This document summarises the results of several research campaigns over the past seven years. The main connecting theme is the physical layer of widely deployed sensors in Pervasive Computing domains. In particular, we have focused on the RF-channel or on ambient audio. The initial problem from which we started this work was that of distributed adaptive transmit beamforming. We have been looking for a simple method to align the phases of jointly transmitting nodes (e.g. sensor or IoT nodes). The algorithmic solution to this problem was to implement a distributed random optimisation method on the participating nodes in which the transmitters and the receiver follow an iterative question-and-answer scheme. We have been able to derive sharp asymptotic bounds on the expected optimisation time of an evolutionary random optimiser and presented an asymptotically optimal approach. One thing that we have learned from the work on these physical layer algorithms was that the signals we work on are fragile and perceptive to physical environmental changes. These could be obstacles such as furniture, opened or closed windows or doors as well as movement of individuals. This observation motivated us to view the wireless interface as a sensor for environmental changes in Pervasive Computing environments. Another use of physical layer RF-signals is for security applications. We are currently working to further push these mentioned directions and novel fields of physical prototyping. In particular, the calculation of mathematical operations on the wireless channel at the time of transmission appears to contain good potential for gains in efficiency for communication and computation in Pervasive Computing domains.
@cite_107: We present three unconventional approaches to keying variable management. The first approach is based on using a public key cryptosystem (PKC) that is breakable in short, but on average less, time than it takes to set up an ultrawide bandwidth modem that is then used to transport a keying variable for a classical cryptosystem. The second concept proposes using the characteristics of an urban UHF radio channel, determined by mutual sounding, as the cryptovariable. The third concept encourages research into ill-conditioned problems as potentially fruitful ground for PKCs not based on finite field arithmetic. >
@cite_205: New approaches that generate secret keys from radio channel characteristics have been recently proposed. The security of these schemes usually relies on the reciprocity principle which states that the channel fluctuations can be a source of common randomness for two communicating peers and these fluctuations can't be measured by any eavesdropper. A validation of these properties for indoor environments is presented in this work. The shared keys are created by measuring the reciprocal channel features and converting this information to binary vectors using a quantization algorithm. This paper addresses the problem of quantization. It identifies an important weakness of existing key generation algorithms and it shows that the secret bits extraction has a significant impact on the robustness and security of these algorithms. A new adaptive quantization algorithm for secret key generation is presented. This method has the advantages to create sufficient long secret keys with a high key agreement ratio between authorized users without revealing information to the attacker. The new scheme is experimentally validated using Ultra Wide Band technology.
@cite_191: In this paper, a single-antenna reciprocity relation is derived for the time domain. First, the antenna is considered on transmission; next, the same antenna is considered when it is receiving an incident plane wave. The two states, transmission and reception, are related by the application of a modified form of the reciprocity theorem for electromagnetic fields with general time dependence due to Cheo. The derivation of the reciprocity relation for the antenna makes use of simple geometric arguments to evaluate the spatial temporal integrals that occur in the theorem. A few extensions of the reciprocity relation are also described.
@cite_116: Securing interactions between devices that do not know each other a priori is an important and challenging task. We present Amigo, a technique to authenticate co-located devices using knowledge of their shared radio environment as proof of physical proximity. We present evaluation results that show that our technique is robust against a range of passive and active attacks. The key advantages of our technique are that it does not require any additional hardware to be present on the devices beyond the radios that are already used for communication, it does not require user involvement to verify the validity of the authentication process, and it is not vulnerable to eavesdropping.
@cite_126: It has been shown that the radio channel impulse response for a pair of legitimate Ultra-wide band (UWB) transceivers can be used to generate secret keys for secure communications. Past proposed secret key generation algorithms under-exploited the available number of secret key bits from the radio channel. This paper proposes a new efficient method for generation of the shared key where the transceivers use LDPC decoders to resolve the differences in their channel impulse response measurements caused by measurement noise. To ensure secret key agreement, a method of public discussion between the two users is performed using the syndrome from Hamming (7,3) binary codes. An algorithm is proposed to check the equality of generated keys for both legitimate users, and ensure error-free secure communication. The security of this algorithm has been verified by AVISPA. Comparisons are performed with previous work on secret key generation and it has been shown that this algorithm reliably generates longer secret keys in standard UWB radio channels.
Another sensor class utilised for context-based device authentication is the RF-channel. present a technique to authenticate co-located devices based on RF-measurements since channel measurements from devices in near proximity are sufficiently similar to authenticate devices against each other @cite_116 . utilise physical layer features to derive secret keys for a pair of devices @cite_107 . In the absence of interference and non-linear components, transmitter and receiver experience identical channel response @cite_191 . This information is utilised to generate a secret key among a node pair. Since channel characteristics are spatially sharply concentrated and not predictable at a remote location @cite_126 , an eavesdropper is not capable of guessing information about the secret. This scheme was validated in an indoor environment in @cite_205 . Although we consider the keys generated by this scheme as strong, it does not preserve spatial properties. A device at arbitrary distance could pretend to be a nearby communication partner.