The European Central Bank is preparing for the potential issuance of a central bank digital currency (CBDC), called the digital euro. A recent regulatory proposal by the European Commission defines several requirements for the digital euro, such as support for both online and offline payments. Offline payments are expected to enable cash-like privacy, local payment settlement, and the enforcement of holding limits. While other central banks have expressed similar desired functionality, achieving such offline payments poses a novel technical challenge. We observe that none of the existing research solutions, including offline E-cash schemes, are fully compliant. Proposed solutions based on secure elements offer no guarantees in case of compromise and can therefore lead to significant payment fraud. The main contribution of this paper is PayOff, a novel CBDC design motivated by the digital euro regulation, which focuses on offline payments. We analyze the security implications of local payment settlement and identify new security objectives. PayOff protects user privacy, supports complex regulations such as holding limits, and implements safeguards to increase robustness against secure element failure. Our analysis shows that PayOff provides strong privacy and identifies residual leakages that may arise in real-world deployments. Our evaluation shows that offline payments can be fast and that the central bank can handle high payment loads with moderate computing resources. However, the main limitation of PayOff is that offline payment messages and storage requirements grow in the number of payments that the sender makes or receives without going online in between.

The Android Virtualization Framework enables the execution of security-sensitive workloads in protected virtual machines using trusted hypervisors. We present Aster, an in-depth analysis of the Android Virtualization Framework security model as defined in the Android Compatibility Definition Document. It explores the design space for deploying protected virtual machines across Arm Trusted Execution Environments. Our analysis shows that executing Android in the normal world and protected virtual machines in the realm world using Arm Confidential Computing Architecture achieves the best tradeoff between security and implementation overheads. Aster strengthens Android Virtualization Framework isolation guarantees by introducing improved memory protection to mitigate physical attacks, enhancing independent memory management, deploying per-VM memory encryption, and enforcing stricter privilege separation. We implement and validate Aster on two platforms: functional emulator that supports Android, and a performance prototype on an Arm board that captures microarchitectural aspects. Our in-depth evaluation of impact of Aster on protected virtual machines execution under stress benchmarks (CPU, system, IO) as well as representative applications (public key generation, One-Time-Password, isolated compilation) show the minimal runtime performance impact.

To mitigate interrupt-based stepping attacks (notably using SGX-Step), Intel introduced AEX-Notify, an ISA extension to Intel SGX that aims to prevent deterministic single-stepping. In this work, we introduce AEX-NStep, the first interrupt counting attack on AEX Notify-enabled Enclaves. We show that deterministic single-stepping is not required for interrupt counting attacks to be practical and that, therefore, AEX-Notify does not entirely prevent such attacks. We specifically show that one of AEX-Notify's security guarantees, obfuscated forward progress, does not hold, and we introduce two new probabilistic interrupt counting attacks. We use these attacks to construct a practical ECDSA key leakage attack on an AEX-Notiy-enabled SGX enclave. Our results extend the original security analysis of AEX-Notify and inform the design of future mitigations.

Time-of-arrival (ToA) estimation via cross-correlation is an essential building block of time-of-flight ranging. However, in narrowband systems, it is notoriously difficult to protect against distance-decreasing attacks such as Early-Detect/Late-Commit (ED/LC). We present and analyze two new attacks that reshape ranging signals to compromise correlation-based ToA estimation. The first attack multiplies the signal by a symbol-periodic waveform in the time domain, while the second passes it through a negative group delay (NGD) filter. In contrast to ED/LC, our attacks do not require real-time symbol detection or adaptive compensation; they are completely symbol-agnostic. We describe implementation strategies for both attacks and discuss NGD filtering in the context of Bluetooth Channel Sounding (CS), a recent narrowband ranging system. To this end, we simulate an NGD circuit in LTspice and a ToA estimator in MATLAB, demonstrating that the attack can result in distance reductions of up to 18 m against Bluetooth CS RTT ranging. Finally, we verify the feasibility of the NGD approach by building a prototype using commercial off-the-shelf components.

The widespread availability of cellular devices introduces new threat vectors that allow users or attackers to bypass security policies and physical barriers and bring unauthorized devices into sensitive areas. These threats can arise from user non-compliance or deliberate actions aimed at data exfiltration/infiltration via hidden devices, drones, etc. We identify a critical gap in this context: the absence of low-latency systems for high-quality and instantaneous monitoring of cellular transmissions. Such low-latency systems are crucial to allow for timely detection, decision (e.g., geofencing or localization), and disruption of unauthorized communication in sensitive areas. Operator-based monitoring systems, built for purposes such as people counting or tracking, lack real-time capability, require cooperation across multiple operators, and thus are hard to deploy. Operator-independent monitoring approaches proposed in the literature either lack low-latency capabilities or do not scale. We propose WaveTag, the first low-latency, operator-independent, and scalable system designed to monitor 5G and LTE connections across all operators prior to any user data transmission. WaveTag consists of several downlink receivers and a distributed network of uplink receivers that measure both downlink protocol information and uplink signal characteristics at multiple locations to gain a detailed spatial image of uplink signals. WaveTag then aggregates the recorded information, processes it, and provides a decision about the connection before the UE completes connection establishment. To evaluate WaveTag, we deployed it in the context of geofencing, where WaveTag was able to determine whether the signals originate from inside or outside of an area within 2.3 ms of the initial base station-to-device message, therefore enabling prompt and targeted suppression of communication before any

Cloud-centric workloads increasingly leverage domain-specific accelerators (DSAs) such as GPU, NPU, FPGA, etc., to achieve massive speedup over general-purpose CPUs. These workloads compute sensitive data; furthermore, the programs can be proprietary business secrets such as high-performance AI models. Therefore, several confidential cloud solutions have recently emerged to protect against the attacker-controlled software stack (OS/VMM) and the cloud service providers or CSPs themselves. CPU-centric trusted execution environments, or TEEs, have been around for decades and are deployed commercially. However, despite some recent proposals, most nodes lack TEE capability and, therefore, are unprotected against malicious CSP and software stack.We address this gap by proposing a new dedicated hardware module, the security controller (SC), that acts as the TEE proxy for the legacy non-TEE DSA nodes in a data center across racks. SC enforces access control and attestation mechanisms and protects the non-TEE nodes even from a physical attacker. This way, SC enables new-generation TEE-enabled nodes and legacy non-TEE nodes to be used in a data center simultaneously while ensuring security. We implement and synthesize SC hardware and evaluate it with real-world cloud-centric workloads with heterogeneous DSAs. Our evaluation shows that, on average, SC introduces 1.5-5% overhead while running AI, Redis, and file system workloads and scales well with an increasing number of DSA nodes (up to 2236 concurrent NPUs running CNNs).

A common form of phishing training in organizations is the use of simulated phishing emails to test employees' susceptibility to phishing attacks, and the immediate delivery of training material to those who fail the test. This widespread practice is dubbed embedded training; however, its effectiveness in decreasing the likelihood of employees falling for phishing again in the future is questioned by the contradictory findings of several recent field studies.

Trusted Execution Environments (TEEs) have been proposed as a solution to protect code confidentiality in scenarios where computation is outsourced to an untrusted operator. We study the resilience of such solutions to side-channel attacks in two commonly deployed scenarios: when the confidential code is a native binary that is shipped and executed within a TEE and when the confidential code is an intermediate representation (IR) executed on top of a runtime within a TEE. We show that executing IR code such as WASM bytecode on a runtime executing in a TEE leaks most IR instructions with high accuracy and therefore reveals the confidential code. Contrary to IR execution, native execution is much less susceptible to leakage and largely resists even the most powerful side-channel attacks. We evaluate native execution leakage in Intel SGX and AMD SEV and experimentally demonstrate end-to-end instruction extraction on Intel SGX, with WASM bytecode as IR executed within two popular WASM runtimes: WAMR and wasmi. Our experiments show that IR code leakage from such systems is practical and therefore question the security claims of several commercial solutions which rely on TEEs+WASM for code confidentiality.

Due to the popularity of blockchain-based cryptocurrencies, the increasing digitalization of payments, and the constantly reducing role of cash in society, central banks have shown an increased interest in deploying central bank digital currencies (CBDCs) that could serve as a digital cash-equivalent. While most recent research on CBDCs focuses on blockchain technology, it is not clear that this choice of technology provides the optimal solution. In particular, the centralized trust model of a CBDC offers opportunities for different designs. In this paper, we depart from blockchain designs and instead build on ideas from traditional e-cash schemes. We propose a new style of building digital currencies that combines the transaction processing model of e-cash with an account-based fund management model. We argue that such a style of building digital currencies is especially well-suited to CBDCs. We also design the first such digital currency system, called Platypus, that provides strong privacy, high scalability, and expressive but simple regulation, which are all critical features for a CBDC. Platypus achieves these properties by adapting techniques similar to those used in anonymous blockchain cryptocurrencies like Zcash to fit our account model and applying them to the e-cash context.

In cellular networks, attacks on the communication link between a mobile device and the core network significantly impact privacy and availability. Up until now, fake base stations have been required to execute such attacks. Since they require a continuously high output power to attract victims, they are limited in range and can be easily detected both by operators and dedicated apps on users' smartphones.

In this paper, we present findings from a largescale and long-term phishing experiment that we conducted in collaboration with a partner company. Our experiment ran for 15 months during which time more than 14,000 study participants (employees of the company) received different simulated phishing emails in their normal working context. We also deployed a reporting button to the company’s email client which allowed the participants to report suspicious emails they received. We measured click rates for phishing emails, dangerous actions such as submitting credentials, and reported suspicious emails. The results of our experiment provide three types of contributions. First, some of our findings support previous literature with improved ecological validity. One example of such results is good effectiveness of warnings on emails. Second, some of our results contradict prior literature and common industry practices. Surprisingly, we find that embedded training during simulated phishing exercises, as commonly deployed in the industry today, does not make employees more resilient to phishing, but instead it can have unexpected side effects that can make employees even more susceptible to phishing. And third, we report new findings. In particular, we are the first to demonstrate that using the employees as a collective phishing detection mechanism is practical in large organizations. Our results show that such crowd-sourcing allows fast detection of new phishing campaigns, the operational load for the organization is acceptable, and the employees remain active over long periods of time.

In this talk, I will discuss physical layer attacks on wireless networks, including attacks on GNSS systems, UWB ranging and cellular networks. Although these attacks differ in their goals and adversarial models, they share some commonalities - they attack the mechanisms that cannot be solely protected by cryptographic means. In particular, I will show how physical layer attacks can defeat the security of recently deployed commercial UWB ranging systems that are used in modern phones and vehicles. I will then show how these attacks can be used to deploy powerful DoS and tracking attacks on modern cellular networks. Finally, I will discuss the new research opportunities in this broad domain that emerge with the deployment of new types of networks and communication systems.

OFDM is a widely used modulation scheme. It transmits data over multiple subcarriers in parallel, which provides high resilience against frequency-dependent channel drops (fading) and achieves high throughput. Due to the proliferation of OFDM-enabled devices and the increasing need for location information, the research community has suggested using OFDM symbols for secure (time-of-flight) distance measurements. However, a consequence of relying on multiple subcarriers is long symbols (time-wise). This makes OFDM systems not a natural fit for secure ranging, as long symbols allow an attacker longer observation and reaction times to mount a so-called early-detect/late-commit attack. Despite these concerns, a recent standardization effort (IEEE 802.11az [5]) envisions the use of OFDM-based signals for secure ranging. This paper lays the groundwork for analyzing OFDM time-of-flight measurements and studies the security guarantees of OFDM-based ranging against a physical-layer attacker. We use BPSK and 4-QAM, the most robust configurations, as examples to present a strategy that increases the chances for early-detecting the transmitted symbols. Our theoretical analysis and simulations show that such OFDM systems are vulnerable to early-detection/late-commit attacks, irrespective of frame length and number of subcarriers. We identify the underlying causes and explore a possible countermeasure, consisting of orthogonal noise and randomized phase.

IEEE 802.15.4z, a standard for Ultra-Wide Band (UWB) secure distance measurement, was adopted in 2020 and the chips that implement this standard are already deployed in mobile phones and in the automotive industry (for Passive Keyless Entry and Start). The standard specifies two different modes---LRP and HRP. Whereas the security of LRP mode has been analyzed, there is no publicly available security analysis of the HRP mode, which is used in different chips like NXP Trimension SR150/SR040, Samsung smartphones, and U1 chip deployed in Apple iPhones.

Smart contracts are programmable, decentralized and transparent financial applications. Because smart contract platforms typically support Turing-complete programming languages, such systems are often said to enable arbitrary applications. However, the current permissionless smart contract systems impose heavy restrictions on the types of computations that can be implemented. For example, the globally-replicated and sequential execution model of Ethereum requires low gas limits that make many computations infeasible.

Intel SGX enables protected enclaves on untrusted computing platforms. An important part of SGX is its remote attestation mechanism that allows a remote verifier to check that the expected enclave was correctly initialized before provisioning secrets to it. However, SGX attestation is vulnerable to relay attacks where the attacker, using malicious software on the target platform, redirects the attestation and therefore the provisioning of confidential data to a platform that he physically controls. Although relay attacks have been known for a long time, their consequences have not been carefully examined. In this paper, we analyze relay attacks and show that redirection increases the adversary's abilities to compromise the enclave in several ways, enabling for instance physical and digital side-channel attacks that would not be otherwise possible.

Secure distance measurement and therefore secure Time-of-Arrival (ToA) measurement is critical for applications such as contactless payments, passive-keyless entry and start systems, and navigation systems. This paper initiates the study of Message Time of Arrival Codes (MTACs) and their security. MTACs represent a core primitive in the construction of systems for secure ToA measurement. By surfacing MTACs in this way, we are able for the first time to formally define the security requirements of physical-layer measures that protect ToA measurement systems against attacks. Our viewpoint also enables us to provide a unified presentation of existing MTACs (such as those proposed in distance-bounding protocols and in a secure distance measurement standard) and to propose basic principles for protecting ToA measurement systems against attacks that remain unaddressed by existing mechanisms. We also use our perspective to systematically explore the tradeoffs between security and performance that apply to all signal modulation techniques enabling ToA measurements.

Recent research has demonstrated that Intel's SGX is vulnerable to software-based side-channel attacks. In a common attack, the adversary monitors CPU caches to infer secret-dependent data accesses patterns. Known defenses have major limitations, as they require either error-prone developer assistance, incur extremely high runtime overhead, or prevent only specific attacks.

Attacker models are the cornerstone of any security assessment. As attacker's capabilities evolve over time, it is key to re-evaluate periodically if attacker models that were deemed unrealistic in the past might not pose a possible threat today. In this work, we evaluate the threat of wireless radio signal cancellation attacks in the face of recent advancements in software-defined radio attacker capabilities. Unlike classical radio interference or jamming attacker models which add noise to the legitimate communication, signal cancellation attacks aim at interfering destructively with the legitimate signal in order to remove those signals from the spectrum. While signal cancellation attacks were deemed unrealistic in the analogue domain, we analyse the system requirements to perform such attacks digitally using SDRs and evaluate the feasibility to launch such attacks against wireless communication systems such as GPS. Our evaluation reveals that signal cancellation attacks that manage to attenuate up to 40 dB of the signal at the receiver are feasible over the air. We further show that even complex CDMA signals such as GPS can be attenuated by 30 dB, even below a receiver's noise floor. These results indicate that digital signal cancellation attacks - especially against systems like GPS - should not be considered impossible per se, but deserve consideration when assessing the threat of attacks on wireless communication systems.

Two factor authentication (2FA) schemes provide strong user authentication guarantees and increase the security of a wide range of web services. However, 2FA schemes still largely remain vulnerable to phishing attacks in which attackers also phish users' second factor (e.g., their OTP tokens). We propose 2FA-PP, a phishing detection scheme that protects users' 2nd authentication factor from phishing attacks. 2FA-PP uses novel browser APIs that support direct communication between browsers and external devices (e.g., mobile phones) and enables the user's phone to check the domain to which the user is connected. The second factor is then only made available to the user if he is accessing the correct domain. 2FA-PP can be combined with different 2FA schemes, both interactive, based on OTP, QR codes and non-interactive, based on device pairing or proximity.

Recent news reveal a powerful attacker which breaks data confidentiality by acquiring cryptographic keys, by means of coercion or backdoors in cryptographic software. Once the encryption key is exposed, the only viable measure to preserve data confidentiality is to limit the attacker's access to the ciphertext. This may be achieved, for example, by spreading ciphertext blocks across servers in multiple administrative domains-thus assuming that the adversary cannot compromise all of them. Nevertheless, if data is encrypted with existing schemes, an adversary equipped with the encryption key, can still compromise a single server and decrypt the ciphertext blocks stored therein. In this paper, we study data confidentiality against an adversary which knows the encryption key and has access to a large fraction of the ciphertext blocks. To this end, we propose Bastion, a novel and efficient scheme that guarantees data confidentiality even if the encryption key is leaked and the adversary has access to almost all ciphertext blocks. We analyze the security of Bastion, and we evaluate its performance by means of a prototype implementation. We also discuss practical insights with respect to the integration of Bastion in commercial dispersed storage systems. Our evaluation results suggest that Bastion is well-suited for integration in existing systems since it incurs less than 5 percent overhead compared to existing semantically secure encryption modes.

Low-power wide area networks (LPWANs), such as LoRa, are fast emerging as the preferred networking technology for large-scale Internet of Things deployments (e.g., smart cities). Due to long communication range and ultra low power consumption, LPWAN-enabled sensors are today being deployed in a variety of application scenarios where sensitive information is wirelessly transmitted. In this work, we study the privacy guarantees of LPWANs, in particular LoRa. We show that, although the event-based duty cycling of radio communication, i.e., transmission of radio signals only when an event occurs, saves power, it inherently leaks information. This information leakage is independent of the implemented crypto primitives. We identify two types of information leakage and show that it is hard to completely prevent leakage without incurring significant additional communication and computation costs.

Cloud storage platforms promise a convenient way for users to share files and engage in collaborations, yet they require all files to have a single owner who unilaterally makes access control decisions. Existing clouds are, thus, agnostic to the notion of shared ownership. This can be a significant limitation in much collaboration because, for example, one owner can delete files and revoke access without consulting the other collaborators. In this paper, we first formally define a notion of shared ownership within a file access control model. We then propose two possible instantiations of our proposed shared ownership model. Our first solution, called Commune, relies on secure file dispersal and collusion-resistant secret sharing to ensure that all access grants in the cloud require the support of an agreed threshold of owners. As such, Commune can be used in existing clouds without modifications to the platforms. Our second solution, dubbed Comrade, leverages the blockchain technology in order to reach consensus on access control decision. Unlike Commune, Comrade requires that the cloud is able to translate access control decisions that reach consensus in the blockchain into storage access control rules, thus requiring minor modifications to existing clouds. We analyze the security of our proposals and compare/evaluate their performance through implementations using Amazon S3.

Many privacy-enhancing technologies, in particular anonymous communication networks (ACNs) as a key building block, suffer from a lack of a sufficient number of participants. Without high user participation, ACNs are vulnerable to traffic analysis attacks. The only ACN with a high number of participants (around 1.5 million users) is Tor. Yet, Tor is prone to traffic analysis attacks traffic pattern attacks. While other ACNs have been proposed that are even secure against global attackers, they are not scalable and suffer from a low number of participants, since even a perfect ACN can at most hide a user among all participating users. These ACNs are in a vicious circle: the lack of participants leads to low degree of anonymity, and a low degree of anonymity makes these ACNs unattractive for users. In this work, we break this vicious cycle by studying the question: Can an anonymous communication network be strengthened by "forced" participation? What privacy guarantees and performance can such an ACN provide? We develop CoverUp, a system that "forces" visitors of highly accessed websites (entry servers) to become involuntary participants of an ACN. CoverUp triggers users to participate in a centralized, constant-rate mix by leveraging basic functionality of their browsers to execute (JavaScript) code served by the entry servers. Candidates for entry servers could be universities or news sites. They would let a distinct CoverUp server provide (via an iframe) JavaScript code to the end-users' browsers, which in turn makes them participate in the ACN via a mix server. Visitors of these entry servers' websites become (involuntary) participants of an ACN, creating cover traffic for voluntary participants. For voluntary participants, we developed a browser extension that renders their CoverUp requests indistinguishable from the cover traffic of involuntary participants.

Mobile application spoofing is an attack where a malicious mobile app mimics the visual appearance of another one. A common example of mobile application spoofing is a phishing attack where the adversary tricks the user into revealing her password to a malicious app that resembles the legitimate one. In this paper, we propose a novel spoofing detection approach, tailored to the protection of mobile app login screens, using screenshot extraction and visual similarity comparison. We use deception rate as a novel similarity metric for measuring how likely the user is to consider a potential spoofing app as one of the protected applications. We conducted a large-scale online study where participants evaluated spoofing samples of popular mobile app login screens, and used the study results to implement a detection system that accurately estimates deception rate. We show that efficient detection is possible with low overhead.

We show that the new hover (floating touch) technology, available in a number of today's smartphone models, can be abused by malicious Android applications to record all touchscreen input into applications system-wide. Leveraging this attack, a malicious application running on the system is able to capture sensitive input such as passwords and PINs, record all user's social interactions, as well as profile user's behavior. To evaluate our attack we implemented Hoover, a proof-of-concept malicious application that runs in the background and records all input to all foreground applications. We evaluated Hoover with 20 users, across two different Android devices and two input methods, stylus and finger. In the case of touchscreen input by finger, Hoover estimated the positions of users' clicks within an error of 100 pixels and keyboard input with an accuracy of 79%. Hoover captured users' input by stylus even more accurately, estimating users' clicks within 2 pixels and keyboard input with an accuracy of 98%. Differently from existing well-known side channel attacks, this is the first work that proves the security implications of the hover technology and its potential to steal all user inputs with high granularity. We discuss ways of mitigating this attack and show that this cannot be done by simply restricting access to permissions or imposing additional cognitive load on the users since this would significantly constrain the intended use of the hover technology.

Proof of Work (PoW) powered blockchains currently account for more than 90% of the total market capitalization of existing digital cryptocurrencies. Although the security provisions of Bitcoin have been thoroughly analysed, the security guarantees of variant (forked) PoW blockchains (which were instantiated with different parameters) have not received much attention in the literature. This opens the question whether existing security analysis of Bitcoin's PoW applies to other implementations which have been instantiated with different consensus and/or network parameters.

Mobile application phishing happens when a malicious mobile application masquerades as a legitimate one to steal user credentials. Personalized security indicators may help users to detect phishing attacks, but rely on the user's alertness. Previous studies in the context of website phishing have shown that users tend to ignore personalized security indicators and fall victim to attacks despite their deployment. Consequently, the research community has deemed personalized security indicators an ineffective phishing detection mechanism. We revisit the question of personalized security indicator effectiveness and evaluate them in the previously unexplored and increasingly important context of mobile applications. We conducted a user study with 221 participants and found that the deployment of personalized security indicators decreased the phishing attack success rate to 50%. Personalized security indicators can, therefore, help phishing detection in mobile applications and their reputation as an anti-phishing mechanism in the mobile context should be reconsidered.

Multilateration techniques have been proposed to verify the integrity of unprotected location claims in wireless localization systems. A common assumption is that the adversary is equipped with only a single device from which it transmits location spoofing signals. In this paper, we consider a more advanced model where the attacker is equipped with multiple devices and performs a geographically distributed coordinated attack on the multilateration system. The feasibility of a distributed multi-device attack is demonstrated experimentally with a self-developed attack implementation based on multiple COTS software-defined radio (SDR) devices. We launch an attack against the OpenSky Network, an air traffic surveillance system that implements a time-difference-of-arrival (TDoA) multi-lateration method for aircraft localization based on ADS-B signals. Our experiments show that the timing errors for distributed spoofed signals are indistinguishable from the multilateration errors of legitimate aircraft signals, indicating that the threat of multi-device spoofing attacks is real in this and other similar systems. In the second part of this work, we investigate physical-layer features that could be used to detect multi-device attacks. We show that the frequency offset and transient phase noise of the attacker's radio devices can be exploited to discriminate between a received signal that has been transmitted by a single (legitimate) transponder or by multiple (malicious) spoofing sources. Based on that, we devise a multi-device spoofing detection system that achieves zero false positives and a false negative rate below 1%.

Global Positioning System (GPS) is used ubiquitously in a wide variety of applications ranging from navigation and tracking to modern smart grids and communication networks. However, it has been demonstrated that modern GPS receivers are vulnerable to signal spoofing attacks. For example, today it is possible to change the course of a ship or force a drone to land in a hostile area by simply spoofing GPS signals. Several countermeasures have been proposed in the past to detect GPS spoofing attacks. These counter-measures offer protection only against naive attackers. They are incapable of detecting strong attackers such as those capable of seamlessly taking over a GPS receiver, which is currently receiving legitimate satellite signals, and spoofing them to an arbitrary location. Also, there is no hardware platform that can be used to compare and evaluate the effectiveness of existing countermeasures in real-world scenarios.

The Location Service (LCS) proposed by the telecommunication industry is an architecture that allows the location of mobile devices to be accessed in various applications. We explore the use of LCS in location-enhanced server authentication, which traditionally relies on certificates. Given recent incidents involving certificate authorities, various techniques to strengthen server authentication were proposed. They focus on improving the certificate validation process, such as pinning, revocation, or multi-path probing. In this paper, we propose using the server's geographic location as a second factor of its authenticity. Our solution, SALVE, achieves location-based server authentication by using secure DNS resolution and by leveraging LCS for location measurements. We develop a TLS extension that enables the client to verify the server's location in addition to its certificate. Successful server authentication therefore requires a valid certificate and the server's presence at a legitimate geographic location, e.g., on the premises of a data center. SALVE prevents server impersonation by remote adversaries with mis-issued certificates or stolen private keys of the legitimate server. We develop a prototype implementation and our evaluation in real-world settings shows that it incurs minimal impact to the average server throughput. Our solution is backward compatible and can be integrated with existing approaches for improving server authentication in TLS.

Web applications rely on web servers to protect the integrity of sensitive information. However, an attacker gaining access to web servers can tamper with the data and query computation results, and thus serve corrupted web pages to the user. Violating the integrity of the web page can have serious consequences, affecting application functionality and decision-making processes. Worse yet, data integrity violation may affect physical safety, as in the case of medical web applications which enable physicians to assign treatment to patients based on diagnostic information stored at the web server. This paper presents Verena, a web application platform that provides end-to-end integrity guarantees against attackers that have full access to the web and database servers. In Verena, a client's browser can verify the integrity of a web page by verifying the results of queries on data stored at the server. Verena provides strong integrity properties such as freshness, completeness, and correctness for a common set of database queries, by relying on a small trusted computing base. In a setting where there can be many users with different write permissions, Verena allows a developer to specify an integrity policy for query results based on our notion of trust contexts, and then enforces this policy efficiently. We implemented and evaluated Verena on top of the Meteor framework. Our results show that Verena can support real applications with modest overhead.

Cloud platforms that use logical partitions to allocate dedicated resources to VMs can benefit from small and therefore secure hypervisors. Many-core platforms, with their abundant resources, are an attractive basis to create and deploy logical partitions on a large scale. However, many-core platforms are designed for efficient cross-core data sharing rather than isolation, which is a key requirement for logical partitions. Typically, logical partitions leverage hardware virtualization extensions that require complex CPU core enhancements. These extensions are not optimal for many-core platforms, where it is preferable to keep the cores as simple as possible.

Today, contactless smart cards are used to provide physical access control and authentication in a wide variety of applications. Prior research have demonstrated the vulnerability of contactless smart cards to relay attacks. For example, an attacker can relay the communication between the card reader and the smart card to steal a car or pay for goods in a supermarket. To solve this problem, smart cards need to be enhanced with secure proximity verification, i.e., distance bounding, which enables the card reader and the card to verify their mutual distance. However, existing technologies do not support the deployment of distance bounding in such systems: NFC cannot provide sufficient distance resolution, and hardware complexity of the proposed (e.g., UWB-based) distance bounding radios prevents their use in contactless smart cards.

Given the increasing adoption of Bitcoin, the number of transactions and the block sizes within the system are only expected to increase. To sustain its correct operation in spite of its ever-increasing use, Bitcoin implements a number of necessary optimizations and scalability measures. These measures limit the amount of information broadcast in the system to the minimum necessary. In this paper, we show that current scalability measures adopted by Bitcoin come at odds with the security of the system. More specifically, we show that an adversary can exploit these measures in order to effectively delay the propagation of transactions and blocks to specific nodes for a considerable amount of time---without causing a network partitioning in the system. Notice that this attack alters the information received by Bitcoin nodes, and modifies their views of the ledger state. Namely, we show that this allows the adversary to considerably increase its mining advantage in the network, and to double-spend transactions in spite of the current countermeasures adopted by Bitcoin. Based on our results, we propose a number of countermeasures in order to enhance the security of Bitcoin without deteriorating its scalability.

Cloud storage platforms promise a convenient way for users to share files and engage in collaborations, yet they require all files to have a single owner who unilaterally makes access control decisions. Existing clouds are, thus, agnostic to shared ownership. This can be a significant limitation in many collaborations because, for example, one owner can delete files and revoke access without consulting the other collaborators.

Distance bounding protocols enable one device (the verifier) to securely establish an upper bound on its distance to another device (the prover). These protocols can be used for secure location verification and detection of relay attacks, even in presence of strong attackers. The rapid-bit-exchange is the core of distance bounding protocols---the verifier sends single bit challenges, which the prover is expected to answer with minimal and stable processing delay. Based on the measured round trip time of flight, the verifier calculates its upper bound to the prover. Although several aspects of distance bounding implementations have been discussed in the past, no full implementation of a wireless distance bounding system has been presented so far.

Bitcoin is a decentralized payment system that relies on Proof-of-Work (PoW) to resist double-spending through a distributed timestamping service. To ensure the operation and security of Bitcoin, it is essential that all transactions and their order of execution are available to all Bitcoin users. Unavoidably, in such a setting, the security of transactions comes at odds with transaction privacy. Motivated by the fact that transaction confirmation in Bitcoin requires tens of minutes, we analyze the conditions for performing successful double-spending attacks against fast payments in Bitcoin, where the time between the exchange of currency and goods is short (in the order of a minute). We show that unless new detection techniques are integrated in the Bitcoin implementation, double-spending attacks on fast payments succeed with considerable probability and can be mounted at low cost. We propose a new and lightweight countermeasure that enables the detection of double-spending attacks in fast transactions. In light of such misbehavior, accountability becomes crucial. We show that in the specific case of Bitcoin, accountability complements privacy. To illustrate this tension, we provide accountability and privacy definition for Bitcoin, and we investigate analytically and empirically the privacy and accountability provisions in Bitcoin.

Lightweight Bitcoin clients are gaining increasing adoption among Bitcoin users, owing to their reduced resource and bandwidth consumption. These clients support a simplified payment verification (SPV) mode as they are only required to download and verify a part of the block chain---thus supporting the usage of Bitcoin on constrained devices, such as smartphones. SPV clients rely on Bloom filters to receive transactions that are relevant to their local wallet. These filters embed all the Bitcoin addresses used by the SPV clients, and are outsourced to more powerful Bitcoin nodes which then only forward to those clients transactions relevant to their outsourced Bloom filters.

On primary storage systems content is often replicated, converted or modified, and the users quickly lose control over its dispersal on the system. Deleting content related to a particular project from the system therefore becomes a labor-intensive task for the user. In this paper we present IRCUS, a system that assists the user in securely removing project-related content, but does not require changes to the user's behavior or to any of the system components, such as the file system, kernel or applications. IRCUS transparently integrates within the user's system, operates in user-space and stores the resulting metadata alongside the files. We implemented and evaluated our system and show that its overhead and accuracy are acceptable for practical use and deployment.

Web search queries reveal extensive information about users' personal lives to the search engines and Internet eavesdroppers. Obfuscating search queries through adding dummy queries is a practical and user-centric protection mechanism to hide users' search intentions and interests. Despite few such obfuscation methods and tools, there is no generic quantitative methodology for evaluating users' web-search privacy. In this paper, we provide such a methodology. We formalize adversary's background knowledge and attacks, the users' privacy objectives, and the algorithms to evaluate effectiveness of query obfuscation mechanisms. We build upon machine-learning algorithms to learn the linkability between user queries. This encompasses the adversary's knowledge about the obfuscation mechanism and the users' web-search behavior. Then, we quantify privacy of users with respect to linkage attacks. Our generic attack can run against users for which the adversary does not have any background knowledge, as well as for the cases where some prior queries from the target users are already observed. We quantify privacy at the query level (the link between user's queries) and the semantic level (user's topics of interest). We design a generic tool that can be used for evaluating generic obfuscation mechanisms, and users with different web search behavior. To illustrate our approach in practice, we analyze and compare privacy of users for two example obfuscation mechanisms on a set of real web-search logs.

Recent incidents of data-breaches from the cloud suggest that users should not trust the cloud provider to enforce access control on their data. We focus on mitigating trust to the cloud in scenarios where granting access to data not only considers user identities (as in conventional access policies), but also contextual information such as the user's location and time of access. Previous work in this context assumes a fully trusted cloud that is further capable of locating users. We introduce LoTAC, a novel framework that seamlessly integrates the operation of a cloud provider and a localization infrastructure to enforce location- and time-based access control to cloud-stored data. In LoTAC, the two entities operate independently and are only trusted to offer their basic services: the cloud provider is used and trusted only to reliably store data, the localization infrastructure is used and trusted only to accurately locate users. Furthermore, neither the cloud provider nor the localization infrastructure can access the data, even if they collude. LoTAC protocols require no changes to the cloud provider and minimal changes to the localization infrastructure. We evaluate our protocols using a cellular network as the localization infrastructure and show that they incur in low communication and computation costs and scale well with a large number of users and policies.

Power companies are deploying a multitude of sensors to monitor the energy grid. Measurements at different locations should be aligned in time to obtain the global state of the grid, and the industry therefore uses GPS as a common clock source. However, these sensors are exposed to GPS time spoofing attacks that cause misaligned aggregated measurements, leading to inaccurate monitoring that affects power stability and line fault contingencies. In this paper, we analyze the resilience of phasor measurement sensors, which record voltages and currents, to GPS spoofing performed by an adversary external to the system. We propose a solution that leverages the characteristics of multiple sensors in the power grid to limit the feasibility of such attacks. In order to increase the robustness of wide-area power grid monitoring, we evaluate mechanisms that allow collaboration among GPS receivers to detect spoofing attacks. We apply multilateration techniques to allow a set of GPS receivers to locate a false GPS signal source. Using simulations, we show that receivers sharing a local clock can locate nearby spoofing adversaries with sufficient confidence.

In this article, we propose a new micropayment model for nonspecialized commodity web-services based on microcomputations. In our model, a user that wishes to access online content (offered by a website) does not need to register or pay to access the website; instead, he will accept to run microcomputations on behalf of the service provider in exchange for access to the content. These microcomputations can, for example, support ongoing computing projects that have clear social benefits (e.g., projects relating to medical research) or can contribute towards commercial computing projects. We analyze the security and privacy of our proposal and we show that it preserves the privacy of users. We argue that this micropayment model is economically and technically viable and that it can be integrated in existing distributed computing frameworks (e.g., the BOINC platform). In this respect, we implement a prototype of a system based on our model and we deploy our prototype on Amazon Mechanical Turk to evaluate its performance and usability given a large number of users. Our results show that our proposed scheme does not affect the browsing experience of users and is likely to be used by a non-trivial proportion of users. Finally, we empirically show that our scheme incurs comparable bandwidth and CPU consumption to the resource usage incurred by online advertisements featured in popular websites.

Secure deletion is the task of deleting data irrecoverably from a physical medium. In this work, we present a general approach to the design and analysis of secure deletion for persistent storage that relies on encryption and key wrapping. We define a key disclosure graph that models the adversarial knowledge of the history of key generation and wrapping. We introduce a generic update function and prove that it achieves secure deletion of data against a coercive attacker; instances of the update function implement the update behaviour of all arborescent data structures including B-Trees, extendible hash tables, linked lists, and others. We implement a B-Tree instance of our solution. Our implementation is at the block-device layer, allowing any block-based file system to be used on top of it. Using different workloads, we find that the storage and communication overhead required for storing and retrieving B-Tree nodes is small and that this therefore constitutes a viable solution for many applications requiring secure deletion from persistent media.

End-users have become accustomed to the ease with which online systems allow them to exchange messages, pictures, and other files with colleagues, friends, and family. This con- venience, however, sometimes comes at the expense of hav- ing their data be viewed by a number of unauthorized par- ties, such as hackers, advertisement companies, other users, or governmental agencies. A number of systems have been proposed to protect data shared online; yet these solutions typically just shift trust to another third party server, are platform specific (e.g., work for Facebook only), or fail to hide that confidential communication is taking place. In this paper, we present a novel system that enables users to exchange data over any web-based sharing platform, while both keeping the communicated data confidential and hiding from a casual observer that an exchange of confidential data is taking place. We provide a proof-of-concept implementa- tion of our system in the form of a publicly available Fire- fox plugin, and demonstrate the viability of our approach through a performance evaluation.

Secure data deletion is the task of deleting data irrecoverably from a physical medium. In the digital world, data is not securely deleted by default; instead, many approaches add secure deletion to existing physical medium interfaces. Interfaces to the physical medium exist at different layers, such as user-level applications, the file system, the device driver, etc. Depending on which interface is used, the properties of an approach can differ significantly. In this paper, we survey the related work in detail and organize existing approaches in terms of their interfaces to physical media. We further present a taxonomy of adversaries differing in their capabilities as well as a systematization for the characteristics of secure deletion approaches. Characteristics include environmental assumptions, such as how the interface's use affects the physical medium, as well as behavioural properties of the approach such as the deletion latency and physical wear. We perform experiments to test a selection of approaches on a variety of file systems and analyze the assumptions made in practice.

Wireless communication provides unique security challenges, but also enables novel ways to defend against attacks. In the past few years, a number of works discussed the use of friendly jamming to protect the confidentiality of the communicated data as well as to enable message authentication and access control. In this work, we analytically and experimentally evaluate the confidentiality that can be achieved by the use of friendly jamming, given an attacker with multiple receiving antennas. We construct a MIMO-based attack that allows the attacker to recover data protected by friendly jamming and refine the conditions for which this attack is most effective. Our attack shows that friendly jamming cannot provide strong confidentiality guarantees in all settings. We further test our attack in a setting where friendly jamming is used to protect the communication to medical implants.

The packet-pair technique is a widely adopted method to estimate the capacity of a path. The use of the packet-pair technique has been suggested in numerous applications including network management and end-to-end admission control. Recent observations also indicate that this technique can be used to fingerprint Internet paths. However, given that packet-pair measurements are performed in an open environment, end-hosts might try to alter these measurements to increase their gain in the network. In this paper, we explore the security of measurements based on the packet-pair technique. More specifically, we analyze the major threats against bandwidth estimation using the packet-pair technique and we demonstrate empirically that current implementations of this technique are vulnerable to a wide range of bandwidth manipulation attacks-in which end-hosts can accurately modify their claimed bandwidths. We propose lightweight countermeasures to detect attacks on bandwidth measurements; our technique can detect whether delays were inserted within the transmission of a packet-pair (e.g., by bandwidth shapers). We further propose a novel scheme for remote path identification using the distribution of packet-pair dispersions and we evaluate its accuracy, robustness, and potential use. Our findings suggest that the packet-pair technique can reveal valuable information about the identity/locations of remote hosts.

Modern smartphones that implement permission-based security mechanisms suffer from attacks by colluding applications. Users are not made aware of possible implications of application collusion attacks---quite the contrary---on existing platforms, users are implicitly led to believe that by approving the installation of each application independently, they can limit the damage that an application can cause.

The growing complexity and increased networking of security and safety-critical systems expose them to the risk of adversarial compromise through remote attacks. These attacks can result in full system compromise, but often the attacker gains control only over some system components (e.g., a peripheral) and over some applications running on the system. We consider the latter scenario and focus on enabling on-schedule execution of critical applications that are running on a partially compromised system --- we call this trusted scheduling. We identify the essential properties needed for the realization of a trusted scheduling system and we design an embedded system that achieves these properties. We show that our system protects not only against misbehaving applications but also against attacks by compromised peripherals. We evaluate the feasibility and performance of our system through a prototype implementation based on the AVR ATmega103 microcontroller.

Notwithstanding the latest buzzword (grid, cloud, utility computing, SaaS, etc.), large-scale computing and cloud-like infrastructures are here to stay. How exactly they will look like tomorrow is still for the markets to decide, yet one thing is certain: clouds bring with them new untested deployment and associated adversarial models and vulnerabilities. It is essential that our community becomes involved at this early stage. The CCSW workshop aims to bring together researchers and practitioners in all security aspects of cloud-centric and outsourced computing to act as a fertile ground for creative debate and interaction in security-sensitive areas of computing impacted by clouds.

Bitcoin is a decentralized payment system that relies on Proof-of-Work (PoW) to verify payments. Nowadays, Bitcoin is increasingly used in a number of fast payment scenarios, where the time between the exchange of currency and goods is short (in the order of few seconds). While the Bitcoin payment verification scheme is designed to prevent double-spending, our results show that the system requires tens of minutes to verify a transaction and is therefore inappropriate for fast payments. An example of this use of Bitcoin was recently reported in the media: Bitcoins were used as a form of \emph{fast} payment in a local fast-food restaurant. Until now, the security of fast Bitcoin payments has not been studied. In this paper, we analyze the security of using Bitcoin for fast payments. We show that, unless appropriate detection techniques are integrated in the current Bitcoin implementation, double-spending attacks on fast payments succeed with overwhelming probability and can be mounted at low cost. We further show that the measures recommended by Bitcoin developers for the use of Bitcoin in fast payments are not always effective in detecting double-spending; we show that if those recommendations are integrated in future Bitcoin implementations, double-spending attacks on Bitcoin will still be possible. Finally, we propose and implement a modification to the existing Bitcoin implementation that ensures the detection of double-spending attacks against fast payments.

Deleting a file from a storage medium serves two purposes: it reclaims storage resources and ensures that any sensitive information contained in the file becomes inaccessible. When done for the latter purpose, it is critical that the file is securely deleted, meaning that its content does not persist on the storage medium after deletion. Secure deletion is the act of deleting data from a storage medium such that the data is afterwards irrecoverable from the storage medium. The time between deleting data and it becoming irrecoverable is called the deletion latency.

After several years of theoretical research on distance bounding protocols, the first implementations of such protocols have recently started to appear. These protocols are typically analyzed with respect to three types of attacks, which are historically known as Distance Fraud, Mafia Fraud, and Terrorist Fraud. We define and analyze a fourth main type of attack on distance bounding protocols, called Distance Hijacking. This type of attack poses a serious threat in many practical scenarios. We show that many proposed distance bounding protocols are vulnerable to Distance Hijacking, and we propose solutions to make these protocols resilient to this type of attack. We show that verifying distance bounding protocols using existing informal and formal frameworks does not guarantee the absence of Distance Hijacking attacks. We extend a formal framework for reasoning about distance bounding protocols to include overshadowing attacks. We use the resulting framework to prove the absence of all of the found attacks for protocols to which our countermeasures have been applied.

Chirp signals have been extensively used in radar and sonar systems to determine distance, velocity and angular position of objects and in wireless communications as a spread spectrum technique to provide robustness and high processing gain. Recently, several standards have adopted chirp spread spectrum (CSS) as an underlying physical-layer scheme for precise, low-power and low-complexity real-time localization. While CSS-based ranging and localization solutions have been implemented and deployed, their security has so far not been analyzed.

The deployment of RFID poses a number of security and privacy threats such as cloning, unauthorized tracking, etc. Although the literature contains many investigations of these issues on the logical level, few works have explored the security implications of the physical communication layer. Recently, related studies have shown the feasibility of identifying RFID-enabled devices based on physical-layer fingerprints. In this work, we leverage on these findings and demonstrate that physical-layer identification of HF RFID devices is also practical, that is, can achieve high accuracy and stability. We propose an improved hardware setup and enhanced techniques for fingerprint extraction and matching. Our new system enables device identification with an Equal Error Rate as low as 0.005 (0.5%) on a set 50 HF RFID smart cards of the same manufacturer and type. We further investigate the fingerprint stability over an extended period of time and across different acquisition setups. In the latter case, we propose a solution based on channel equalization that preserves the fingerprint quality across setups. Our results strengthen the practical use of physical-layer identification of RFID devices in product and document anti-counterfeiting solutions.

The integration of Trusted Computing technologies into virtualized computing environments enables the hardware-based protection of private information and the detection of malicious software. Their use in virtual platforms, however, requires appropriate virtualization of their main component, the Trusted Platform Module (TPM) by means of virtual TPMs (vTPM). The challenge here is that the use of TPM virtualization should not impede classical platform processes such as virtual machine (VM) migration.

An increasing number of wireless applications rely on GPS signals for localization, navigation, and time synchronization. However, civilian GPS signals are known to be susceptible to spoofing attacks which make GPS receivers in range believe that they reside at locations different than their real physical locations. In this paper, we investigate the requirements for successful GPS spoofing attacks on individuals and groups of victims with civilian or military GPS receivers. In particular, we are interested in identifying from which locations and with which precision the attacker needs to generate its signals in order to successfully spoof the receivers. We will show, for example, that any number of receivers can easily be spoofed to one arbitrary location; however, the attacker is restricted to only few transmission locations when spoofing a group of receivers while preserving their constellation. In addition, we investigate the practical aspects of a satellite-lock takeover, in which a victim receives spoofed signals after first being locked on to legitimate GPS signals. Using a civilian GPS signal generator, we perform a set of experiments and find the minimal precision of the attacker's spoofing signals required for covert satellite-lock takeover.

Several studies ranked security and privacy to be major areas of concern and impediments of cloud adoption for companies, but none have looked into end-users' attitudes and practices. Not much is known about consumers' privacy beliefs and expectations for cloud storage, such as web-mail, document and photo sharing platforms, or about users' awareness of contractual terms and conditions. We conducted 36 in-depth interviews in Switzerland and India (two countries with different privacy perceptions and expectations); and followed up with an online survey with 402 participants in both countries. We study users' privacy attitudes and beliefs regarding their use of cloud storage systems. Our results show that privacy requirements for consumer cloud storage differ from those of companies. Users are less concerned about some issues, such as guaranteed deletion of data, country of storage and storage outsourcing, but are uncertain about using cloud storage. Our results further show that end-users consider the Internet intrinsically insecure and prefer local storage for sensitive data over cloud storage. However, users desire better security and are ready to pay for services that provide strong privacy guarantees. Participants had misconceptions about the rights and guarantees their cloud storage providers offers. For example, users believed that their provider is liable in case of data loss, does not have the right to view and modify user data, and cannot disable user accounts. Finally, our results show that cultural differences greatly influence user attitudes and beliefs, such as their willingness to store sensitive data in the cloud and their acceptance that law enforcement agencies monitor user accounts. We believe that these observations can help in improving users privacy in cloud storage systems.

Currently, several online businesses deem that advertising revenues alone are not sufficient to generate profits and are therefore set to charge for online content. In this paper, we explore a complement to the current advertisement model; more specifically, we propose a micropayment model for non-specialized commodity web-services based on microcomputations. In our model, a user that wishes to access online content offered by a website does not need to register or pay to access the website; instead, he will accept to run microcomputations on behalf of the website in exchange for access to the content. These microcomputations can, for example, support ongoing computing projects that have clear social benefits (e.g., projects relating to HIV, dengue, cancer, etc.) or can contribute towards commercial computing projects. We argue that this micropayment model is economically and technically viable and that it can be integrated in existing distributed computing frameworks (e.g., the BOINC platform). We implement a preliminary prototype of a system based on our model through which we evaluate its performance and usability. Finally, we analyze the security and privacy of our proposal and we show that it ensures payment for the content while preserving the privacy of users.

Traditional security protocols are mainly concerned with authentication and key establishment and rely on predistributed keys and properties of cryptographic operators. In contrast, new application areas are emerging that establish and rely on properties of the physical world. Examples include protocols for secure localization, distance bounding, and secure time synchronization. We present a formal model for modeling and reasoning about such physical security protocols. Our model extends standard, inductive, trace-based, symbolic approaches with a formalization of physical properties of the environment, namely communication, location, and time. In particular, communication is subject to physical constraints, for example, message transmission takes time determined by the communication medium used and the distance between nodes. All agents, including intruders, are subject to these constraints and this results in a distributed intruder with restricted, but more realistic, communication capabilities than those of the standard Dolev-Yao intruder. We have formalized our model in Isabelle/HOL and have used it to verify protocols for authenticated ranging, distance bounding, broadcast authentication based on delayed key disclosure, and time synchronization.

We address the problem of confidentiality in scenarios where the attacker is not only able to observe the communication between principals, but can also fully compromise the communicating parties (their devices, not only their long term secrets) after the confidential data has been exchanged. We formalize this problem and explore solutions that provide confidentiality after the full compromise of devices and user passwords. We propose two new solutions that use explicit key deletion and forward-secret protocols combined with key storage on porter devices. Our solutions provide the users with control over their privacy. We analyze the proposed solutions using an automatic verification tool. We also implement a prototype using a mobile phone as a porter device to illustrate how the solution can be realized on modern platforms.

In this work, we study physical-layer identification of passive UHF RFID tags. We collect signals from a population of 70 tags using a purpose-built reader and we analyze time domain and spectral features of the collected signals. We show that, based on timing features of the signals, UHF RFID tags can be classified, independently of the location and distance to the reader (evaluated up to 6 meters), with an accuracy of approx. 71% (within our population). Additionally, we show that is possible to uniquely identify a maximum of approx. 26 UHF RFID tags independently of the population size. We analyze the implications of these results on tag holder privacy. We further show that, in controlled environments, UHF RFID tags can be uniquely identified based on their signal spectral features with an Equal Error Rate of 0% (within our population); we discuss the application of those techniques to cloning detection in RFID-enabled supply chains.

Recent years have seen a proliferation of secure device pairing methods that try to improve both the usability and security of today's de-facto standard -- PIN-based authentication. Evaluating such improvements is difficult. Most comparative laboratory studies have so far mainly focused on completeness, trying to find the single best method among the dozens of proposed approaches -- one that is both rated the most usable by test subjects, and which provides the most robust security guarantees. This search for the "best" pairing method, however, fails to take into account the variety of situations in which such pairing protocols may be used in real life. The comparative study reported here, therefore, explicitly situates pairing tasks in a number of more realistic situations. Our results indicate that people do not always use the easiest or most popular method -- they instead prefer different methods in different situations, based on the sensitivity of data involved, their time constraints, and the social conventions appropriate for a particular place and setting. Our study also provides qualitative data on factors influencing the perceived security of a particular method, the users' mental models surrounding security of a method, and their security needs.

Physical-layer identification of wireless devices, commonly referred to as Radio Frequency (RF) fingerprinting, is the process of identifying a device based on transmission imperfections exhibited by its radio transceiver. It can be used to improve access control in wireless networks, revent device cloning and complement message authentication protocols. This paper studies the feasibility of performing impersonation attacks on the modulation-based and transient-based fingerprinting techniques. Both techniques are vulnerable to impersonation attacks; however, transient-based techniques are more difficult to reproduce due to the effects of the wireless channel and antenna in their recording process. We assess the feasibility of performing impersonation attacks by extensive measurements as well as simulations using collected data from wireless devices. We discuss the implications of our findings and how they affect current device identification techniques and related applications.

Jamming-resistant communication is crucial for safety-critical applications such as emergency alert broadcasts or the dissemination of navigation signals in adversarial settings. In such applications, mission-critical messages are broadcast to a large and unknown number of (potentially untrusted) receivers that rely on the availability, integrity, and authenticity of the messages; here, availability primarily refers to the ability to communicate in the presence of jamming. Common techniques to counter jamming-based denial-of-service attacks such as Frequency Hopping (FH) and Direct Sequence Spread Spectrum (DSSS) cannot be applied in such settings because they depend on secret pairwise or group keys shared between the sender and the receivers before the communication. This dependency entails serious or unsolvable scalability and keysetup problems or weak jamming-resistance (a single malicious receiver can compromise the whole system). As a solution, in this work, we propose uncoordinated spread spectrum techniques that enable anti-jamming broadcast communication without shared secrets. Uncoordinated spread spectrum techniques can handle an unlimited amount of (malicious) receivers. We present two instances (Uncoordinated FH and Uncoordinated DSSS) and analyze differences in their performance as well as their combination. We further discuss the applications of these techniques to anti-jamming navigation broadcast, bootstrapping of coordinated spread spectrum communication, and anti-jamming emergency alerts.

Despite years of intensive research, the main deterrents of widely deploying secure communication between wireless nodes remains the cumbersome key setup process. In this paper, we address this problem and we introduce Integrity (I) regions, a novel security primitive that enables message authentication in wireless networks without the use of preestablished or precertified keys. Integrity regions are based on the verification of entity proximity through time-of-arrival ranging techniques. IRegions can be efficiently implemented with ultrasonic ranging, in spite of the fact that ultrasound ranging techniques are vulnerable to distance enlargement and reduction attacks. We further show how IRegions can be used for key establishment in mobile peer-to-peer wireless networks and we propose a novel automatic key establishment approach, largely transparent to users, by leveraging on IRegions and nodes' mobility. We analyze our proposals against a multitude of security threats and we validate our findings via extensive simulations.

An integral part of most security- and safety-critical applications is a dependable and timely alarm notification. However, owing to the resource constraints of wireless sensor nodes (i.e., their limited power and spectral diversity), ensuring a timely and jamming-resistant delivery of alarm messages in applications that rely on wireless sensor networks is a challenging task. With current alarm forwarding schemes, blocking of an alarm by jamming is straightforward and jamming is very likely to remain unnoticed. In this work, we propose a novel jamming detection scheme as a solution to this problem. Our scheme is able to identify the cause of bit errors for individual packets by looking at the received signal strength during the reception of these bits and is well-suited for the protection of reactive alarm systems with very low network traffic. We present three different techniques for the identification of bit errors based on: predetermined knowledge, error correcting codes, and limited node wiring. We perform a detailed evaluation of the proposed solution and validate our findings experimentally with Chipcon CC1000 radios. The results show that our solution effectively detects sophisticated jamming attacks that cannot be detected with existing techniques and enables the formation of robust sensor networks for dependable delivery of alarm notifications. Our scheme also meets the high demands on the energy efficiency of reactive surveillance applications as it can operate without introducing additional wireless network traffic.

We propose a proximity-based access control scheme for implantable medical devices (IMDs). Our scheme is based on ultrasonic distance-bounding and enables an implanted medical device to grant access to its resources only to those devices that are in its close proximity. We demonstrate the feasibility of our approach through tests in an emulated patient environment. We show that, although implanted, IMDs can successfully verify the proximity of other devices with high accuracy. We propose a set of protocols that support our scheme, analyze their security in detail and discuss possible extensions. We make new observations about the security of implementations of ultrasonic distance-bounding protocols. Finally, we discuss the integration of our scheme with existing IMD devices and with their existing security measures.

We present a formal model for modeling and reasoning about security protocols. Our model extends standard, inductive, trace-based, symbolic approaches with a formalization of physical properties of the environment, namely communication, location, and time. In particular, communication is subject to physical constraints, for example, message transmission takes time determined by the communication medium used and the distance traveled. All agents, including intruders, are subject to these constraints and this results in a distributed intruder with restricted, but more realistic, communication capabilities than those of the standard Dolev-Yao intruder. We have formalized our model in Isabelle/HOL and used it to verify protocols for authenticated ranging, distance bounding, and broadcast authentication based on delayed key disclosure.

We address the problem of jamming-resistant communication in scenarios in which the communicating parties do not share secret keys. This includes scenarios where the communicating parties are not known in advance or where not all parties can be trusted (e.g., jamming-resistant key establishment or anti-jamming broadcast to a large set of unknown receivers). In these cases, the deployment of shared secret keys is unrealistic, and therefore this problem cannot be solved using existing anti-jamming solutions like FHSS and DSSS that depend on pre-shared keys. Recently, a solution to this problem has been proposed that introduces Uncoordinated Frequency Hopping (UFH), a new spread-spectrum anti-jamming technique that does not rely on secret keys. In this work, we investigate the efficiency of UFH-based communication: we identify optimal strategies for the UFH frequency channel selection and we propose a set of new UFH-based anti-jamming schemes that, compared to the original UFH proposal, reduce the communication latency up to one-half (i.e., increase UFH communication throughput up to two times).

In this work, we study the security of public WLAN-based positioning systems. Specifically, we investigate the Skyhook positioning system, available on PCs and used on a number of mobile platforms, including Apple's iPod touch and iPhone. By implementing and analyzing several kinds of attacks, we demonstrate that this system is vulnerable to location spoofing and location database manipulation. In both, the attacker can arbitrarily change the result of the localization at the victim device, by either impersonating remote infrastructure or by tampering with the service database. Our attacks can easily be replicated and we conjecture that--without appropriate countermeasures--public WLAN-based positioning should therefore be used with caution in safety-critical contexts. We further discuss several approaches for securing WLAN-based positioning systems.

Peer-to-peer (P2P) architectures are gaining increasing popularity in disseminating content to a large number of nodes. In this paper, we show that small coalitions between peers can further enhance the performance of current P2P architectures. Small coalitions bridge the gap between inefficient non-cooperative and fully cooperative architectures by establishing a robust tradeoff between the complexity and performance of the resource distribution process. Owing to their small size, small coalitions are inherently resilient to the churn in existing P2P systems and embed natural incentives for peers to self-organize in order to improve their download times. We evaluate several coalition strategies analytically and empirically via simulations and we show that our solutions considerably improve the download performance in current P2P systems.

Distance bounding protocols have been proposed for many security critical applications as a means of getting an upper bound on the physical distance to a communication partner. As such, distance bounding protocols are executed frequently, e.g., to keep node locations up to date, etc. We analyze distance bounding protocols in terms of their location privacy and we show that they leak information about the location and distance between communicating partners even to passive attackers. This location and distance information may be highly sensitive since it can form the basis for access control, key establishment, or be used as input to location aware applications. We analyze, in a number of scenarios, how much information distance bounding protocols leak. We further discuss several straightforward countermeasures and show why they do not provide adequate protection against distance leakage. Finally, we propose a location private distance bounding protocol that maintains the properties of existing distance bounding protocols while leaking no information about the distance measured between the communicating parties.

We consider the following problem: how can two devices that do not share any secrets establish a shared secret key over a wireless radio channel in the presence of a communication jammer? An inherent challenge in solving this problem is that known anti-jamming techniques (e.g.,frequency hopping or direct-sequence spread spectrum) which should support device communication during the key establishment require that the devices share a secret spreading key (or code) prior to the start of their communication. This requirement creates a circular dependency between anti-jamming spread-spectrum communication and key establishment, which has so far not been addressed. In this work, we propose an Uncoordinated Frequency Hopping (UFH) scheme that breaks this dependencyand enables key establishment in the presence of a communication jammer. We perform a detailed analysis of our UFH scheme and show its feasibility, both in terms of execution time and resource requirements.

Inspired by unidirectional error detecting codes that are used in situations where only one kind of bit errors are possible (e.g., it is possible to change a bit "0" into a bit "1", but not the contrary), we propose integrity codes (I-codes) for a radio communication channel, which enable integrity protection of messages exchanged between entities that do not hold any mutual authentication material (i.e. public keys or shared secret keys). The construction of I-codes enables a sender to encode any message such that if its integrity is violated in transmission over a radio channel, the receiver is able to detect it. In order to achieve this, we rely on the physical properties of the radio channel and on unidirectional error detecting codes. We analyze in detail the use of I-codes on a radio communication channel and we present their implementation on a wireless platform as a "proof of concept". We further introduce a novel concept called "authentication through presence", whose broad applications include broadcast authentication, key establishment and navigation signal protection. We perform a detailed analysis of the security of our coding scheme and we show that it is secure within a realistic attacker model.

Time synchronization is critical in sensor networks at many layers of their design. It enables better duty-cycling of the radio, accurate and secure localization, beamforming, and other collaborative signal processing tasks. These benefits make time-synchronization protocols a prime target of malicious adversaries who want to disrupt the normal operation of a sensor network. In this article, we analyze attacks on existing time synchronization protocols for wireless sensor networks and we propose a secure time synchronization toolbox to counter these attacks. This toolbox includes protocols for secure pairwise and group synchronization of nodes that either lie in the neighborhood of each other or are separated by multiple hops. We provide an in-depth analysis of the security and the energy overhead of the proposed protocols. The efficiency of these protocols has been tested through an experimental study on Mica2 motes.

Secure bootstrapping of mobile ad hoc networks (MANETs) is a challenging problem in scenarios in which network users (or nodes) do not share trust relationships prior to the network deployment. In recent years, a number of schemes have been proposed to solve this problem, assuming either no or limited trust between the nodes prior to their deployment. Despite numerous proposals, there is no common understanding of the proposed schemes and of the trade-offs that they provide. This has consequences for both researchers and practitioners, who do not have a clear idea how to compare the schemes and how to select a scheme for a given application. In this article, we present a framework that helps in understanding and comparing schemes for secure bootstrapping of MANETs. The framework is general because it is policy-neutral and can accommodate many existing bootstrapping schemes. The proposed framework can equally serve as a good basis for the development of new MANET bootstrapping schemes; we show how the development of the framework leads to two new (classes of) distributed bootstrapping schemes. Within the framework, we not only investigate and characterize the properties of the relevant bootstrapping schemes, but also give methods for practitioners to select the relevant system parameters in the Random Walk and the (Restricted) Random Waypoint mobility models.

In this work, we propose and analyze a new approach for securing localization and location verification in wireless networks based on hidden and mobile base stations. Our approach enables secure localization with a broad spectrum of localization techniques: ultrasonic or radio, based on received signal strength or signal time of flight. Through several examples we show how this approach can be used to secure node-centric and infrastructure-centric localization schemes. We further show how this approach can be applied to secure localization in mobile ad-hoc and sensor networks.

We propose SecNav, a new protocol for securing wireless navigation systems. This protocol secures localization and time synchronization in wireless networks by relying on devices' awareness of presence in the power-range (coverage area) of navigation stations. We perform a detailed security analysis of SecNav and show that, compared to existing secure navigation approaches, it prevents the widest range of attacks on navigation. Our implementation of SecNav, using 802.11b devices, shows that this scheme can be efficiently implemented with existing technologies.

Due to their very nature, wireless sensor networks are probably the category of wireless networks most vulnerable to "radio channel jamming"-based denial-of-service (DoS) attacks. An adversary can easily mask the events that the sensor network should detect by stealthily jamming an appropriate subset of the nodes; in this way, he prevents them from reporting what they are sensing to the network operator. Therefore, even if an event is sensed by one or several nodes (and the sensor network is otherwise fully connected), the network operator cannot be informed on time. We show how the sensor nodes can exploit channel diversity in order to create wormholes that lead out of the jammed region, through which an alarm can be transmitted to the network operator. We propose three solutions. The first is based on wired pairs of sensors, the second relies on frequency hopping, and the third is based on a novel concept called uncoordinated channel hopping. We develop appropriate mathematical models to study the proposed solutions

Abstract — Until recently, the problem of localization in wireless networks has been mainly studied in a nonadversarial setting. Only recently, a number of solutions have been proposed that aim to detect and prevent attacks on localization systems. In this work, we propose a new approach to secure localization based on hidden and mobile base stations. Our approach enables secure localization with a broad spectrum of localization techniques: ultrasonic or radio, based on received signal strength or signal time of flight. Through several examples we show how this approach can be used to secure nodecentric and infrastructure-centric localization schemes. We further show how this approach can be applied to secure localization in sensor networks. I.

Inspired by unidirectional error detecting codes that are used in situations where only one kind of bit errors are possible (e.g., it is possible to change a bit "0" into a bit "1", but not the contrary), we propose integrity codes (I-codes) for a radio communication channel, which enable integrity protection of messages exchanged between entities that do not hold any mutual authentication material (i.e. public keys or shared secret keys). The construction of I-codes enables a sender to encode any message such that if its integrity is violated in transmission over a radio channel, the receiver is able to detect it. In order to achieve this, we rely on the physical properties of the radio channel. We analyze in detail the use of I-codes on a radio communication channel and we present their implementation on a Mica2 wireless sensor platform as a "proof of concept". We finally introduce a novel concept called "authentication through presence" that can be used for several applications, including for key establishment and for broadcast authentication over an insecure radio channel. We perform a detailed analysis of the security of our coding scheme and we show that it is secure with respect to a realistic attacker model.

So far, the problem of positioning in wireless networks has been studied mainly in a nonadversarial setting. In this paper, we analyze the resistance of positioning techniques to position and distance spoofing attacks. We propose a mechanism for secure positioning of wireless devices, that we call verifiable multilateration. We then show how this mechanism can be used to secure positioning in sensor networks. We analyze our system through simulations.

We present a set of simple techniques for key establishment over a radio link in peer-to-peer networks. Our approach is based on the Diffie-Hellmankey agreement protocol, which is known to be vulnerable to the "man-in-the-middle" attack if the two users involved in the protocol do not share any authenticated information about each other (e.g., public keys, certificates, passwords,shared keys, etc.) prior to the protocol execution. In this paper, we solve the problem by leveraging on the natural ability of users to authenticate each other by visual and verbal contact. We propose three techniques. The first is based on visual comparison of short strings, the second on distance bounding, and the third on integrity codes; in each case, the users do not need to enter any password or other data, nor do they need physical or infrared connectivity between their devices. We base our analysis on a well-established methodology that leads us to a rigorous modularization and a thorough robustness proof of our proposal.

We propose a straightforward technique to provide peer-to-peer security in mobile networks. We show that far from being a hurdle, mobility can be exploited to set up security associations among users. We leverage on the temporary vicinity of users, during which appropriate cryptographic protocols are run. We illustrate the operation of the solution in two scenarios, both in the framework of mobile ad hoc networks. In the first scenario, we assume the presence of an offline certification authority and we show how mobility helps to set up security associations for secure routing; in this case, the security protocol runs over one-hop radio links. We further show that mobility can be used for the periodic renewal of vital security information (e.g., the distribution of hash chain/Merkle tree roots). In the second scenario, we consider fully self-organized security: Users authenticate each other by visual contact and by the activation of an appropriate secure side channel of their personal device; we show that the process can be fuelled by taking advantage of trusted acquaintances. We then show that the proposed solution is generic: It can be deployed on any mobile network and it can be implemented either with symmetric or with asymmetric cryptography. We provide a performance analysis by studying the behavior of the solution in various scenarios.

So far, the problem of positioning in wireless networks has been mainly studied in a non-adversarial setting. In this work, we analyze the resistance of positioning techniques to position and distance spoofing attacks. We propose a mechanism for secure positioning of wireless devices, that we call verifiable multilateration. We then show how this mechanism can be used to secure positioning in sensor networks. We analyze our system through simulations.

We address the problem of secure location determination, known as secure localization, and the problem of verifying the location claim of a node, known as location verification, in wireless sensor networks (WSN). We propose a robust positioning system we call ROPE that allows sensors to determine their location without any centralized computation. In addition, ROPE provides a location verification mechanism that verifies the location claims of the sensors before data collection. We show that ROPE bounds the ability of an attacker to spoof sensors' locations, with relatively low density deployment of reference points. We confirm the robustness of ROPE against attacks analytically and via simulations.

Recently, researchers have proposed a number of ranging and positioning techniques for wireless networks. However, they all studied these techniques in non-adversarial settings. Distance estimation and positioning techniques are, nevertheless, highly vulnerable to attacks from dishonest nodes and malicious attackers. Internal attackers can report false position and distance information in order to cheat on their locations and external attackers can modify the measured positions and distances of wireless nodes. In this work, we demonstrate two attacks on ultrasonic ranging systems: the wormhole attack, by which the attackers reducethe distance measured between two honest nodes, and the pulsedelay attack, by which the attackers enlarge the measured distance. With these attacks, we show that the attackers can arbitrarily modify distances measured with ultrasonic ranging, despite the authentication and integrity protection of the messages used in the ranging protocol. Based on this difference, B estimates its distance to A.

We propose a formal model of rational exchange and exchange protocols in general, which is based on game theory. In this model, an exchange protocol is represented as a set of strategies in a game that is played by the protocol parties and the network that they use to communicate with each other. W ithin this model, we give a formal definition for rational exchange and various other properties of exchange protocols, including fairness. In particular, rational exchange is defined in terms of a Nash equilibrium in the protocol game. We also study the relationship between rational and fair exchange, and prove that fairness implies rationality, but not vice versa. Finally, we illustrate the usage of our formal model for the analysis of existing rational exchange protocols by analyzing a protocol proposed by Syverson. We show that the protocol is rational only under the assumption that the network is reliable.

Contrary to the common belief that mobility makes security more di#cult to achieve, we show that node mobility can, in fact, be useful to provide security in ad hoc networks. We propose a technique in which security associations between nodes are established, when they are in the vicinity of each other, by exchanging appropriate cryptographic material. We show that this technique is generic, by explaining its application to fully self-organized ad hoc networks and to ad hoc networks placed under an (o#-line) authority. We also propose an extension of this basic mechanism, in which a security association can be established with the help of a "friend". We show that our mechanism can work in any network configuration and that the time necessary to set up the security associations is strongly influenced by several factors, including the size of the deployment area, the mobility patterns, and the number of friends; we provide a detailed investigation of this influence.

In contrast with conventional networks, mobile ad hoc networks usually do not provide online access to trusted authorities or to centralized servers, and they exhibit frequent partitioning due to link and node failures and to node mobility. For these reasons, traditional security solutions that require online trusted authorities or certificate repositories are not well-suited for securing ad hoc networks. We propose a fully self-organized public-key management system that allows users to generate their public-private key pairs, to issue certificates, and to perform authentication regardless of the network partitions and without any centralized services. Furthermore, our approach does not require any trusted authority, not even in the system initialization phase.

In this paper, we provide a formal analysis of a rational exchange protocol proposed by Syverson. A rational exchange protocol guarantees that misbehavior cannot generate benefits, and is therefore discouraged. The analysis is performed using our formal model, which is based on game theory. In this model, rational exchange is defined in terms of a Nash equilibrium.

We propose a new approach to securing self-organized mobile ad hoc networks. In this approach, security is achieved in a fully self-organized manner; by this we mean that the security system does not require any kind of certification authority or centralized server, even for the initialization phase. In our work, we were inspired by PGP [15] because its operation relies solely on the acquaintances between users. We show that the small-world phenomenon naturally emerges in the PGP system as a consequence of the self-organization of users. We show this by studying the PGP certificate graph properties and by quantifying its small-world characteristics. We argue that the certificate graphs of self-organized security systems will exhibit a similar small-world phenomenon, and we provide a way to model self-organized certificate graphs. The results of the PGP certificate graph analysis and graph modelling can be used to build new self-organized security systems and to test the performance of the existing proposals. In this work, we refer to such an example.

So far, research on mobile ad hoc networks has been forcused primarily on routing issues. Security, on the other hand, has been given a lower priority. This paper provides an overview of security problems for mobile ad hoc networks, distinguishing the threats on basic mechanisms and on security mechanisms. It then describes our solution to protect the security mechanisms. The original features of this solution include that (i) it is fully decentralized and (ii) all nodes are assigned equivalent roles.