model for simulating outdoor Wi-Fi networks

We present a measurement-based model of the frame error process on a Wi-Fi channel in rural environments. Measures are obtained in controlled conditions, and careful statistical analysis is performed on the data, providing information which the network simulation literature is lacking. Results indicate that most network simulators use a frame loss model that can miss important transmission impairments even at a short distance, particularly when considering antenna radiation pattern anisotropy and multi-rate switching.

Mobile Application Security for Video Streaming Authentication and Data Integrity Combining Digital Signature and Watermarking Techniques

Satellite link presents peculiar characteristics like no packet reordering and low bit error rate. In this paper we leverage these characteristics combined with watermarking techniques to propose a novel authentication algorithm for multicast video streaming. This algorithm combines a single digital signature with a hash chain pre-computed on the transmitter side; the hash chain is embedded in the video stream by means of a watermarking technique. Our proposal shows several interesting features: authentication is enforced, as well as integrity of the received multicast stream; received blocks can be authenticated on the fly; no storage is required on the receiver side, except for the amount of memory needed to store a single hash; overhead computations required on the receiver sum up to single hash per block, while a digital signature verification is amortized over the whole received stream. Finally, note that the bandwidth overhead introduced is negligible, since the applied watermarking technique introduces virtually no modifications (at least, not recognizable by humans) on the original video stream pictures.

A cyber-physical approach to secret key generation in smart environments

Encrypted communication in wireless sensor networks oftentimes requires additional randomness and frequent re-keying in order to avoid known-plain text attacks. Conventional approaches for shared secret generation suffer however from various disadvantages, such as necessity of a trusted third party, protocol scalability, and especially, the computational resources needed for performance-demanding public-key protocols. To appropriately respond to the increasing disproportions between a computationally powerful adversary and lightweight wireless devices, a cyber-physical approach has recently attracted much attention. The general idea is to leverage the properties of the physical world and include them in a design of lightweight security protocols. Especially valuable physical property is the erratic and unpredictable nature of multi-path signal propagation which has already shown itself as a rich source of randomness. This work presents a new cyber-physical approach in order to make secure wireless sensor communications and proposes a secret key extraction algorithm that leverages signal strength fluctuations resulting from dynamic physical environments, e.g. environments experiencing human movements. In particular, this work presents a systematic experimental evaluation by using a real-world sensor network, and analyzes the impact of different moving patterns on legitimate devices and an eavesdropper. Finally, this work quantifies the main factors that influence the key establishment algorithm and propose a protocol which allows secret sharing in an effective and efficient way.

COKE Crypto-Less Over-the-Air Key Establishment

In this paper, we present a novel probabilistic protocol (COKE) to allow two wireless communicating parties to commit over-the-air (OTA) on a shared secret, even in the presence of a globally eavesdropping adversary. The proposed solution leverages no crypto but just plaintext messages exchange. Indeed, the security of the solution relies on the difficulty for the adversary to correctly identify, for each one-bit transmission, the sender of that bit-not its value, which is indeed exchanged in cleartext. Due to the low requirements of COKE (essentially, the capability to send a few wireless messages), it is particularly suited for resource constrained wireless devices (e.g., WNSs, wireless embedded systems), as well as for those scenarios where just energy saving is at premium, such as smartphones.

Silence is Golden: Exploiting Jamming and Radio Silence to Communicate

Jamming techniques require only moderate resources to be deployed, while their effectiveness in disrupting communications is unprecedented. In this article, we introduce several contributions to jamming mitigation. In particular, we introduce a novel adversary model that has both (unlimited) jamming reactive capabilities as well as powerful (but limited) proactive jamming capabilities. Under this adversary model, to the best of our knowledge more powerful than any other adversary model addressed in the literature, the communication bandwidth provided by current anti-jamming solutions drops to zero. We then present Silence is Golden (SiG): a novel anti-jamming protocol that, introducing a tunable, asymmetric communication channel, is able to mitigate the adversary capabilities, enabling the parties to communicate. For instance, with SiG it is possible to deliver a 128-bits-long message with a probability greater than 99% in 4096 time slots despite the presence of a jammer that jams all on-the-fly communications and 74% of the silent radio spectrum—while competing proposals simply fail. Moreover, when SiG is used in a scenario in which the adversary can jam only a subset of all the available frequencies, performance experiences a boost: a 128-bits-long message is delivered within just 17 time slots for an adversary able to jam 90% of the available frequencies. We present a thorough theoretical analysis for the solution, which is supported by extensive simulation results, showing the viability of our proposal.

Robust and efficient authentication of video stream broadcasting

We present a novel video stream authentication scheme which combines signature amortization by means of hash chains and an advanced watermarking technique. We propose a new hash chain construction, the Duplex Hash Chain, which allows us to achieve bit-by-bit authentication that is robust to low bit error rates. This construction is well suited for wireless broadcast communications characterized by low packet losses such as in satellite networks. Moreover, neither hardware upgrades nor specific end-user equipment are needed to enjoy the authentication services. The computation overhead experienced on the receiver only sums to two hashes per block of pictures and one digital signature verification for the whole received stream. This overhead introduces a provably negligible decrease in video quality. A thorough analysis of the proposed solution is provided in conjunction with extensive simulations.

AmbiSec: securing smart spaces using entropy harvesting

Following the vision of Ambient Intelligence (AmI), this paper introduces and evaluates a novel security scheme that takes the advantage of the unpredictable and erratic behavior of wireless communication to generate secret keys. The main advantage is that the secret key generation is applicable to every wireless device, independently of their hardware characteristics as it only requires a wireless interface and a human movement, which inherently affects the signal propagation within the physical environment. To analyze the applicability of this scheme, we implement and systematically evaluate the key generation using a wireless sensor network deployed in a real-world scenario. The analysis clarifies how different factors influence the amount of randomness collected from the physical environment, and it also shows that guessing attacks from an eavesdropper are negligible even if it is able to eavesdrop the complete wireless communication.