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## Über dieses Buch

This book constitutes the thoroughly refereed post-conference proceedings of the 20th International Conference on Financial Cryptography and Data Security, FC 2016, held in Christ church, Barbados, in February 2016.

The 27 revised full papers and 9 short papers were carefully selected and reviewed from 137 full papers submissions. The papers are grouped in the following topical sections: fraud and deception; payments, auctions, and e-voting; multiparty computation; mobile malware; social interaction and policy; cryptanalysis; surveillance and anonymity; Web security and data privacy; Bitcoin mining; cryptographic protocols; payment use and abuse.

## Inhaltsverzeichnis

### Understanding Craigslist Rental Scams

Fraudulently posted online rental listings, rental scams, have been frequently reported by users. However, our understanding of the structure of rental scams is limited. In this paper, we conduct the first systematic empirical study of online rental scams on Craigslist. This study is enabled by a suite of techniques that allowed us to identify scam campaigns and our automated system that is able to collect additional information by conversing with scammers. Our measurement study sheds new light on the broad range of strategies different scam campaigns employ and the infrastructure they depend on to profit. We find that many of these strategies, such as credit report scams, are structurally different from the traditional advanced fee fraud found in previous studies. In addition, we find that Craigslist remove less than half of the suspicious listings we detected. Finally, we find that many of the larger-scale campaigns we detected depend on credit card payments, suggesting that a payment level intervention might effectively demonetize them.

Youngsam Park, Damon McCoy, Elaine Shi

### Graph Analytics for Real-Time Scoring of Cross-Channel Transactional Fraud

We present a new approach to cross channel fraud detection: build graphs representing transactions from all channels and use analytics on features extracted from these graphs. Our underlying hypothesis is community based fraud detection: an account (holder) performs normal or trusted transactions within a community that is “local” to the account. We explore several notions of community based on graph properties. Our results show that properties such as shortest distance between transaction endpoints, whether they are in the same strongly connected component, whether the destination has high page rank, etc., provide excellent discriminators of fraudulent and normal transactions whereas traditional social network analysis yields poor results. Evaluation on a large dataset from a European bank shows that such methods can substantially reduce false positives in traditional fraud scoring. We show that classifiers built purely out of graph properties are very promising, with high AUC, and can complement existing fraud detection approaches.

Ian Molloy, Suresh Chari, Ulrich Finkler, Mark Wiggerman, Coen Jonker, Ted Habeck, Youngja Park, Frank Jordens, Ron van Schaik

### Android UI Deception Revisited: Attacks and Defenses

App-based deception attacks are increasingly a problem on mobile devices and they are used to steal passwords, credit card numbers, text messages, etc. Current versions of Android are susceptible to these attacks. Recently, Bianchi et al. proposed a novel solution “What the App is That” that included a host-based system to identify apps to users via a security indicator and help assure them that their input goes to the identified apps [7]. Unfortunately, we found that the solution has a significant side channel vulnerability as well as susceptibility to clickjacking that allow non-privileged malware to completely compromise the defenses, and successfully steal passwords or other keyboard input. We discuss the vulnerabilities found, propose possible defenses, and then evaluate the defenses against different types of UI deception attacks.

Earlence Fernandes, Qi Alfred Chen, Justin Paupore, Georg Essl, J. Alex Halderman, Z. Morley Mao, Atul Prakash

### Introducing Reputation Systems to the Economics of Outsourcing Computations to Rational Workers

Outsourcing computation to remote parties (“workers”) is an increasingly common practice, owing in part to the growth of cloud computing. However, outsourcing raises concerns that outsourced tasks may be completed incorrectly, whether by accident or because workers cheat to minimize their cost and optimize their gain. The goal of this paper is to explore, using game theory, the conditions under which the incentives for all parties can be configured to efficiently disincentivize worker misbehavior, either inadvertent or deliberate. By formalizing multiple scenarios with game theory, we establish conditions to discourage worker cheating that take into account the dynamics of multiple workers, workers with limited capacity, and changing levels of trust. A key novelty of our work is modeling the use of a reputation system to decide how computation tasks are allocated to workers based on their reliability, and we provide insights on strategies for using a reputation system to increase the expected quality of results. Overall, our results contribute to make outsourcing computation more reliable, consistent, and predictable.

Jassim Aljuraidan, Lujo Bauer, Michael K. Reiter, Matthias Beckerle

### Accountable Privacy for Decentralized Anonymous Payments

Decentralized ledger-based currencies such as Bitcoin provide a means to construct payment systems without requiring a trusted bank. Removing this trust assumption comes at the significant cost of transaction privacy. A number of academic works have sought to improve the privacy offered by ledger-based currencies using anonymous electronic cash (e-cash) techniques. Unfortunately, this strong degree of privacy creates new regulatory concerns, since the new private transactions cannot be subject to the same controls used to prevent individuals from conducting illegal transactions such as money laundering. We propose an initial approach to addressing this issue by adding privacy preserving policy-enforcement mechanisms that guarantee regulatory compliance, allow selective user tracing, and admit tracing of tainted coins (e.g., ransom payments). To accomplish this new functionality we also provide improved definitions for Zerocash and, of independent interest, an efficient construction for simulation sound zk-SNARKs.

Christina Garman, Matthew Green, Ian Miers

### Private eCash in Practice (Short Paper)

Most electronic payment systems for applications, such as eTicketing and eToll, involve a single entity acting as both merchant and bank. In this paper, we propose an efficient privacy-preserving post-payment eCash system suitable for this particular use case that we refer to, afterwards, as private eCash. To this end, we introduce a new partially blind signature scheme based on a recent Algebraic MAC scheme due to Chase et al. Unlike previous constructions, it allows multiple presentations of the same signature in an unlinkable way. Using it, our system is the first versatile private eCash system where users must only hold a sole reusable token (i.e. a reusable coin spendable to a unique merchant). It also enables identity and token revocations as well as flexible payments. Indeed, our payment tokens are updated in a partially blinded way to collect refunds without invading user’s privacy. By implementing it on a Global Platform compliant SIM card, we show its efficiency and suitability for real-world use cases, even for delay-sensitive applications and on constrained devices as a transaction can be performed in only 205 ms.

Amira Barki, Solenn Brunet, Nicolas Desmoulins, Sébastien Gambs, Saïd Gharout, Jacques Traoré

### Practically Efficient Secure Single-Commodity Multi-market Auctions

We study the problem of securely building single-commodity multi-markets auction mechanisms. We introduce a novel greedy algorithm and its corresponding privacy preserving implementation using secure multi-party computation. More specifically, we determine the quantity of supply and demand bids maximizing welfare. Each bid is attached to a specific market, but exchanges between different markets are allowed up to some upper limit. The general goal is for the players to bid their intended valuations without concerns about what the other players can learn. This problem is inspired by day-ahead electricity markets where there are substantial transmission capacity between the different markets, but applies to other commodity markets like gas. Furthermore, we provide computational results with a specific C++ implementation of our algorithm and the necessary MPC primitives. We can solve problems of 1945 bids and 4 markets in 1280 s when online/offline phases are considered. Finally, we report on possible set-ups, workload distributions and possible trade-offs for real-life applications of our results based on this experimentation and prototyping.

Abdelrahaman Aly, Mathieu Van Vyve

### How to Challenge and Cast Your e-Vote

An electronic voting protocol provides cast-as-intended verifiability if the voter can verify that her encrypted vote contains the voting options that she selected. There are some proposals of protocols with cast-as-intended verifiability in the literature, but all of them have drawbacks either in terms of usability or in terms of security. In this paper, we propose a new voting scheme with cast-as-intended verifiability which allows to audit the vote to be cast, while providing measures for avoiding coercion by allowing the voter to create fake proofs of the content of her vote. We provide an efficient implementation and formally analize its security properties.

Sandra Guasch, Paz Morillo

### VD-PSI: Verifiable Delegated Private Set Intersection on Outsourced Private Datasets

Private set intersection (PSI) protocols have many real world applications. With the emergence of cloud computing the need arises to carry out PSI on outsourced datasets where the computation is delegated to the cloud. However, due to the possibility of cloud misbehavior, it is essential to verify the integrity of any outsourced datasets, and results of any delegated computation. Verifiable Computation on private datasets that does not leak any information about the data is very challenging, especially when the datasets are outsourced independently by different clients. In this paper we present VD-PSI, a protocol that allows multiple clients to outsource their private datasets and delegate computation of set intersection to the cloud, while being able to verify the correctness of the result. Clients can independently prepare and upload their datasets, and with their agreement can verifiably delegate the computation of set intersection an unlimited number of times, without the need to download or maintain a local copy of their data. The protocol ensures that the cloud learns nothing about the datasets and the intersection. VD-PSI is efficient as its verification cost is linear to the intersection cardinality, and its computation and communication costs are linear to the (upper bound of) dataset cardinality. Also, we provide a formal security analysis in the standard model.

Aydin Abadi, Sotirios Terzis, Changyu Dong

### Confidential Benchmarking Based on Multiparty Computation

We report on the design and implementation of a system that uses multiparty computation to enable banks to benchmark their customers’ confidential performance data against a large representative set of confidential performance data from a consultancy house. The system ensures that both the banks’ and the consultancy house’s data stays confidential, the banks as clients learn nothing but the computed benchmarking score. In the concrete business application, the developed prototype helps Danish banks to find the most efficient customers among a large and challenging group of agricultural customers with too much debt. We propose a model based on linear programming for doing the benchmarking and implement it using the SPDZ protocol by Damgård et al., which we modify using a new idea that allows clients to supply data and get output without having to participate in the preprocessing phase and without keeping state during the computation. We ran the system with two servers doing the secure computation using a database with information on about 2500 users. Answers arrived in about 25 s.

Ivan Damgård, Kasper Damgård, Kurt Nielsen, Peter Sebastian Nordholt, Tomas Toft

### Efficiently Making Secure Two-Party Computation Fair

Secure two-party computation cannot be fair against malicious adversaries, unless a trusted third party (TTP) or a gradual-release type super-constant round protocol is employed. Existing optimistic fair two-party computation protocols with constant rounds are either too costly to arbitrate (e.g., the TTP may need to re-do almost the whole computation), or require the use of electronic payments. Furthermore, most of the existing solutions were proven secure and fair via a partial simulation, which, we show, may lead to insecurity overall. We propose a new framework for fair and secure two-party computation that can be applied on top of any secure two party computation protocol based on Yao’s garbled circuits and zero-knowledge proofs. We show that our fairness overhead is minimal, compared to all known existing work. Furthermore, our protocol is fair even in terms of the work performed by Alice and Bob. We also prove our protocol is fair and secure simultaneously, through one simulator, which guarantees that our fairness extensions do not leak any private information. Lastly, we ensure that the TTP never learns the inputs or outputs of the computation. Therefore, even if the TTP becomes malicious and causes unfairness by colluding with one party, the security of the underlying protocol is still preserved.

Handan Kılınç, Alptekin Küpçü

### Fast Optimistically Fair Cut-and-Choose 2PC

Secure two party computation (2PC) is a well-studied problem with many real world applications. Due to Cleve’s result on general impossibility of fairness, however, the state-of-the-art solutions only provide security with abort. We investigate fairness for 2PC in presence of a trusted Arbiter, in an optimistic setting where the Arbiter is not involved if the parties act fairly. Existing fair solutions in this setting are by far less efficient than the fastest unfair 2PC.We close this efficiency gap by designing protocols for fair 2PC with covert and malicious security that have competitive performance with the state-of-the-art unfair constructions. In particular, our protocols only requires the exchange of a few extra messages with sizes that only depend on the output length; the Arbiter’s load is independent of the computation size; and a malicious Arbiter can only break fairness, but not covert/malicious security even if he colludes with a party. Finally, our solutions are designed to work with the state-of-the-art optimizations applicable to garbled circuits and cut-and-choose 2PC such as free-XOR, half-gates, and the cheating-recovery paradigm.

Alptekin Küpçü, Payman Mohassel

### CuriousDroid: Automated User Interface Interaction for Android Application Analysis Sandboxes

Mobile computing has experienced enormous growth in market share and computational power in recent years. As a result, mobile malware is becoming more sophisticated and more prevalent, leading to research into dynamic sandboxes as a widespread approach for detecting malicious applications. However, the event-driven nature of Android applications renders critical the capability to automatically generate deterministic and intelligent user interactions to drive analysis subjects and improve code coverage. In this paper, we present CuriousDroid, an automated system for exercising Android application user interfaces in an intelligent, user-like manner. CuriousDroid operates by decomposing application user interfaces on-the-fly and creating a context-based model for interactions that is tailored to the current user layout. We integrated CuriousDroid with Andrubis, a well-known Android sandbox, and conducted a large-scale evaluation of 38,872 applications taken from different data sets. Our evaluation demonstrates significant improvements in both end-to-end sample classification as well as increases in the raw number of elicited behaviors at runtime.

Patrick Carter, Collin Mulliner, Martina Lindorfer, William Robertson, Engin Kirda

### DroydSeuss: A Mobile Banking Trojan Tracker (Short Paper)

After analyzing several Android mobile banking trojans, we observed the presence of repetitive artifacts that describe valuable information about the distribution of this class of malicious apps. Motivated by the high threat level posed by mobile banking trojans and by the lack of publicly available analysis and intelligence tools, we automated the extraction of such artifacts and created a malware tracker named DroydSeuss. DroydSeuss first processes applications both statically and dynamically, extracting relevant strings that contain traces of communication endpoints. Second, it prioritizes the extracted strings based on the APIs that manipulate them. Finally, DroydSeuss correlates the endpoints with descriptive metadata from the samples, providing aggregated statistics, raw data, and cross-sample information that allow researchers to pinpoint relevant groups of applications.We connected DroydSeuss to the VirusTotal daily feed, consuming Android samples that perform banking-trojan activity. We manually analyzed its output and found supporting evidence to confirm its correctness. Remarkably, the most frequent itemset unveiled a campaign currently spreading against Chinese and Korean bank customers.Although motivated by mobile banking trojans, DroydSeuss can be used to analyze the communication behavior of any suspicious application.

Alberto Coletta, Victor van der Veen, Federico Maggi

### DroidAuditor: Forensic Analysis of Application-Layer Privilege Escalation Attacks on Android (Short Paper)

Smart mobile devices process and store a vast amount of security- and privacy-sensitive data. To protect this data from malicious applications mobile operating systems, such as Android, adopt fine-grained access control architectures. However, related work has shown that these access control architectures are susceptible to application-layer privilege escalation attacks. Both automated static and dynamic program analysis promise to proactively detect such attacks. Though while state-of-the-art static analysis frameworks cannot adequately address native and highly obfuscated code, dynamic analysis is vulnerable to malicious applications using logic bombs to avoid early detection.In contrast, the long-term observation of application behavior could help users and security analysts better understand malicious apps. In this paper we present the design and implementation of DroidAuditor, which observes application behavior on real Android devices and generates a graph-based representation. It visualizes this behavior graph, which enables users to develop an intuitive understanding of application internals. Our solution further allows security analysts to query the behavior graph for malicious patterns. We present the design of the DroidAuditor framework and instantiate it using the Android Security Modules (ASM) access control architecture. We evaluate its capability to detect application-layer privilege escalation attacks, such as confused deputy and collusion attacks. In addition, we demonstrate how our architecture can be used to analyze malicious spyware applications.

Stephan Heuser, Marco Negro, Praveen Kumar Pendyala, Ahmad-Reza Sadeghi

### Discrete Choice, Social Interaction, and Policy in Encryption Technology Adoption (Short Paper)

We introduce a model for examining the factors that lead to the adoption of new encryption technologies. Building on the work of Brock and Durlauf, the model describes how agents make choices, in the presence of social interaction, between competing technologies given their relative cost, functionality, and usability. We apply the model to examples about the adoption of encryption in communication (email and messaging) and storage technologies (self-encrypting drives) and also consider our model’s predictions for the evolution of technology adoption over time.

Tristan Caulfield, Christos Ioannidis, David Pym

### Failures of Security APIs: A New Case

We report novel API attacks on a Captcha web service, and discuss lessons that we have learned. In so doing, we expand the horizon of security APIs research by extending it to a new setting. We also show that system architecture analysis is useful both for identifying vulnerabilities in security APIs and for fixing them.

Abdalnaser Algwil, Jeff Yan

### Explicit Optimal Binary Pebbling for One-Way Hash Chain Reversal

We present explicit optimal binary pebbling algorithms for reversing one-way hash chains. For a hash chain of length $$2^k$$ , the number of hashes performed in each output round does not exceed $$\lceil k/2 \rceil$$ , whereas the number of hash values stored (pebbles) throughout is at most k. This is optimal for binary pebbling algorithms characterized by the property that the midpoint of the hash chain is computed just once and stored until it is output, and that this property applies recursively to both halves of the hash chain.We introduce a framework for rigorous comparison of explicit binary pebbling algorithms, including simple speed-1 binary pebbling, Jakobsson’s speed-2 binary pebbling, and our optimal binary pebbling algorithm. Explicit schedules describe for each pebble exactly how many hashes need to be performed in each round. The optimal schedule turns out to be essentially unique and exhibits a nice recursive structure, which allows for fully optimized implementations that can readily be deployed. In particular, we develop the first in-place implementations with minimal storage overhead (essentially, storing only hash values), and fast implementations with minimal computational overhead. Moreover, we show that our approach is not limited to hash chains of length $$n=2^k$$ , but accommodates hash chains of arbitrary length $$n\ge 1$$ , without incurring any overhead. Finally, we show how to run a cascade of pebbling algorithms along with a bootstrapping technique, facilitating sequential reversal of an unlimited number of hash chains growing in length up to a given bound.

Berry Schoenmakers

### Factoring as a Service

The difficulty of integer factorization is fundamental to modern cryptographic security using RSA encryption and signatures. Although a 512-bit RSA modulus was first factored in 1999, 512-bit RSA remains surprisingly common in practice across many cryptographic protocols. Popular understanding of the difficulty of 512-bit factorization does not seem to have kept pace with developments in computing power. In this paper, we optimize the CADO-NFS and Msieve implementations of the number field sieve for use on the Amazon Elastic Compute Cloud platform, allowing a non-expert to factor 512-bit RSA public keys in under four hours for $75. We go on to survey the RSA key sizes used in popular protocols, finding hundreds or thousands of deployed 512-bit RSA keys in DNSSEC, HTTPS, IMAP, POP3, SMTP, DKIM, SSH, and PGP. Luke Valenta, Shaanan Cohney, Alex Liao, Joshua Fried, Satya Bodduluri, Nadia Heninger ### The Self-blindable U-Prove Scheme from FC’14 Is Forgeable (Short Paper) Recently an unlinkable version of the U-Prove attribute-based credential scheme was proposed at Financial Crypto’14 [9]. Unfortunately, the new scheme is forgeable: if sufficiently many users work together then they can construct new credentials, containing any set of attributes of their choice, without any involvement of the issuer. In this note we show how they can achieve this and we point out the error in the unforgeability proof. Eric Verheul, Sietse Ringers, Jaap-Henk Hoepman ### A Sound for a Sound: Mitigating Acoustic Side Channel Attacks on Password Keystrokes with Active Sounds Keyboard acoustic side channel attacks have been shown to utilize the audio leakage from typing on the keyboard to infer the typed words up to a certain degree of accuracy. Researchers have continued to improve upon the accuracy of such attacks by employing different techniques and attack vectors such as feature extraction and classification, keyboard geometry and triangulation.While research is still ongoing towards further improving acoustic side channel attacks, much work has been lacking in building a working defense mechanism against such class of attacks. In this paper, we set out to propose a practical defense mechanism against keyboard acoustic attacks specifically on password typing and test its performance against several attack vectors. Our defense involves the use of various background sounds to mask the audio leakage from the keyboard thereby preventing the side channel attacks from gaining usable information about the typed password. The background sounds are generated by the device that is used to input the passwords. We also evaluate the usability of our approach and show that the addition of background sounds does not hamper users’ capability to input passwords. S. Abhishek Anand, Nitesh Saxena ### Surveillance and Anonymity #### Frontmatter ### Leaky Birds: Exploiting Mobile Application Traffic for Surveillance Over the last decade, mobile devices and mobile applications have become pervasive in their usage. Although many privacy risks associated with mobile applications have been investigated, prior work mainly focuses on the collection of user information by application developers and advertisers. Inspired by the Snowden revelations, we study the ways mobile applications enable mass surveillance by sending unique identifiers over unencrypted connections. Applying passive network fingerprinting, we show how a passive network adversary can improve his ability to target mobile users’ traffic.Our results are based on a large-scale automated study of mobile application network traffic. The framework we developed for this study downloads and runs mobile applications, captures their network traffic and automatically detects identifiers that are sent in the clear. Our findings show that a global adversary can link 57% of a user’s unencrypted mobile traffic. Evaluating two countermeasures available to privacy aware mobile users, we find their effectiveness to be very limited against identifier leakage. Eline Vanrykel, Gunes Acar, Michael Herrmann, Claudia Diaz ### Footprint Scheduling for Dining-Cryptographer Networks In many communication scenarios it is not sufficient to protect only the content of the communication, it is necessary to also protect the identity of communicating parties. Various protocols and technologies have been proposed to offer such protection, for example, anonymous proxies, mix-networks, or onion routing. The protocol that offers the strongest anonymity guarantees, namely unconditional sender and recipient untraceability, is the Dining Cryptographer (DC) protocol proposed by Chaum in 1988. Unfortunately the strong anonymity guarantees come at the price of limited performance and scalability and multiple issues that make deployment complicated in practice.In this paper we address one of those issues, namely slot reservation. We propose footprint scheduling as a new technique for participants to negotiate communication slots without losing anonymity and at the same time hiding the number of actively sending users. Footprint scheduling is at the same time simple, efficient and yields excellent results, in particular in very dynamic networks with a frequently changing set of participants and frequently changing activity rate. Anna Krasnova, Moritz Neikes, Peter Schwabe ### Web Security and Data Privacy #### Frontmatter ### How Anywhere Computing Just Killed Your Phone-Based Two-Factor Authentication Exponential growth in smartphone usage combined with recent advances in mobile technology is causing a shift in (mobile) app behavior: application vendors no longer restrict their apps to a single platform, but rather add synchronization options that allow users to conveniently switch from mobile to PC or vice versa in order to access their services. This process of integrating apps among multiple platforms essentially removes the gap between them. Current, state of the art, mobile phone-based two-factor authentication (2FA) mechanisms, however, heavily rely on the existence of such separation. They are used in a variety of segments (such as consumer online banking services or enterprise secure remote access) to protect against malware. For example, with 2FA in place, attackers should no longer be able to use their PC-based malware to instantiate fraudulent banking transactions.In this paper, we analyze the security implications of diminishing gaps between platforms and show that the ongoing integration and desire for increased usability results in violation of key principles for mobile phone 2FA. As a result, we identify a new class of vulnerabilities dubbed 2FA synchronization vulnerabilities. To support our findings, we present practical attacks against Android and iOS that illustrate how a Man-in-the-Browser attack can be elevated to intercept One-Time Passwords sent to the mobile phone and thus bypass the chain of 2FA mechanisms as used by many financial services. Radhesh Krishnan Konoth, Victor van der Veen, Herbert Bos ### Security Keys: Practical Cryptographic Second Factors for the Modern Web “Security Keys” are second-factor devices that protect users against phishing and man-in-the-middle attacks. Users carry a single device and can self-register it with any online service that supports the protocol. The devices are simple to implement and deploy, simple to use, privacy preserving, and secure against strong attackers. We have shipped support for Security Keys in the Chrome web browser and in Google’s online services. We show that Security Keys lead to both an increased level of security and user satisfaction by analyzing a two year deployment which began within Google and has extended to our consumer-facing web applications. The Security Key design has been standardized by the FIDO Alliance, an organization with more than 250 member companies spanning the industry. Currently, Security Keys have been deployed by Google, Dropbox, and GitHub. An updated and extended tech report is available at https://github.com/google/u2f-ref-code/docs/SecurityKeys_TechReport.pdf. Juan Lang, Alexei Czeskis, Dirk Balfanz, Marius Schilder, Sampath Srinivas ### Include Me Out: In-Browser Detection of Malicious Third-Party Content Inclusions Modern websites include various types of third-party content such as JavaScript, images, stylesheets, and Flash objects in order to create interactive user interfaces. In addition to explicit inclusion of third-party content by website publishers, ISPs and browser extensions are hijacking web browsing sessions with increasing frequency to inject third-party content (e.g., ads). However, third-party content can also introduce security risks to users of these websites, unbeknownst to both website operators and users. Because of the often highly dynamic nature of these inclusions as well as the use of advanced cloaking techniques in contemporary malware, it is exceedingly difficult to preemptively recognize and block inclusions of malicious third-party content before it has the chance to attack the user’s system.In this paper, we propose a novel approach to achieving the goal of preemptive blocking of malicious third-party content inclusion through an analysis of inclusion sequences on the Web. We implemented our approach, called Excision, as a set of modifications to the Chromium browser that protects users from malicious inclusions while web pages load. Our analysis suggests that by adopting our in-browser approach, users can avoid a significant portion of malicious third-party content on the Web. Our evaluation shows that Excision effectively identifies malicious content while introducing a low false positive rate. Our experiments also demonstrate that our approach does not negatively impact a user’s browsing experience when browsing popular websites drawn from the Alexa Top 500. Sajjad Arshad, Amin Kharraz, William Robertson ### A Sensitivity-Adaptive -Uncertainty Model for Set-Valued Data Set-valued data brings enormous opportunities to data mining tasks for various purposes. Many anonymous methods for set-valued data have been proposed to effectively protect an individual’s privacy against identify linkable attacks and item linkage attacks. In these methods, sensitive items are protected by a privacy threshold to limit the re-identification probability of sensitive items. However, lots of set-valued data have diverse sensitivity on data items. This leads to the over-protection problem when these existing privacy-preserving methods are applied to process the data items with diverse sensitivity, and it reduces the utility of data. In this paper, we propose a sensitivity-adaptive $${\rho }$$-uncertainty model to prevent over-generalization and over-suppression by using adaptive privacy thresholds. Thresholds, which accurately capture the hidden privacy features of the set-valued dataset, are defined by uneven distribution of different sensitive items. Under the model, we develop a fine-grained privacy preserving technique through Local Generalization and Partial Suppression, which optimizes a balance between privacy protection and data utility. Experiments show that our method effectively improves the utility of anonymous data. Liuhua Chen, Shenghai Zhong, Li-e Wang, Xianxian Li ### Bitcoin Mining #### Frontmatter ### Incentive Compatibility of Bitcoin Mining Pool Reward Functions In this paper we introduce a game-theoretic model for reward functions in Bitcoin mining pools. Our model consists only of an unordered history of reported shares and gives participating miners the strategy choices of either reporting or delaying when they discover a share or full solution. We defined a precise condition for incentive compatibility to ensure miners strategy choices optimize the welfare of the pool as a whole. With this definition we show that proportional mining rewards are not incentive compatible in this model. We introduce and analyze a novel reward function which is incentive compatible in this model. Finally we show that the popular reward function pay-per-last-N-shares is also incentive compatible in a more general model. Okke Schrijvers, Joseph Bonneau, Dan Boneh, Tim Roughgarden ### When Cryptocurrencies Mine Their Own Business Bitcoin and hundreds of other cryptocurrencies employ a consensus protocol called Nakamoto consensus which rewards miners for maintaining a public blockchain. In this paper, we study the security of this protocol with respect to rational miners and show how a minority of the computation power can incentivize the rest of the network to accept a blockchain of the minority’s choice. By deviating from the mining protocol, a mining pool which controls at least 38.2% of the network’s total computational power can, with modest financial capacity, gain mining advantage over honest mining. Such an attack creates a longer valid blockchain by forking the honest blockchain, and the attacker’s blockchain need not disrupt any “legitimate” non-mining transactions present on the honest blockchain. By subverting the consensus protocol, the attacking pool can double-spend money or simply create a blockchain that pays mining rewards to the attacker’s pool. We show that our attacks are easy to encode in any Nakamoto-consensus-based cryptocurrency which supports a scripting language that is sufficiently expressive to encode its own mining puzzles. Jason Teutsch, Sanjay Jain, Prateek Saxena ### Optimal Selfish Mining Strategies in Bitcoin The Bitcoin protocol requires nodes to quickly distribute newly created blocks. Strong nodes can, however, gain higher payoffs by withholding blocks they create and selectively postponing their publication. The existence of such selfish mining attacks was first reported by Eyal and Sirer, who have demonstrated a specific deviation from the standard protocol (a strategy that we name SM1).In this paper we investigate the profit threshold – the minimal fraction of resources required for a profitable attack. Our analysis provides a bound under which the system can be considered secure against such attacks. Our techniques can be adapted to protocol modifications to assess their susceptibility to selfish mining, by computing the optimal attack under different variants. We find that the profit threshold is strictly lower than the one induced by the SM1 scheme. The policies given by our algorithm dominate SM1 by better regulating attack-withdrawals. We further evaluate the impact of some previously suggested countermeasures, and show that they are less effective than previously conjectured.We then gain insight into selfish mining in the presence of communication delays, and show that, under a model that accounts for delays, the profit threshold vanishes, and even small attackers have incentive to occasionally deviate from the protocol. We conclude with observations regarding the combined power of selfish mining and double spending attacks. Ayelet Sapirshtein, Yonatan Sompolinsky, Aviv Zohar ### Cryptographic Protocols #### Frontmatter ### A Short Paper on Blind Signatures from Knowledge Assumptions This paper concerns blind signature schemes. We focus on two moves constructions, which imply concurrent security. There are known efficient blind signature schemes based on the random oracle model and on the common reference string model. However, constructing two move blind signatures in the standard model is a challenging task, as shown by the impossibility results of Fischlin et al. The recent construction by Garg et al. (Eurocrypt’14) bypasses this result by using complexity leveraging, but it is impractical due to the signature size ($$\approx$$ 100 kB). Fuchsbauer et al. (Crypto’15) presented a more practical construction, but with a security argument based on interactive assumptions. We present a blind signature scheme that is two-move, setup-free and comparable in terms of efficiency with the results of Fuchsbauer et al. Its security is based on a knowledge assumption. Lucjan Hanzlik, Kamil Kluczniak ### KBID: Kerberos Bracelet Identification (Short Paper) The most common method for a user to gain access to a system, service, or resource is to provide a secret, often a password, that verifies her identity and thus authenticates her. Password-based authentication is considered strong only when the password meets certain length and complexity requirements, or when it is combined with other methods in multi-factor authentication. Unfortunately, many authentication systems do not enforce strong passwords due to a number of limitations; for example, the time taken to enter complex passwords. We present an authentication system that addresses these limitations by prompting a user for credentials once and then storing an authentication ticket in a wearable device that we call Kerberos Bracelet Identification (KBID). Joseph Carrigan, Paul Martin, Michael Rushanan ### Payment Use and Abuse #### Frontmatter ### The Other Side of the Coin: User Experiences with Bitcoin Security and Privacy We present the first large-scale survey to investigate how users experience the Bitcoin ecosystem in terms of security, privacy and anonymity. We surveyed 990 Bitcoin users to determine Bitcoin management strategies and identified how users deploy security measures to protect their keys and bitcoins. We found that about 46% of our participants use web-hosted solutions to manage at least some of their bitcoins, and about half of them use exclusively such solutions. We also found that many users do not use all security capabilities of their selected Bitcoin management tool and have significant misconceptions on how to remain anonymous and protect their privacy in the Bitcoin network. Also, 22% of our participants have already lost money due to security breaches or self-induced errors. To get a deeper understanding, we conducted qualitative interviews to explain some of the observed phenomena. Katharina Krombholz, Aljosha Judmayer, Matthias Gusenbauer, Edgar Weippl ### Refund Attacks on Bitcoin’s Payment Protocol BIP70 is a community-accepted Payment Protocol standard that governs how merchants and customers perform payments in Bitcoin. This standard is supported by most major wallets and the two dominant Payment Processors: Coinbase and BitPay, who collectively provide the infrastructure for accepting Bitcoin as a form of payment to more than 100,000 merchants. In this paper, we present new attacks on the Payment Protocol, which affect all BIP70 merchants. The Silkroad Trader attack highlights an authentication vulnerability in the Payment Protocol while the Marketplace Trader attack exploits the refund policies of existing Payment Processors. Both attacks have been experimentally verified on real-life merchants using a modified Bitcoin wallet. The attacks have been acknowledged by both Coinbase and Bitpay with temporary mitigation measures put in place. However, to fully address the identified issues will require revising the BIP70 standard. We present a concrete proposal to revise BIP70 by providing the merchant with publicly verifiable evidence to prevent both attacks. Patrick McCorry, Siamak F. Shahandashti, Feng Hao ### Are Payment Card Contracts Unfair? (Short Paper) Fraud victims are often refused a refund by their bank on the grounds that they failed to comply with their bank’s terms and conditions about PIN safety. We, therefore, conducted a survey of how many PINs people have, and how they manage them. We found that while only a third of PINs are ever changed, almost half of bank customers write at least one PIN down. We also found bank conditions that are too vague to test, or even contradictory on whether PINs could be shared across cards. Yet, some hazardous practices are not forbidden by many banks: of the 22.9% who re-use PINs across devices, half also use their bank PINs on their mobile phones. We conclude that many bank contracts fail a simple test of reasonableness, and ‘strong authentication’, as required by the Payment Services Directive II, should include usability testing. Steven J. Murdoch, Ingolf Becker, Ruba Abu-Salma, Ross Anderson, Nicholas Bohm, Alice Hutchings, M. Angela Sasse, Gianluca Stringhini ### The Bitcoin Brain Drain: Examining the Use and Abuse of Bitcoin Brain Wallets In the cryptocurrency Bitcoin, users can deterministically derive the private keys used for transmitting money from a password. Such “brain wallets” are appealing because they free users from storing their private keys on untrusted computers. Unfortunately, they also enable attackers to conduct unlimited offline password guessing. In this paper, we report on the first large-scale measurement of the use of brain wallets in Bitcoin. Using a wide range of word lists, we evaluated around 300 billion passwords. Surprisingly, after excluding activities by researchers, we identified just 884 brain wallets worth around$100K in use from September 2011 to August 2015. We find that all but 21 wallets were drained, usually within 24 h but often within minutes. We find that around a dozen “drainers” are competing to liquidate brain wallets as soon as they are funded. We find no evidence that users of brain wallets loaded with more bitcoin select stronger passwords, but we do find that brain wallets with weaker passwords are cracked more quickly.

Marie Vasek, Joseph Bonneau, Ryan Castellucci, Cameron Keith, Tyler Moore

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