Protecting communications’ metadata can be as important as protecting their content, i.e., recognizing someone contacting a medical service may already allow to infer sensitive information. There are numerous proposals to implement anonymous communications, yet none provides it in a strong (but feasible) threat model in an efficient way. We propose Hydra, an anonymity system that is able to efficiently provide metadata security for a wide variety of applications. Main idea is to use latency-aware, padded, and onion-encrypted circuits even for connectionless applications. This allows to implement strong metadata security for contact discovery and text-based messages with relatively low latency. Furthermore, circuits can be upgraded to support voice calls, real-time chat sessions, and file transfers—with slightly reduced anonymity in presence of global observers. We evaluate Hydra using an analytical model as well as call simulations. Compared to other systems for text-based messaging, Hydra is able to decrease end-to-end latencies by an order of magnitude without degrading anonymity. Using a dataset generated by performing latency measurements in the Tor network, we further show that Hydra is able to support anonymous voice calls with acceptable quality of service in real scenarios. A first prototype of Hydra is published as open source.

RAN sharing allows multiple operators to efficiently share parts of a 5G radio network. However, it also poses a risk to security, as some shared components have the ability to eavesdrop and modify the communication of all connected devices, regardless of the operator they are connected to. This demo paper introduces a method to architecturally improve tenant separation in 5G/6G networks utilizing RAN sharing. We introduce an F1AP proxy, which allows each operator to control their own Central Unit (CU), requiring only the Distributed Units (DU) and proxies to be shared. Since the keys needed to decrypt or modify user plane and control plane traffic are only stored and processed at the CU, our approach enables a strict cryptographic separation while not requiring changes to existing NG-RAN implementations. We demonstrate the feasibility of the approach with a prototypical implementation of the proxy in an otherwise unmodified OpenAirInterface setup.

: Conventional asymmetric cryptography is threatened by the ongoing development of quantum computers. A mandatory countermeasure in the context of virtual private networks (VPNs) is to use post-quantum cryptography (PQC) as a drop-in replacement for the authenticated key exchange in the Internet Key Exchange (IKE) protocol. However, the results of the ongoing cryptanalysis of PQC cannot be predicted. Consequently, this article discusses orthogonal methods for quantum-resistant key exchanges, like quantum key distribution (QKD) and multipath key reinforcement (MKR). As each method has limitations when used on its own, we conclude that it is best to maximize security by combining all available sources of symmetric key material to protect traffic inside a VPN. As one possible realization, we propose a lightweight proxy concept that uses available symmetric keys, like QKD and MKR keys, to implement a transparent cryptographic tunnel for all IKE packets, and consequently for PQC key exchanges. In contrast to combining PQC and symmetric key material within the IKE protocol, our approach provides security in depth: If secure symmetric keys are available, attacks on IKE and hence on PQC algorithms are infeasible. But even otherwise, the security properties of IKE and thus PQC are not weakened, so the overall security of the VPN is guaranteed to increase.

Assuming a threat model of a global observer, statistical disclosure attacks have been proposed to efficiently de-anonymize communication relationships in text-based mix networks over time. It is commonly assumed that such attacks are also able to disclose call relationships in anonymous communication networks (ACNs) that support voice calls. One straightforward countermeasure is to expect users to permanently send and receive packets that mimic a Voice over IP (VoIP) call. However, this is not practical in real world scenarios, like on mobile devices. In this article, we adapt one specific statistical disclosure attack (Z-SDA-MD) to voice calls and quantitatively study less resource-intensive countermeasures. As base countermeasure, we evaluate a round-based communication model, corresponding to a timed mix. A simulation study of this scenario shows that the Z-SDA-MD is not well suited for a general disclosure of call relationships because of too many false positives. Nevertheless, the attack is able to correctly identify the most frequent relationships. Still, the accuracy in that regard may significantly be decreased by increasing the duration of one round, by decoupling actions (call setup and teardown) of caller and callee by a random number of rounds, and by occasional fake calls to a fixed set of “fake friends”. Overall, our study shows that anonymous voice calls may be implemented with an acceptable trade-off between anonymity, call setup time, and bandwidth overhead.

This dataset supplements our following research paper: [1] Schatz, David; Rossberg, Michael; Schaefer, Guenter: Optimizing Packet Scheduling and Path Selection for Anonymous Voice Calls. Accepted at ARES 2021. Measurement Method The basic idea to estimate the latency (one-way delay) between any two Tor relays u, v is as follows [2]: Use a measurement agent a (a custom Tor client [3]). Pick an arbitrary entry relay e and build a three hop circuit (e, u, v), i.e. network packets would normally traverse the path (a, e, u, v) and back. Agent a asks the relay v to open a connection to localhost, which will be refused with a special error message. The agent a can measure the RTT (round-trip time) of this failed attempt and divide it by 2 to get an estimate of the one-way latency dl of the path (a, e, u, v). Due to the "leaky pipe" design of Tor, the agent a can do the same with relay u to estimate the one-way latency ds of the path (a, e, u). The estimation for the one-way delay between relays u and v is du,v = dl - ds. Measurement Series As agent, we used a PC at the TU Ilmenau in Germany, connected to the DFN network. As entry relays, we always used Tor relays from Germany as well. To later reduce the influence of random jitter in our estimations, we measured both dl and ds for a pair u, v 100 times (once overy 0.5 seconds). The measurements took place from 2019-12-19 to 2021-01-06. Dataset Content The dataset consists of three files: paths: This file contains the measurement results for dl and ds for each pair u, v we measured. Each line consists of 4 entries (separated by a space): Our internal ids for u and v, an indication if it is dl (value = 3) or ds (value = 2), and a comma separated list of the 100 (or less due to potential packet loss) measurements. Each measurement is the one-way latency in microseconds. probedRelays: This file contains the mapping from our internal ids of relays to their Tor fingerprints, one in each line, separated by a space. relayInfo: This file contains a subset of the relay descriptors for all active relays during our measurement period, one per line. Each line contains 4 entries: The fingerprint, the IP address, the consensus weight and a comma seperated list of flags. Note that we only measured relays which include the flags stable, running, and valid. The file was last updated on 2021-01-06 (relays that were not online at this date show their last known infos before that date). Our Pre-Processing For our research [1], we used the minium values for dl and ds (of the 100 each) to get one latency estimate du,v for each pair u, v. We further filtered 90 duplicates, i.e. 90 internal ids that actually mapped to the same fingerprint as some other id, leaving a total of 4102 probed relays. Further filtering was done as described in [1]. Limitations Unfortunately, we did not record the timestamp of measurements. Nevertheless, the paths file lists our measurements in chronological order and the measurement "speed" was constant during the ~2 years of measurements. Furthermore, we did not record the fingerprint of the selected entry relays. Further note that the long measurement period of ~2 years implies that estimated latencies do not capture a "snapshot" of the Tor network. For example, if a relay v got an "upgraded" access links during the two years, early estimates containing v will be higher than later estimates. References [2] Panchenko, Andriy; Renner, Johannes. Path Selection Metrics for Performance-Improved Onion Routing. SAINT 2009. Pages 114–120. [3] To build custom Tor circuits, we used the Python library stem, which uses the control port of a local Tor client.

Recent developments in computer networks increased flexibility, making them more dynamic and programmable, e.g., by SDN and NFV. However, this also increased complexity and volatility of network components. This is a challenge for highly regulated environments such as critical infrastructure networks where certified components are used to guarantee security requirements of infrastructures, e.g., through mandatory filtering or encryption of network traffic. This demo paper presents a setup where programmable and volatile components are separated from trusted, and thus certified, components. In particular, programmable Network Operating Systems (NOSes) and SDN controllers are deployed to steer the network flows in a VPN overlay. Yet, these flexible components do not have to be included into a certification process.

Automotive radar systems are indispensable for advanced driver assistance systems and traffic safety. Besides existing monostatic radar techniques, bi-static radar sensing like passive coherent location offers additional options to improve the radar visibility of vulnerable road users. Regarding the testing and evaluation of signal processing algorithms including parameter estimation, it is essential to provide electromagnetically shielded and reproducible measurement conditions, in addition to field tests in real traffic scenarios. This study describes the possibility to emulate relevant performance parameters for bi-static radar scenarios in the frequency range from 1 GHz to 6 GHz in a metal-shielded semi-anechoic chamber. Of special interest are the bi-static angle between transmitter, target, and receiver, and the resulting bi-static Doppler frequencies of a realistic vehicular traffic scenario. According to the concept of cooperative passive coherent location, Doppler scattering measurements are presented and compared to electromagnetic simulations. The authors find promising agreement between measured and ground truth data in the delay-Doppler spectrum.

Onion routing is a promising approach to implement anonymous voice calls. Uniform-sized voice packets are routed via multiple relays and encrypted in layers to avoid a correlation of packet content in different parts in the network. By using pre-built circuits, onion encryption may use efficient symmetric ciphers. However, if packets are forwarded by relays as fast as possible-to minimize end-to-end latency-network flow watermarking may still de-anonymize users. A recently proposed countermeasure synchronizes the start time of many calls and batch processes voice packets with the same sequence number in relays. However, if only a single link with high latency is used, it will also negatively affect latency of all other calls. This article explores the limits of this approach by formulating a mixed integer linear program (MILP) that minimizes latency "bottlenecks" in path selection. Furthermore, we suggest a different scheduling strategy for voice packets, i.e. implementing independent de-jitter buffers for all flows. In this case, a MILP is used to minimize the average latency of selected paths. For comparison, we solve the MILPs using latency and bandwidth datasets obtained from the Tor network. Our results show that batch processing cannot reliably achieve acceptable end-to-end latency (below 400 ms) in such a scenario, where link latencies are too heterogeneous. In contrast, when using de-jitter buffers for packet scheduling, path selection benefits from low latency links without degrading anonymity. Consequently, acceptable end-to-end latency is possible for a large majority of calls.

Large Wi-Fi installations may make use of Wi-Fi controllers managing client mobility and different Virtual Local Area Networks (VLANs). However, as they might form a bottleneck, controller-less solutions are on the rise, e.g., cloud-managed solutions or mesh networks. IEEE 802.11 defines a framework for providing roaming assistance through neighbor reports and transition requests (IEEE 802.11k and IEEE 802.11v), but leaves generating meaningful candidates included in these frames open to the implementor. Without central controllers, deriving these candidates for Stations (STAs) is much more difficult. Access Points (APs) with more autonomy need to know other APs located in their proximity to provide roaming assistance for STAs. In this paper, we design and evaluate a network-assisted roaming architecture for controller-less Wi-Fi networks. APs learn roaming events in their vicinity and steer STAs by means of refined, yet standard-compliant neighbor reports and transition requests to better suited APs. Our results using a real-system prototype indicate a noticeable decrease of roaming times when comparing various reference approaches to our proposal.

The IPsec protocol family, although not always undisputed, has shown to be extremely reliable over the last two decades. However, given the fact that communication networks evolved tremendously since ESP was standardized, this paper proposes changes to the security protocol to accommodate for the needs of modern wide area and data center networks. In particular it addresses optimizations for high-speed software implementations as well as use cases in data center networks. The evaluation shows that rather small yet targeted changes are sufficient to allow for more flexible and scalable implementations.

There is a continuous interest in multi-hop multichannel Wireless Mesh Networks (WMNs) based on IEEE 802.11 for years now. Many design problems have been proposed to calculate performance metrics like the maximum flow to improve network capacity. However, these usually presume global coordination of transmissions in a slotted time model, a highly questionable presumption for IEEE 802.11. Additionally to this shortcoming, existing data rate models and link correlations are not realistic, rendering results imprecise. Furthermore, they are not incorporating other means of communication, e.g., wired connections, which could exist in real-world scenarios due to geographical proximity of selected nodes or availability of access to a wired LAN or WAN infrastructure - leading to the use case of hybrid WMNs. This paper proposes a model for maximum flow in hybrid multi-channel WMNs based on shared airtime. It is tailored to incorporate an exact representation of data rates and link correlations in IEEE 802.11 networks to produce dependable results and it furthermore supports wired connections. Packet-level simulation results verify key aspects of our model, supporting its validity with only a small relative error of less than 5% on average and no severe outliers.

Finding a widest path to transmit the maximum possible data rate is well-known in the field of computer science. For computational complexity, the network type has a significant impact: It is relatively easy to solve for simple graphs, whereas it is NP-complete for wireless networks based on slotted time models. These models neither facilitate the problem of finding widest paths, nor are they best suited to reflect realistic networks based on IEEE 802.11. Therefore, this paper studies the widest path problem for hybrid multi-channel Wireless Mesh Networks (WMNs) without slotted time. In our model, wireless data rates are equally shared among edges within interference range. We prove NP-completeness of the widest path problem (even for a simpler model), but heuristics already demonstrated good results in practical settings.

Automotive radar systems are indispensable for advanced driver assistance systems and traffic safety. Besides existing monostatic radar techniques, bi-static radar sensing like passive coherent location offers additional options to improve the radar visibility of vulnerable road users. Regarding the testing and evaluation of signal processing algorithms including parameter estimation, it is essential to provide electromagnetically shielded and reproducible measurement conditions, in addition to field tests in real traffic scenarios. This study describes the possibility to emulate relevant performance parameters for bi-static radar scenarios in the frequency range from 1 GHz to 6 GHz in a metal-shielded semi-anechoic chamber. Of special interest are the bi-static angle between transmitter, target, and receiver, and the resulting bi-static Doppler frequencies of a realistic vehicular traffic scenario. According to the concept of cooperative passive coherent location, Doppler scattering measurements are presented and compared to electromagnetic simulations. The authors find promising agreement between measured and ground truth data in the delay-Doppler spectrum.

IEEE 802.11s enables rapid deployment of Wireless Mesh Networks (WMNs) to supply basic wireless connectivity for client hardware. Application of distance vector based routing protocols proved advantageous in WMNs for efficiency, i.e., low overhead. However, it remains unclear, if plain distance vector routing protocols allow for adequate robustness against outages in large installations. Particularly, the required time for routing re-convergence may be too long, as in practice, the count-to-infinity phenomenon needs to be dealt with. This paper proposes Spare Forwarding Entries (SFEs), resulting in a proactive mechanism improving robustness against outages when compared to the reactive behavior of distance vector protocols. It works as an extension of distance vector operation, i.e., efficiency can be preserved. We integrate SFE into the well-known Babel routing protocol and also propose measures to increase scalability. Simulation studies indicate that SFEs improve robustness against node outages significantly. In certain scenarios, the Packet Delivery Ratio (PDR) was doubled with our mechanisms in place. Further modifications let Babel-based WMNs scale up to 300 nodes. Moreover, convergence times and overhead are significantly smaller.

Seamless roaming in distributed Wireless Mesh Networks (WMNs) is still a complex problem in practice. We employ a 20-node testbed that enables mobile clients to stream live video with real-time properties despite frequent roaming events. A Linux-based prototype implements enhancements to the Babel distance vector routing and novel network-assisted roaming improvements.

Hidden terminal situations, in which two transmitters may interfere at a single receiver, but the transmitters cannot notice each other’s interference, can have severe effects. Traditional access technologies use RTS/CTS to remedy this. This is infeasible in vehicular scenarios, because the broadcast communication would lead to many RTS/CTS dialogues and every potential receiver might not be known to the transmitter. Hence, there are no mitigations in IEEE 802.11p, ITS-G5 or LTE-V2X mode 4.In this paper, we describe a method to leverage the periodicity of the semi-persistent scheduling in LTE-V2X mode 4 to proactively consider and reactively detect hidden terminal problems. Our approach uses the padding bits of periodic messages, e.g., CAMs, to indicate which resource has been successfully decoded by the terminal. This information can be used by terminals to detect whether its own transmissions have not been decodable by others, and react to this situation by selecting new resources. Additionally, we present an extended candidate resource selection procedure that considers these acknowledgements to extend the station’s sensing range. Due to semi-persistent scheduling, already acknowledged resources are likely to result in hidden terminal situations in future. Our approach significantly outperforms standard LTE-V2X mode 4 in our evaluation.

IEEE802.11s enables rapid deployment of Wireless Mesh Networks (WMNs) to supply basic wireless connectivity for client hardware. However, if state-of-the-art routing protocols are up to the task in practice is not systematically evaluated, especially when considering seamless client integration without using central Wi-Fi controllers. This paper analyzes and enhances the Babel routing protocol for WMNs with attached clients. In particular, we improve roaming times by enhancing client route handling and integrating clients as if they were announcing themselves. We present client roaming performance based on real-world experiments leveraging a 20-node testbed.

5G promises many new vertical service areas beyond simple communication and data transfer. We propose cooperative passive coherent location (CPCL), a distributed MIMO radar service, which can be offered by mobile radio network operators as a service for public user groups. CPCL comes as an inherent part of the radio network and takes advantage of the most important key features proposed for 5G. It extends the well-known idea of passive radar (also known as passive coherent location, PCL) by introducing cooperative principles. These range from cooperative, synchronous radio signaling and MAC up to radar data fusion on the sensor and scenario levels. By using software-defined radio and network paradigms, as well as real-time mobile edge computing facilities intended for 5G, CPCL promises to become a ubiquitous radar service that may be adaptive, reconfigurable, and perhaps cognitive. As CPCL makes double use of radio resources (in terms of both frequency bands and hardware), it can be considered a green technology. Although we introduce the CPCL idea from the viewpoint of vehicle-to-vehicle/infrastructure communication, it can definitely also be applied to many other applications in industry, transport, logistics, and for safety and security applications.

IEEE 802.11p (ITS-G5 in Europe) and LTE-V2X are the dominant wireless access technologies for vehicle-to-vehicle communication. While the former has been extensively tested and evaluated for years, the cellular approach is relatively new. Previous research reported safety-critical issues of the distributed MAC mode such as re-occurring packet collisions, caused by repeated resource reuse due to the semi-persistent scheduling (SPS). While the evaluation of LTE-V2X is in its early stages, modifications and additions are simultaneously being released. The ETSI recently published a distributed congestion control (DCC) mechanism to limit the channel load and ensure that the random-access based MAC protocol can work as intended. By dropping packets and reducing the predictability of resource allocations achieved by SPS, this DCC mechanism raises concerns about the compatibility with the MAC protocol. In this paper, we present a modification to the MAC protocol that limits the impact of re-occurring collisions while retaining the benefits of SPS. Additionally, the proposed modification allows to implement an improved DCC version, which is better compatible with the MAC protocol of LTE-V2X. The modifications and DCC variant are evaluated in a system-level simulation and show significant benefits compared to the standard-compliant versions.

Currently deployed Wireless Mesh Networks (WMNs) are mostly hybrid, i.e., some Mesh Points (MPs) also employ additional Access Point (AP) radios to connect non-mesh stations (STAs). Today's Wi-Fi security protocols are unsuited in the use case of WMNs, as they can neither derive key material without central authentication servers nor tolerate compromised MPs, as it is required in outdoor deployments. To establish high security standards while embracing the distributed nature of WMNs, we need a novel security architecture, that does not rely on central entities and protects traffic between MPs with End-to-End Encryption (E2EE). We propose and evaluate a distributed security architecture for WMNs with attached APs, which uses certificates early in the authentication process. The architecture provides E2EE between MPs and authentic MAC addresses of all STAs and MPs. STAs, e.g., resource constrained Internet of Things (IoT) devices, cannot participate in the end-to-end encryption, but need to be securely attached to the WMN with mobility and other requirements in mind. The evaluation in our Wi-Fi testbed shows the authentication protocol's suitability for fast (re-)authentication in mobile scenarios.