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AVs' protection is a full-time task against malware: This is similar to a game where malware ... [more ▼]To protect their digital assets from malware attacks, most users and companies rely on antivirus (AV) software. AVs' protection is a full-time task against malware: This is similar to a game where malware, e.g., through obfuscation and polymorphism, denial of service attacks, and malformed packets and parameters, tries to circumvent AV defences or make them crash. However, AVs react by complementing signature-based detection with anomaly or behavioral analysis, and by using OS protection, standard code, and binary protection techniques. Further, malware counter-acts, for instance, by using adversarial inputs to avoid detection, and so on. In this cat-and-mouse game, a winning strategy is trying to anticipate the move of the adversary by looking into one's own weaknesses, seeing how the adversary can penetrate them, and building up appropriate defences or attacks. In this article, we play the role of malware developers and anticipate two novel moves for the malware side to demonstrate the weakness in the AVs and to improve the defences in AVs' side. The first one consists in simulating mouse events to control AVs, namely, to send them mouse "clicks" to deactivate their protection. We prove that many AVs can be disabled in this way, and we call this class of attacks Ghost Control. The second one consists in controlling whitelisted applications, such as Notepad, by sending them keyboard events (such as "copy-and-paste") to perform malicious operations on behalf of the malware. We prove that the anti-ransomware protection feature of AVs can be bypassed if we use Notepad as a "puppet" to rewrite the content of protected files as a ransomware would do. Playing with the words, and recalling the cat-and-mouse game, we call this class of attacks Cut-and-Mouse. We tested these two attacks on 29 AVs, and the results show that 14 AVs are vulnerable to Ghost Control attack while all 29 AV programs tested are found vulnerable to Cut-and-Mouse. Furthermore, we also show some weaknesses in additional protection mechanisms of AVs, such as sandboxing and CAPTCHA verification. We have engaged with the affected AV companies, and we reported the disclosure communication with them and their responses. [less ▲]Detailed reference viewed: 17 (0 UL) Analysis, Detection, and Prevention of Cryptographic RansomwareGenç, Ziya Alper Doctoral thesis (2020)Cryptographic ransomware encrypts files on a computer system, thereby blocks access to victim’s data, until a ransom is paid. The quick return in revenue together with the practical difficulties in ... [more ▼]Cryptographic ransomware encrypts files on a computer system, thereby blocks access to victim’s data, until a ransom is paid. The quick return in revenue together with the practical difficulties in accurately tracking cryptocurrencies used by victims to perform the ransom payment, have made ransomware a preferred tool for cybercriminals. In addition, exploiting zero-day vulnerabilities found in Windows Operating Systems (OSs), the most widely used OS on desktop computers, has enabled ransomware to extend its threat and have detrimental effects at world-wide level. For instance, WannaCry and NotPetya have affected almost all countries, impacted organizations, and the latter alone caused damage which costs more than $10 billion. In this thesis, we conduct a theoretical and experimental study on cryptographic ransomware. In the first part, we explore the anatomy of a ransomware, and in particular, analyze the key management strategies employed by notable families. We verify that for a long-term success, ransomware authors must acquire good random numbers to seed Key Derivation Functions (KDFs). The second part of this thesis analyzes the security of the current anti-ransomware approaches, both in academic literature and real-world systems, with the aim to anticipate how such future generations of ransomware will work, and in order to start planning on how to stop them. We argue that among them, there will be some which will try to defeat current anti-ransomware; thus, we can speculate over their working principles by studying the weak points in the strategies that six of the most advanced anti-ransomware currently implements. We support our speculations with experiments, proving at the same time that those weak points are in fact vulnerabilities and that the future ransomware that we have imagined can be effective. Next, we analyze existing decoy strategies and discuss how they are effective in countering current ransomware by defining a set of metrics to measure their robustness. To demonstrate how ransomware can identify existing deception-based detection strategies, we implement a proof-of-concept decoy-aware ransomware that successfully bypasses decoys by using a decision engine with few rules. We also discuss existing issues in decoy-based strategies and propose practical solutions to mitigate them. Finally, we look for vulnerabilities in antivirus (AV) programs which are the de facto security tool installed at computers against cryptographic ransomware. In our experiments with 29 consumer-level AVs, we discovered two critilcal vulnerabilities. The first one consists in simulating mouse events to control AVs, namely to send them mouse “clicks” to deactivate their protection. We prove that 14 out of 29 AVs can be disabled in this way, and we call this class of attacks Ghost Control. The second one consists in controlling whitelisted applications, such as Notepad, by sending them keyboard events (such as “copy-and-paste”) to perform malicious operations on behalf of the malware. We prove that the anti-ransomware protection feature of AVs can be bypassed if we use Notepad as a “puppet” to rewrite the content of protected files as a ransomware would do. Playing with the words, and recalling the cat-and-mouse game, we call this class of attacks Cut-and-Mouse. In the third part of the thesis, we propose a strategy to mitigate cryptographic ransomware attacks. Based on our insights from the first part of the thesis, we present UShallNotPass which works by controlling access to secure randomness sources, i.e., Cryptographically Secure Pseudo-Random Number Generator (CSPRNG) Appliction Programming Interfaces (APIs). We tested UShallNotPass against 524 real-world ransomware samples, and observe that UShallNotPass stops 94% of them, including WannaCry, Locky, CryptoLocker and CryptoWall. Remarkably, it also nullifies NotPetya, the offspring of the family which so far has eluded all defenses. Next, we present NoCry, which shares the same defense strategy but implements an improved architecture. We show that NoCry is more secure (with components that are not vulnerable to known attacks), more effective (with less false negatives in the class of ransomware addressed) and more efficient (with minimal false positive rate and negligible overhead). To confirm that the new architecture works as expected, we tested NoCry against a new set of 747 ransomware samples, of which, NoCry could stop 97.1%, bringing its security and technological readiness to a higher level. Finally, in the fourth part, we present the potential future of the cryptographic ransomware. We identify new possible ransomware targets inspired by the cybersecurity incidents occurred in real-world scenarios. In this respect, we described possible threats that ransomware may pose by targeting critical domains, such as the Internet of Things and the Socio-Technical systems, which will worrisomely amplify the effectiveness of ransomware attacks. Next, we looked into whether ransomware authors re-use the work of others, available at public platforms and repositories, and produce insecure code (which might enable to build decryptors). By methodically reverse-engineering malware executables, we have found that, out of 21 ransomware samples, 9 contain copy-paste code from public resources. From this fact, we recall critical cases of code disclosure in the recent history of ransomware and, reflect on the dual-use nature of this research by arguing that ransomware are components in cyber-weapons. We conclude by discussing the benefits and limits of using cyber-intelligence and counter-intelligence strategies that could be used against this threat. [less ▲]Detailed reference viewed: 204 (14 UL) "The Simplest Protocol for Oblivious Transfer'' RevisitedGenç, Ziya Alper ; Iovino, Vincenzo; Rial, Alfredo in Information Processing Letters (2020)In 2015, Chou and Orlandi presented an oblivious transfer protocol that already drew a lot of attention both from theorists and practitioners due to its extreme simplicity and high efficiency. Chou and ... [more ▼]In 2015, Chou and Orlandi presented an oblivious transfer protocol that already drew a lot of attention both from theorists and practitioners due to its extreme simplicity and high efficiency. Chou and Orlandi claimed that their protocol is universally composable secure (UC-secure) in the random oracle model under dynamic corruptions. UC-security is a very strong security guarantee that assures that, not only the protocol in itself is secure, but can be also used safely in larger protocols. Unfortunately, in this work we point out a flaw in their security proof for the case of a corrupt sender. In more detail, we define a decisional problem and we prove that, if a correct security proof for the Chou and Orlandi's protocol is provided, then this problem can be solved correctly with overwhelming probability. Therefore, the protocol of Chou and Orlandi cannot be instantiated securely with groups for which our decisional problem cannot be solved correctly with overwhelming probability. Consequently, the protocol of Chou and Orlandi cannot be instantiated with {\em all} groups$\G$in which the CDH problem is intractable, but only with groups in which both the CDH problem is intractable and our decisional problem can be solved with overwhelming probability. After the appearance of our work, Chou and Orlandi acknowledged the problems we pointed out in their security proof and subsequent works showed additional issues, removing the claims of UC security of their protocol. [less ▲]Detailed reference viewed: 82 (6 UL) Dual-Use Research In Ransomware Attacks: A Discussion on Ransomware Defence IntelligenceGenç, Ziya Alper ; Lenzini, Gabriele in Proceedings of the 6th International Conference on Information Systems Security and Privacy (2020)Previous research has shown that developers rely on public platforms and repositories to produce functional but insecure code. We looked into the matter for ransomware, enquiring whether also ransomware ... [more ▼]Previous research has shown that developers rely on public platforms and repositories to produce functional but insecure code. We looked into the matter for ransomware, enquiring whether also ransomware engineers re-use the work of others and produce insecure code. By methodically reverse-engineering 128 malware executables, we have found that, out of 21 ransomware samples, 9 contain copy-paste code from public resources. Thanks to this finding, we managed to retrieve the decryption keys with which to nullify the ransomware attacks. From this fact, we recall critical cases of code disclosure in the recent history of ransomware and, arguing that ransomware are components in cyber-weapons, reflect on the dual-use nature of this research. We further discuss benefits and limits of using cyber-intelligence and counter-intelligence strategies that could be used against this threat. [less ▲]Detailed reference viewed: 273 (9 UL) A Critical Security Analysis of the Password-Based Authentication Honeywords System Under Code-Corruption AttackGenç, Ziya Alper ; Lenzini, Gabriele ; Ryan, Peter et alin Information Systems Security and Privacy (2019, July)Password-based authentication is a widespread method to access into systems, thus password files are a valuable resource often target of attacks. To detect when a password file has been stolen, Juels and ... [more ▼]Password-based authentication is a widespread method to access into systems, thus password files are a valuable resource often target of attacks. To detect when a password file has been stolen, Juels and Rivest introduced the Honeywords System in 2013. The core idea is to store the password with a list of decoy words that are indistinguishable'' from the password, called honeywords. An adversary that obtains the password file and, by dictionary attack, retrieves the honeywords can only guess the password when attempting to log in: but any incorrect guess will set off an alarm, warning that file has been compromised. In a recent conference paper, we studied the security of the Honeywords System in a scenario where the intruder also manages to corrupt the server's code (with certain limiting assumptions); we proposed an authentication protocol and proved it secure despite the corruption. In this extended journal version, we detail the analysis and we extend it, under the same attacker model, to the other two protocols of the original Honeywords System, the setup and change of password. We formally verify the security of both of them; further, we discuss that our design suggests a completely new approach that diverges from the original idea of the Honeywords System but indicates an alternative way to authenticate users which is robust to server's code-corruption. [less ▲]Detailed reference viewed: 171 (8 UL) NoCry: No More Secure Encryption Keys for Cryptographic RansomwareGenç, Ziya Alper ; Lenzini, Gabriele ; Ryan, Peter in Proceedings of the Second International Workshop on Emerging Technologies for Authorization and Authentication (2019)Since the appearance of ransomware in the cyber crime scene, researchers and anti-malware companies have been offering solutions to mitigate the threat. Anti-malware solutions differ on the specific ... [more ▼]Since the appearance of ransomware in the cyber crime scene, researchers and anti-malware companies have been offering solutions to mitigate the threat. Anti-malware solutions differ on the specific strategy they implement, and all have pros and cons. However, three requirements concern them all: their implementation must be secure, be effective, and be efficient. Recently, Genç et al. proposed to stop a specific class of ransomware, the cryptographically strong one, by blocking unauthorized calls to cryptographically secure pseudo-random number generators, which are required to build strong encryption keys. Here, in adherence to the requirements, we discuss an implementation of that solution that is more secure (with components that are not vulnerable to known attacks), more effective (with less false negatives in the class of ransomware addressed) and more efficient (with minimal false positive rate and negligible overhead) than the original, bringing its security and technological readiness to a higher level. [less ▲]Detailed reference viewed: 160 (9 UL) A Game of "Cut and Mouse": Bypassing Antivirus by Simulating User InputsGenç, Ziya Alper ; Lenzini, Gabriele ; Sgandurra, Danielein Proceedings of the 35th Annual Computer Security Applications Conference (2019)To protect their digital assets from malware attacks, most users and companies rely on anti-virus (AV) software. But AVs' protection is a full-time task and AVs are engaged in a cat-and-mouse game where ... [more ▼]To protect their digital assets from malware attacks, most users and companies rely on anti-virus (AV) software. But AVs' protection is a full-time task and AVs are engaged in a cat-and-mouse game where malware, e.g., through obfuscation and polymorphism, denial of service attacks and malformed packets and parameters, try to circumvent AV defences or make them crash. On the other hand, AVs react by complementing signature-based with anomaly or behavioral detection, and by using OS protection, standard code, and binary protection techniques. Further, malware counter-act, for instance by using adversarial inputs to avoid detection, et cetera. This paper investigates two novel moves for the malware side. The first one consists in simulating mouse events to control AVs, namely to send them mouse "clicks" to deactivate their protection. We prove that many AVs can be disabled in this way, and we call this class of attacks Ghost Control. The second one consists in controlling high-integrity white-listed applications, such as Notepad, by sending them keyboard events (such as "copy-and-paste") to perform malicious operations on behalf of the malware. We prove that the anti-ransomware protection feature of some AVs can be bypassed if we use Notepad as a "puppet" to rewrite the content of protected files as a ransomware would do. Playing with the words, and recalling the cat-and-mouse game, we call this class of attacks Cut-and-Mouse. [less ▲]Detailed reference viewed: 62 (7 UL) On Deception-Based Protection Against Cryptographic RansomwareGenç, Ziya Alper ; Lenzini, Gabriele ; Sgandurra, Danielein Proceedings of the 16th Conference on Detection of Intrusions and Malware & Vulnerability Assessment (2019)In order to detect malicious file system activity, some commercial and academic anti-ransomware solutions implement deception-based techniques, specifically by placing decoy files among user files. While ... [more ▼]In order to detect malicious file system activity, some commercial and academic anti-ransomware solutions implement deception-based techniques, specifically by placing decoy files among user files. While this approach raises the bar against current ransomware, as any access to a decoy file is a sign of malicious activity, the robustness of decoy strategies has not been formally analyzed and fully tested. In this paper, we analyze existing decoy strategies and discuss how they are effective in countering current ransomware by defining a set of metrics to measure their robustness. To demonstrate how ransomware can identify existing deception-based detection strategies, we have implemented a proof-of-concept anti-decoy ransomware that successfully bypasses decoys by using a decision engine with few rules. Finally, we discuss existing issues in decoy-based strategies and propose practical solutions to mitigate them. [less ▲]Detailed reference viewed: 162 (14 UL) Case Study: Analysis and Mitigation of a Novel Sandbox-Evasion TechniqueGenç, Ziya Alper ; Lenzini, Gabriele ; Sgandurra, Danielein Proceedings of the Third Central European Cybersecurity Conference (2019)Malware is one of the most popular cyber-attack methods in the digital world. According to the independent test company AV-TEST, 350,000 new malware samples are created every day. To analyze all samples ... [more ▼]Malware is one of the most popular cyber-attack methods in the digital world. According to the independent test company AV-TEST, 350,000 new malware samples are created every day. To analyze all samples by hand to discover whether they are malware does not scale, so antivirus companies automate the process e.g., using sand- boxes where samples can be run, observed, and classified. Malware authors are aware of this fact, and try to evade detection. In this paper we describe one of such evasion technique: unprecedented, we discovered it while analyzing a ransomware sample. Analyzed in a Cuckoo Sandbox, the sample was able to avoid triggering malware indicators, thus scoring significantly below the minimum severity level. Here, we discuss what strategy the sample follows to evade the analysis, proposing practical defense methods to nullify, in our turn, the sample’s furtive strategy. [less ▲]Detailed reference viewed: 134 (35 UL) Crypren DecryptorGenç, Ziya Alper Software (2018)Crypren Decryptor is software which recovers the files encrypted by Crypren ransomware. The decryptor targets the sample with SHA256 digest ... [more ▼]Crypren Decryptor is software which recovers the files encrypted by Crypren ransomware. The decryptor targets the sample with SHA256 digest: ce53233a435923a68a9ca6987f0d6333bb97d5a435b942d20944356ac29df598. Further details about the design of this software can be found at the paper: http://orbilu.uni.lu/handle/10993/36627. [less ▲]Detailed reference viewed: 105 (7 UL) No Random, No Ransom: A Key to Stop Cryptographic RansomwareGenç, Ziya Alper ; Lenzini, Gabriele ; Ryan, Peter in Proceedings of the 15th Conference on Detection of Intrusions and Malware, and Vulnerability Assessment (2018)To be effective, ransomware has to implement strong encryption, and strong encryption in turn requires a good source of random numbers. Without access to true randomness, ransomware relies on the pseudo ... [more ▼]To be effective, ransomware has to implement strong encryption, and strong encryption in turn requires a good source of random numbers. Without access to true randomness, ransomware relies on the pseudo random number generators that modern Operating Systems make available to applications. With this insight, we propose a strategy to mitigate ransomware attacks that considers pseudo random number generator functions as critical resources, controls accesses on their APIs and stops unauthorized applications that call them. Our strategy, tested against 524 active real-world ransomware samples, stops 94% of them, including WannaCry, Locky, CryptoLocker and CryptoWall. Remarkably, it also nullifies NotPetya, the latest offspring of the family which so far has eluded all defenses. [less ▲]Detailed reference viewed: 583 (28 UL) Next Generation Cryptographic RansomwareGenç, Ziya Alper ; Lenzini, Gabriele ; Ryan, Peter in Proceedings of the Secure IT Systems (2018)We are assisting at an evolution in the ecosystem of cryptoware - the malware that encrypts files and makes them unavailable unless the victim pays up. New variants are taking the place once dominated by ... [more ▼]We are assisting at an evolution in the ecosystem of cryptoware - the malware that encrypts files and makes them unavailable unless the victim pays up. New variants are taking the place once dominated by older versions; incident reports suggest that forthcoming ransomware will be more sophisticated, disruptive, and targeted. Can we anticipate how such future generations of ransomware will work in order to start planning on how to stop them? We argue that among them there will be some which will try to defeat current anti-ransomware; thus, we can speculate over their working principle by studying the weak points in the strategies that seven of the most advanced anti-ransomware are currently implementing. We support our speculations with experiments, proving at the same time that those weak points are in fact vulnerabilities and that the future ransomware that we have imagined can be effective. [less ▲]Detailed reference viewed: 294 (12 UL) Security Attacks and Enhancements to Chaotic Map-Based RFID Authentication ProtocolsKardaş, Süleyman; Genç, Ziya Alper in Wireless Personal Communications (2018), 98(1), 1135-1154Radio frequency identification (RFID) technology has been increasingly integrated into numerous applications for authentication of objects or individuals. However, because of its limited computation power ... [more ▼]Radio frequency identification (RFID) technology has been increasingly integrated into numerous applications for authentication of objects or individuals. However, because of its limited computation power, RFID technology may cause several security and privacy issues such as tracking the owner of the tag, cloning of the tags and etc. Recently, two chaotic map-based authentication protocols have been proposed for low-cost RFID tags in order to eliminate these issues. In this paper, we give the security analysis of these protocols and uncover their weaknesses. We prove that these protocols are vulnerable to tag tracing, tag impersonation and desynchronization attacks. The attack complexity of an adversary is polynomial and the success probability of these attacks are substantial. Moreover, we also propose an improved RFID authentication protocol that employs Chebyshev chaotic maps and complies with the EPC global Class 1 Generation 2 standard. Finally, we show that our protocol is resistant against those security issues. [less ▲]Detailed reference viewed: 148 (4 UL) Security Analysis of Key Acquiring Strategies Used by Cryptographic RansomwareGenç, Ziya Alper ; Lenzini, Gabriele ; Ryan, Peter in Advances in Cybersecurity 2018 (2018)To achieve its goals, ransomware needs to employ strong encryption, which in turn requires access to high-grade encryption keys. Over the evolution of ransomware, various techniques have been observed to ... [more ▼]To achieve its goals, ransomware needs to employ strong encryption, which in turn requires access to high-grade encryption keys. Over the evolution of ransomware, various techniques have been observed to accomplish the latter. Understanding the advantages and disadvantages of each method is essential to develop robust defense strategies. In this paper we explain the techniques used by ransomware to derive encryption keys and analyze the security of each approach. We argue that recovery of data might be possible if the ransomware cannot access high entropy randomness sources. As an evidence to support our theoretical results, we provide a decryptor program for a previously undefeated ransomware. [less ▲]Detailed reference viewed: 205 (13 UL) A Security Analysis, and a Fix, of a Code-Corrupted Honeywords SystemGenç, Ziya Alper ; Lenzini, Gabriele ; Ryan, Peter et alin Proceedings of the 4th International Conference on Information Systems Security and Privacy (2018)In 2013 Juels and Rivest introduced the Honeywords System, a password-based authentication system designed to detect when a password file has been stolen. A Honeywords System stores passwords together ... [more ▼]In 2013 Juels and Rivest introduced the Honeywords System, a password-based authentication system designed to detect when a password file has been stolen. A Honeywords System stores passwords together with indistinguishable decoy words so when an intruder steals the file, retrieves the words, and tries to log-in, he does not know which one is the password. By guessing one from the decoy words, he may not be lucky and reveal the leak. Juels and Rivest left a problem open: how to make the system secure even when the intruder corrupted the login server’s code. In this paper we study and solve the problem. However, since “code corruption” is a powerful attack, we first define rigorously the threat and set a few assumptions under which the problem is still solvable, before showing meaningful attacks against the original Honeywords System. Then we elicit a fundamental security requirement, implementing which, we are able to restore the honeywords System’s security despite a corrupted login service. We verify the new protocol’s security formally, using ProVerif for this task. We also implement the protocol and test its performance. Finally, at the light of our findings, we discuss whether it is still worth using a fixed honeywords-based system against such a powerful threat, or whether it is better, in order to be resilient against code corruption attacks, to design afresh a completely different password-based authentication solution. [less ▲]Detailed reference viewed: 400 (53 UL) Examination of a New Defense Mechanism: HoneywordsGenç, Ziya Alper ; Kardaş, Süleyman; Kiraz, in Proceedings of the 11th WISTP International Conference on Information Security Theory and Practice (2017)Past experiences show us that password breach is still one of the main methods of attackers to obtain personal or sensitive user data. Basically, assuming they have access to list of hashed passwords ... [more ▼]Past experiences show us that password breach is still one of the main methods of attackers to obtain personal or sensitive user data. Basically, assuming they have access to list of hashed passwords, they apply guessing attacks, i.e., attempt to guess a password by trying a large number of possibilities. We certainly need to change our way of thinking and use a novel and creative approach in order to protect our passwords. In fact, there are already novel attempts to provide password protection. The Honeywords system of Juels and Rivest is one of them which provides a detection mechanism for password breaches. Roughly speaking, they propose a method for password-based authentication systems where fake passwords, i.e., "honeywords" are added into a password file, in order to detect impersonation. Their solution includes an auxiliary secure server called "honeychecker" which can distinguish a user's real password among her honeywords and immediately sets off an alarm whenever a honeyword is used. However, they also pointed out that their system needs to be improved in various ways by highlighting some open problems. In this paper, after revisiting the security of their proposal, we specifically focus on and aim to solve a highlighted open problem, i.e., active attacks where the adversary modifies the code running on either the login server or the honeychecker. [less ▲]Detailed reference viewed: 211 (5 UL) The Cipher, the Random and the Ransom: A Survey on Current and Future RansomwareGenç, Ziya Alper ; Lenzini, Gabriele ; Ryan, Peter in Advances in Cybersecurity 2017 (2017)Although conceptually not new, ransomware recently regained attraction in the cybersecurity community: notorious attacks in fact have caused serious damage, proving their disruptive effect. This is likely ... [more ▼]Although conceptually not new, ransomware recently regained attraction in the cybersecurity community: notorious attacks in fact have caused serious damage, proving their disruptive effect. This is likely just the beginning of a new era. According to a recent intelligence report by Cybersecurity Ventures, the total cost due to ransomware attacks is predicted to exceed$5 billion in 2017. How can this disruptive threat can be contained? Current anti-ransomware solutions are effective only against existing threats, and the worst is yet to come. Cyber criminals will design and deploy more sophisticated strategies, overcoming current defenses and, as it commonly happens in security, defenders and attackers will embrace a competition that will never end. In this arm race, anticipating how current ransomware will evolve may help at least being prepared for some future damage. In this paper, we describe existing techniques to mitigate ransomware and we discuss their limitations. Discussing how current ransomware could become even more disruptive and elusive is crucial to conceive more solid defense and systems that can mitigate zero-day ransomware, yielding higher security levels for information systems, including critical infrastructures such as intelligent transportation networks and health institutions. [less ▲]Detailed reference viewed: 581 (21 UL) Security and Efficiency Analysis of the Hamming Distance Computation Protocol Based on Oblivious TransferKiraz, Mehmet Sabır; Genç, Ziya Alper ; Kardaş, Süleymanin Security and Communication Networks (2015), 8(18), 4123-4135Bringer et al. proposed two cryptographic protocols for the computation of Hamming distance. Their first scheme uses Oblivious Transfer and provides security in the semi-honest model. The other scheme ... [more ▼]Bringer et al. proposed two cryptographic protocols for the computation of Hamming distance. Their first scheme uses Oblivious Transfer and provides security in the semi-honest model. The other scheme uses Committed Oblivious Transfer and is claimed to provide full security in the malicious case. The proposed protocols have direct implications to biometric authentication schemes between a prover and a verifier where the verifier has biometric data of the users in plain form. In this paper, we show that their protocol is not actually fully secure against malicious adversaries. More precisely, our attack breaks the soundness property of their protocol where a malicious user can compute a Hamming distance which is different from the actual value. For biometric authentication systems, this attack allows a malicious adversary to pass the authentication without knowledge of the honest user’s input with at most O(n) complexity instead of O(2n), where n is the input length. We propose an enhanced version of their protocol where this attack is eliminated. The security of our modified protocol is proven using the simulation-based paradigm. Furthermore, as for efficiency concerns, the modified protocol utilizes Verifiable Oblivious Transfer which does not require the commitments to outputs which improves its efficiency significantly. [less ▲]Detailed reference viewed: 104 (9 UL) 1