109 research outputs found

    Secure Messaging with Strong Compromise Resilience, Temporal Privacy, and Immediate Decryption

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    Recent years have seen many advances in designing secure messaging protocols, aiming at provably strong security properties in theory or high efficiency for real-world practical deployment. However, important trade-off areas of the design space inbetween these elements have not yet been explored. In this work we design the first provably secure protocol that at the same time achieves (i) strong resilience against fine-grained compromise, (ii) temporal privacy, and (iii) immediate decryption with constant-size overhead, notably, in the post-quantum (PQ) setting. Besides these main design goals, we introduce a novel definition of offline deniability suitable for our setting, and prove that our protocol meets it, notably when combined with a PQ offline deniable initial key exchange

    LIPIcs, Volume 251, ITCS 2023, Complete Volume

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    LIPIcs, Volume 251, ITCS 2023, Complete Volum

    Security and Privacy for Modern Wireless Communication Systems

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    The aim of this reprint focuses on the latest protocol research, software/hardware development and implementation, and system architecture design in addressing emerging security and privacy issues for modern wireless communication networks. Relevant topics include, but are not limited to, the following: deep-learning-based security and privacy design; covert communications; information-theoretical foundations for advanced security and privacy techniques; lightweight cryptography for power constrained networks; physical layer key generation; prototypes and testbeds for security and privacy solutions; encryption and decryption algorithm for low-latency constrained networks; security protocols for modern wireless communication networks; network intrusion detection; physical layer design with security consideration; anonymity in data transmission; vulnerabilities in security and privacy in modern wireless communication networks; challenges of security and privacy in node–edge–cloud computation; security and privacy design for low-power wide-area IoT networks; security and privacy design for vehicle networks; security and privacy design for underwater communications networks

    LIPIcs, Volume 261, ICALP 2023, Complete Volume

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    LIPIcs, Volume 261, ICALP 2023, Complete Volum

    On Efficient Zero-Knowledge Arguments

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    Cryptographic Analysis of Secure Messaging Protocols

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    Instant messaging applications promise their users a secure and private way to communicate. The validity of these promises rests on the design of the underlying protocol, the cryptographic primitives used and the quality of the implementation. Though secure messaging designs exist in the literature, for various reasons developers of messaging applications often opt to design their own protocols, creating a gap between cryptography as understood by academic research and cryptography as implemented in practice. This thesis contributes to bridging this gap by approaching it from both sides: by looking for flaws in the protocols underlying real-world messaging applications, as well as by performing a rigorous analysis of their security guarantees in a provable security model.Secure messaging can provide a host of different, sometimes conflicting, security and privacy guarantees. It is thus important to judge applications based on the concrete security expectations of their users. This is particularly significant for higher-risk users such as activists or civil rights protesters. To position our work, we first studied the security practices of protesters in the context of the 2019 Anti-ELAB protests in Hong Kong using in-depth, semi-structured interviews with participants of these protests. We report how they organised on different chat platforms based on their perceived security, and how they developed tactics and strategies to enable pseudonymity and detect compromise.Then, we analysed two messaging applications relevant in the protest context: Bridgefy and Telegram. Bridgefy is a mobile mesh messaging application, allowing users in relative proximity to communicate without the Internet. It was being promoted as a secure communication tool for use in areas experiencing large-scale protests. We showed that Bridgefy permitted its users to be tracked, offered no authenticity, no effective confidentiality protections and lacked resilience against adversarially crafted messages. We verified these vulnerabilities by demonstrating a series of practical attacks.Telegram is a messaging platform with over 500 million users, yet prior to this work its bespoke protocol, MTProto, had received little attention from the cryptographic community. We provided the first comprehensive study of the MTProto symmetric channel as implemented in cloud chats. We gave both positive and negative results. First, we found two attacks on the existing protocol, and two attacks on its implementation in official clients which exploit timing side channels and uncover a vulnerability in the key exchange protocol. Second, we proved that a fixed version of the symmetric MTProto protocol achieves security in a suitable bidirectional secure channel model, albeit under unstudied assumptions. Our model itself advances the state-of-the-art for secure channels

    Non-malleable secret sharing against joint tampering attacks

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    Since thousands of years ago, the goal of cryptography has been to hide messages from prying eyes. In recent times, cryptography two important changes: first, cryptography itself evolved from just being about encryption to a broader class of situations coming from the digital era; second, the way of studying cryptography evolved from creating ``seemingly hard'' cryptographic schemes to constructing schemes which are provably secure. However, once the mathematical abstraction of cryptographic primitives started to be too hard to break, attackers found another way to defeat security. Side channel attacks have been proved to be very effective in this task, breaking the security of otherwise provably secure schemes. Because of this, recent trends in cryptography aim to capture this situation and construct schemes that are secure even against such powerful attacks. In this setting, this thesis specializes in the study of secret sharing, an important cryptographic primitive that allows to balance privacy and integrity of data and also has applications to multi-party protocols. Namely, continuing the trend which aims to protect against side channel attacks, this thesis brings some contributions to the state of the art of the so-called leakage-resilient and non-malleable secret sharing schemes, which have stronger guarantees against attackers that are able to learn information from possibly all the shares and even tamper with the shares and see the effects of the tampering. The main contributions of this thesis are twofold. First, we construct secret sharing schemes that are secure against a very powerful class of attacks which, informally, allows the attacker to jointly leak some information and tamper with the shares in a continuous fashion. Second, we study the capacity of continuously non-malleable secret sharing schemes, that is, the maximum achievable information rate. Roughly speaking, we find some lower bounds to the size that the shares must have in order to achieve some forms of non-malleability

    Malware-Resistant Protocols for Real-World Systems

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    Cryptographic protocols are widely used to protect real-world systems from attacks. Paying for goods in a shop, withdrawing money or browsing the Web; all these activities are backed by cryptographic protocols. However, in recent years a potent threat became apparent. Malware is increasingly used in attacks to bypass existing security mechanisms. Many cryptographic protocols that are used in real-world systems today have been found to be susceptible to malware attacks. One reason for this is that most of these protocols were designed with respect to the Dolev-Yao attack model that assumes an attacker to control the network between computer systems but not the systems themselves. Furthermore, most real-world protocols do not provide a formal proof of security and thus lack a precise definition of the security goals the designers tried to achieve. This work tackles the design of cryptographic protocols that are resilient to malware attacks, applicable to real-world systems, and provably secure. In this regard, we investigate three real-world use cases: electronic payment, web authentication, and data aggregation. We analyze the security of existing protocols and confirm results from prior work that most protocols are not resilient to malware. Furthermore, we provide guidelines for the design of malware-resistant protocols and propose such protocols. In addition, we formalize security notions for malware-resistance and use a formal proof of security to verify the security guarantees of our protocols. In this work we show that designing malware-resistant protocols for real-world systems is possible. We present a new security notion for electronic payment and web authentication, called one-out-of-two security, that does not require a single device to be trusted and ensures that a protocol stays secure as long as one of two devices is not compromised. Furthermore, we propose L-Pay, a cryptographic protocol for paying at the point of sale (POS) or withdrawing money at an automated teller machine (ATM) satisfying one-out-of-two security, FIDO2 With Two Displays (FIDO2D) a cryptographic protocol to secure transactions in the Web with one-out-of-two security and Secure Aggregation Grouped by Multiple Attributes (SAGMA), a cryptographic protocol for secure data aggregation in encrypted databases. In this work, we take important steps towards the use of malware-resistant protocols in real-world systems. Our guidelines and protocols can serve as templates to design new cryptographic protocols and improve security in further use cases

    Malleable zero-knowledge proofs and applications

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    In recent years, the field of privacy-preserving technologies has experienced considerable expansion, with zero-knowledge proofs (ZKPs) playing one of the most prominent roles. Although ZKPs have been a well-established theoretical construct for three decades, recent efficiency improvements and novel privacy applications within decentralized finance have become the main drivers behind the surge of interest and investment in this area. This momentum has subsequently sparked unprecedented technical advances. Non-interactive ZKPs (NIZKs) are now regularly implemented across a variety of domains, encompassing, but not limited to, privacy-enabling cryptocurrencies, credential systems, voting, mixing, secure multi-party computation, and other cryptographic protocols. This thesis, although covering several areas of ZKP technologies and their application, focuses on one important aspect of NIZKs, namely their malleability. Malleability is a quality of a proof system that describes the potential for altering an already generated proof. Different properties may be desired in different application contexts. On the one end of the spectrum, non-malleability ensures proof immutability, an important requirement in scenarios such as prevention of replay attacks in anonymous cryptocurrencies. At the other end, some NIZKs enable proof updatability, recursively and directly, a feature that is integral for a variety of contexts, such as private smart contracts, compact blockchains, ZK rollups, ZK virtual machines, and MPC protocols generally. This work starts with a detailed analysis of the malleability and overarching security of a popular NIZK, known as Groth16. Here we adopt a more definitional approach, studying certain properties of the proof system, and its setup ceremony, that are crucial for its precise modelling within bigger systems. Subsequently, the work explores the malleability of transactions within a private cryptocurrency variant, where we show that relaxing non-malleability assumptions enables a functionality, specifically an atomic asset swap, that is useful for cryptocurrency applications. The work culminates with a study of a less general, algebraic NIZK, and particularly its updatability properties, whose applicability we present within the context of ensuring privacy for regulatory compliance purposes

    Secure Messaging with Strong Compromise Resilience, Temporal Privacy, and Immediate Decryption

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    Recent years have seen many advances in designing secure messaging protocols, aiming at provably strong security properties in theory or high efficiency for real-world practical deployment. However, important trade-off areas of the design space inbetween these elements have not yet been explored. In this work we design the first provably secure protocol that at the same time achieves (i) strong resilience against finegrained compromise, (ii) temporal privacy, and (iii) immediate decryption with constant-size overhead, notably, in the postquantum (PQ) setting. Besides these main design goals, we introduce a novel definition of offline deniability suitable for our setting, and prove that our protocol meets it, notably when combined with a PQ offline deniable initial key exchange
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