Building Security Protocols Against Powerful Adversaries

As our sensitive data is increasingly carried over the Internet and stored remotely, security in communications becomes a fundamental requirement. Yet, today's security practices are designed around assumptions the validity of which is being challenged. In this thesis we design new security mechanisms for certain scenarios where traditional security assumptions do not hold. First, we design secret-agreement protocols for wireless networks, where the security of the secrets does not depend on assumptions about the computational limitations of adversaries. Our protocols leverage intrinsic characteristics of the wireless to enable nodes to agree on common pairwise secrets that are secure against computationally unconstrained adversaries. Through testbed and simulation experimentation, we show that it is feasible in practice to create thousands of secret bits per second. Second, we propose a traffic anonymization scheme for wireless networks. Our protocol aims in providing anonymity in a fashion similar to Tor - yet being resilient to computationally unbounded adversaries - by exploiting the security properties of our secret-agreement. Our analysis and simulation results indicate that our scheme can offer a level of anonymity comparable to the level of anonymity that Tor does. Third, we design a lightweight data encryption protocol for protecting against computationally powerful adversaries in wireless sensor networks. Our protocol aims in increasing the inherent weak security that network coding naturally offers, at a low extra overhead. Our extensive simulation results demonstrate the additional security benefits of our approach. Finally, we present a steganographic mechanism for secret message exchange over untrustworthy messaging service providers. Our scheme masks secret messages into innocuous texts, aiming in hiding the fact that secret message exchange is taking place. Our results indicate that our schemes succeeds in communicating hidden information at non-negligible rates.

Creating Shared Secrets out of Thin Air

Suhas Diggavi, Christina Fragouli, Iris Safaka

Current security systems typically rely on the adversary's computational limitations (e.g., the fact that it cannot invert a hash function or perform large-integer factorization). Wireless networks offer the opportunity for a different, complementary kind of security, which relies not on the adversary's computational limitations, but on its limited network presence (i.e., that the adversary cannot be located at many different points in the network at the same time). We take a first step toward designing and building a wireless security system that leverages this opportunity: We consider the problem where a group of n nodes, connected to the same broadcast wireless network, want to agree on a shared secret (e.g., an encryption key), in the presence of an adversary Eve who tries to listen in and steal the secret. We propose a secret-agreement protocol, where the n nodes of the group keep exchanging bits until they have all agreed on a bit sequence that Eve cannot reconstruct (with very high probability). We provide experimental evidence -- to the best of our knowledge, the first one -- that a group of wireless nodes can generate thousands of new shared secret bits per second, with their secrecy being independent of the adversary's computational capabilities.

Assoc Computing Machinery2012

Routing and search on large scale networks

Dominique Florian Tschopp

In this thesis, we address two seemingly unrelated problems, namely routing in large wireless ad hoc networks and comparison based search in image databases. However, the underlying problem is in essence similar and we can use the same strategy to attack those two problems. In both cases, the intrinsic complexity of the problem is in some sense low, and we can exploit this fact to design efficient algorithms. A wireless ad hoc network is a communication network consisting of wireless devices such as for instance laptops or cell phones. The network does not have any fixed infrastructure, and hence nodes which cannot communicate directly over the wireless medium must use intermediate nodes as relays. This immediately raises the question of how to select the relay nodes. Ideally, one would like to find a path from the source to the destination which is as short as possible. The length of the found path, also called route, typically depends on how much signaling traffic is generated in order to establish the route. This is the fundamental trade-off that we will investigate in this thesis. As mentioned above, we try and exploit the fact that the communication network is intrinsically low-dimensional, or in other words has low complexity. We show that this is indeed the case for a large class of models and that we can design efficient algorithms for routing that use this property. Low dimensionality implies that we can well embed the network in a low-dimensional space, or build simple hierarchical decompositions of the network. We use both those techniques to design routing algorithms. Comparison based search in image databases is a new problem that can be defined as follows. Given a large database of images, can a human user retrieve an image which he has in mind, or at least an image similar to that image, without going sequentially through all images? More precisely, we ask whether we can search a database of images only by making comparisons between images. As a case in point, we ask whether we can find a query image q only by asking questions of the type "does image q look more like image A or image B"? The analogous to signaling traffic for wireless networks would here be the questions we can ask human users in a learning phase anterior to the search. In other words, we would like to ask as few questions as possible to pre-process and prepare the database, while guaranteeing a certain quality of the results obtained in the search phase. As the underlying image space is not necessarily metric, this raises new questions on how to search spaces for which only rank information can be obtained. The rank of A with respect to B is k, if A is B's kth nearest neighbor. In this setup, low-dimensionality is analogous to the homogeneity of the image space. As we will see, the homogeneity can be captured by properties of the rank relationships. In turn, homogeneous spaces can be well decomposed hierarchically using comparisons. Further, it allows us to design good hash functions. To design efficient algorithms for these two problems, we can apply the same techniques mutatis mutandis. In both cases, we relied on the intuition that the problem has a low intrinsic complexity, and that we can exploit this fact. Our results come in the form of simulation results and asymptotic bounds.

EPFL2010

Effectiveness of Distance-Decreasing Attacks Against Impulse Radio Ranging

Manuel Flury, Jean-Pierre Hubaux, Jean-Yves Le Boudec, Panagiotis Papadimitratos, Marcin Poturalski

We expose the vulnerability of an emerging wireless ranging technology, impulse radio ultra-wide band (IR-UWB), to distance-decreasing attacks on the physical communication layer (PHY). These attacks violate the security of secure ranging protocols that allow two wireless devices to securely estimate the distance between them, with the guarantee that the estimate is an upper-bound on the actual distance. Such protocols serve as crucial building blocks in security-sensitive applications such as location tracking, physical access control, or localization. Prior works show the theoretical possibility of PHY attacks bypassing cryptographic mechanisms used by secure ranging protocols. They also demonstrates that for physical layers used in ISO 14443 RFID and wireless sensor networks, some PHY attacks are indeed feasible. IR-UWB was proposed as a possible solution, but we show that the de facto standard for IR-UWB, IEEE 802.15.4a, does not automatically provide security against such attacks. We find that with the mandatory modes of the standard an external attacker can decrease the measured distance by as much as 140 meters with a high probability (above 99%).

2010

Routing and search on large scale networks

Dominique Florian Tschopp

EPFL2010