Rene Guillaume

Reciprocity enhancement and decorrelation schemes for PHY-based key generation

Providing secure communication links between devices of low computational power has been increasingly investigated during recent years. The need for fast and easy-to-implement security for computationally weak wireless devices has lead to the development of physical layer based key generation approaches. The generation of symmetric cryptographic keys out of wireless channel properties turned out to be a promising approach comprising advantages of symmetric as well as asymmetric cryptography. Numerous quantization schemes have been proposed in previous works to increase the Key Generation Rate (KGR). Also by increasing the sampling rate, the input data can be generated faster. However, due to fast sampling rates, redundancy of subsequent bits will increase. This lowers the quality, that is, the randomness of the generated secret key and makes the system more vulnerable to brute-force attacks. We present and analyze different techniques for transforming a temporally correlated sequence into a compressed sequence of decorrelated bits and, therefore, assure non-redundant key sequences.

Bringing PHY-Based Key Generation into the Field: An Evaluation for Practical Scenarios

The need for secured communication between computationally weak wireless devices has driven the development of novel key generation protocols. Various schemes for extracting symmetric cryptographic keys out of wireless channel properties have been proposed during recent years, making the generation protocol more and more efficient for individual applications. However, often these schemes were evaluated based on theoretical models and without considering practical effects. We present a system for PHY-based key generation with two legitimate users as well as a passive attacker of equivalent power and analyze results from practical measurements in real world scenarios. Furthermore we extend practical constraints by considering heterogeneous setups and show the impact onto representative performance indicators.

Secret key generation from static channels with untrusted relays

The generation of symmetric cryptographic keys based on reciprocal channel properties has evolved as a promising approach in recent years, especially for devices with only little computational power. Different schemes individually addressing constraints from special setups have been proposed, but most schemes generally suffer from bad performance in case of (nearly) static scenarios. Addressing this dilemma, we present a novel approach for physical layer based key generation with two static users supported by a third party. In contrast to other solutions, this approach does not rely on special hardware like multiple antennas and does not let the third party know anything about the secret key, as the helper is potentially not fully trusted. The scheme does not evade to other diversity domains, but still exploits temporal and spatial diversity. We present thorough performance and security analysis, also taking a passive attacker into account.

On the Energy Cost of Channel Based Key Agreement

Besides security, energy consumption is a major concern for devices in the Internet of Things (IoT). We compare the energy consumption of two key agreement schemes -- Channel-Based Key Agreement (CBKA) and Elliptic Curve Diffie-Hellman (ECDH) -- in the IoT setting, using Wi-Fi as wireless communication interface. While ECDH is a well-studied protocol, CBKA has received attention only in recent years. Several publications proposed CBKA as a low-energy alternative to ECDH, but they did not address the energy cost of communication. For a fair comparison, we implemented the schemes on a 32-bit ARM Cortex M3-based IoT platform and measured the respective energy consumption for computation and communication. Our results show that the limiting factor for CBKA over Wi-Fi is the energy cost of communication, in particular the cost of acquiring the Received Signal Strength Indicator (RSSI) values. Even in an optimal scenario, CBKA must not measure more than ca. 300 RSSI values to be more energy efficient than ECDH. This is at most 1/5 of RSSI values required by CBKA implementations reported in the literature. As an optimization, we present a refined CBKA protocol which can save up to 25% of the energy compared to existing protocols by exploiting inherent data exchanges for entropy extraction.