104 research outputs found

    Hidden in the Cloud : Advanced Cryptographic Techniques for Untrusted Cloud Environments

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    In the contemporary digital age, the ability to search and perform operations on encrypted data has become increasingly important. This significance is primarily due to the exponential growth of data, often referred to as the "new oil," and the corresponding rise in data privacy concerns. As more and more data is stored in the cloud, the need for robust security measures to protect this data from unauthorized access and misuse has become paramount. One of the key challenges in this context is the ability to perform meaningful operations on the data while it remains encrypted. Traditional encryption techniques, while providing a high level of security, render the data unusable for any practical purpose other than storage. This is where advanced cryptographic protocols like Symmetric Searchable Encryption (SSE), Functional Encryption (FE), Homomorphic Encryption (HE), and Hybrid Homomorphic Encryption (HHE) come into play. These protocols not only ensure the confidentiality of data but also allow computations on encrypted data, thereby offering a higher level of security and privacy. The ability to search and perform operations on encrypted data has several practical implications. For instance, it enables efficient Boolean queries on encrypted databases, which is crucial for many "big data" applications. It also allows for the execution of phrase searches, which are important for many machine learning applications, such as intelligent medical data analytics. Moreover, these capabilities are particularly relevant in the context of sensitive data, such as health records or financial information, where the privacy and security of user data are of utmost importance. Furthermore, these capabilities can help build trust in digital systems. Trust is a critical factor in the adoption and use of digital services. By ensuring the confidentiality, integrity, and availability of data, these protocols can help build user trust in cloud services. This trust, in turn, can drive the wider adoption of digital services, leading to a more inclusive digital society. However, it is important to note that while these capabilities offer significant advantages, they also present certain challenges. For instance, the computational overhead of these protocols can be substantial, making them less suitable for scenarios where efficiency is a critical requirement. Moreover, these protocols often require sophisticated key management mechanisms, which can be challenging to implement in practice. Therefore, there is a need for ongoing research to address these challenges and make these protocols more efficient and practical for real-world applications. The research publications included in this thesis offer a deep dive into the intricacies and advancements in the realm of cryptographic protocols, particularly in the context of the challenges and needs highlighted above. Publication I presents a novel approach to hybrid encryption, combining the strengths of ABE and SSE. This fusion aims to overcome the inherent limitations of both techniques, offering a more secure and efficient solution for key sharing and access control in cloud-based systems. Publication II further expands on SSE, showcasing a dynamic scheme that emphasizes forward and backward privacy, crucial for ensuring data integrity and confidentiality. Publication III and Publication IV delve into the potential of MIFE, demonstrating its applicability in real-world scenarios, such as designing encrypted private databases and additive reputation systems. These publications highlight the transformative potential of MIFE in bridging the gap between theoretical cryptographic concepts and practical applications. Lastly, Publication V underscores the significance of HE and HHE as a foundational element for secure protocols, emphasizing its potential in devices with limited computational capabilities. In essence, these publications not only validate the importance of searching and performing operations on encrypted data but also provide innovative solutions to the challenges mentioned. They collectively underscore the transformative potential of advanced cryptographic protocols in enhancing data security and privacy, paving the way for a more secure digital future

    LIPIcs, Volume 251, ITCS 2023, Complete Volume

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    LIPIcs, Volume 251, ITCS 2023, Complete Volum

    Towards compact bandwidth and efficient privacy-preserving computation

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    In traditional cryptographic applications, cryptographic mechanisms are employed to ensure the security and integrity of communication or storage. In these scenarios, the primary threat is usually an external adversary trying to intercept or tamper with the communication between two parties. On the other hand, in the context of privacy-preserving computation or secure computation, the cryptographic techniques are developed with a different goal in mind: to protect the privacy of the participants involved in a computation from each other. Specifically, privacy-preserving computation allows multiple parties to jointly compute a function without revealing their inputs and it has numerous applications in various fields, including finance, healthcare, and data analysis. It allows for collaboration and data sharing without compromising the privacy of sensitive data, which is becoming increasingly important in today's digital age. While privacy-preserving computation has gained significant attention in recent times due to its strong security and numerous potential applications, its efficiency remains its Achilles' heel. Privacy-preserving protocols require significantly higher computational overhead and bandwidth when compared to baseline (i.e., insecure) protocols. Therefore, finding ways to minimize the overhead, whether it be in terms of computation or communication, asymptotically or concretely, while maintaining security in a reasonable manner remains an exciting problem to work on. This thesis is centred around enhancing efficiency and reducing the costs of communication and computation for commonly used privacy-preserving primitives, including private set intersection, oblivious transfer, and stealth signatures. Our primary focus is on optimizing the performance of these primitives.Im Gegensatz zu traditionellen kryptografischen Aufgaben, bei denen Kryptografie verwendet wird, um die Sicherheit und Integrität von Kommunikation oder Speicherung zu gewährleisten und der Gegner typischerweise ein Außenstehender ist, der versucht, die Kommunikation zwischen Sender und Empfänger abzuhören, ist die Kryptografie, die in der datenschutzbewahrenden Berechnung (oder sicheren Berechnung) verwendet wird, darauf ausgelegt, die Privatsphäre der Teilnehmer voreinander zu schützen. Insbesondere ermöglicht die datenschutzbewahrende Berechnung es mehreren Parteien, gemeinsam eine Funktion zu berechnen, ohne ihre Eingaben zu offenbaren. Sie findet zahlreiche Anwendungen in verschiedenen Bereichen, einschließlich Finanzen, Gesundheitswesen und Datenanalyse. Sie ermöglicht eine Zusammenarbeit und Datenaustausch, ohne die Privatsphäre sensibler Daten zu kompromittieren, was in der heutigen digitalen Ära immer wichtiger wird. Obwohl datenschutzbewahrende Berechnung aufgrund ihrer starken Sicherheit und zahlreichen potenziellen Anwendungen in jüngster Zeit erhebliche Aufmerksamkeit erregt hat, bleibt ihre Effizienz ihre Achillesferse. Datenschutzbewahrende Protokolle erfordern deutlich höhere Rechenkosten und Kommunikationsbandbreite im Vergleich zu Baseline-Protokollen (d.h. unsicheren Protokollen). Daher bleibt es eine spannende Aufgabe, Möglichkeiten zu finden, um den Overhead zu minimieren (sei es in Bezug auf Rechen- oder Kommunikationsleistung, asymptotisch oder konkret), während die Sicherheit auf eine angemessene Weise gewährleistet bleibt. Diese Arbeit konzentriert sich auf die Verbesserung der Effizienz und Reduzierung der Kosten für Kommunikation und Berechnung für gängige datenschutzbewahrende Primitiven, einschließlich private Schnittmenge, vergesslicher Transfer und Stealth-Signaturen. Unser Hauptaugenmerk liegt auf der Optimierung der Leistung dieser Primitiven

    Universally Composable End-to-End Secure Messaging

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    We model and analyze the Signal end-to-end secure messaging protocol within the Universal Composability (UC) framework. Specifically: (1) We formulate an ideal functionality that captures end-to-end secure messaging in a setting with Public Key Infrastructure (PKI) and an untrusted server, against an adversary that has full control over the network and can adaptively and momentarily compromise parties at any time, obtaining their entire internal states. Our analysis captures the forward secrecy and recovery-of-security properties of Signal and the conditions under which they break. (2) We model the main components of the Signal architecture (PKI and long-term keys, the backbone continuous-key-exchange or asymmetric ratchet , epoch-level symmetric ratchets, authenticated encryption) as individual ideal functionalities. These components are realized and analyzed separately, and then composed using the UC and Global-State UC theorems. (3) We show how the ideal functionalities representing these components can be realized using standard cryptographic primitives with minimal hardness assumptions. Our modeling introduces additional innovations that enable arguing about the security of Signal, irrespective of the underlying communication medium, and facilitate the secure composition of dynamically generated modules that share state. These features, in conjunction with the basic modularity of the UC framework, will hopefully facilitate the use of both Signal-as-a-whole and its individual components within cryptographic applications

    Imbalanced Cryptographic Protocols

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    Efficiency is paramount when designing cryptographic protocols, heavy mathematical operations often increase computation time, even for modern computers. Moreover, they produce large amounts of data that need to be sent through (often limited) network connections. Therefore, many research efforts are invested in improving efficiency, sometimes leading to imbalanced cryptographic protocols. We define three types of imbalanced protocols, computationally, communicationally, and functionally imbalanced protocols. Computationally imbalanced cryptographic protocols appear when optimizing a protocol for one party having significantly more computing power. In communicationally imbalanced cryptographic protocols the messages mainly flow from one party to the others. Finally, in functionally imbalanced cryptographic protocols the functional requirements of one party strongly differ from the other parties. We start our study by looking into laconic cryptography, which fits both the computational and communicational category. The emerging area of laconic cryptography involves the design of two-party protocols involving a sender and a receiver, where the receiver’s input is large. The key efficiency requirement is that the protocol communication complexity must be independent of the receiver’s input size. We show a new way to build laconic OT based on the new notion of Set Membership Encryption (SME) – a new member in the area of laconic cryptography. SME allows a sender to encrypt to one recipient from a universe of receivers, while using a small digest from a large subset of receivers. A recipient is only able to decrypt the message if and only if it is part of the large subset. As another example of a communicationally imbalanced protocol we will look at NIZKs. We consider the problem of proving in zero-knowledge the existence of exploits in executables compiled to run on real-world processors. Finally, we investigate the problem of constructing law enforcement access systems that mitigate the possibility of unauthorized surveillance, as a functionally imbalanced cryptographic protocol. We present two main constructions. The first construction enables prospective access, allowing surveillance only if encryption occurs after a warrant has been issued and activated. The second allows retrospective access to communications that occurred prior to a warrant’s issuance

    Unbounded Predicate Inner Product Functional Encryption from Pairings

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    Predicate inner product functional encryption (P-IPFE) is essentially attribute-based IPFE (AB-IPFE) which additionally hides attributes associated to ciphertexts. In a P-IPFE, a message x is encrypted under an attribute w and a secret key is generated for a pair (y, v) such that recovery of ⟨x, y⟩ requires the vectors w, v to satisfy a linear relation. We call a P-IPFE unbounded if it can encrypt unbounded length attributes and message vectors. • zero predicate IPFE. We construct the first unbounded zero predicate IPFE (UZP-IPFE) which recovers ⟨x,y⟩ if ⟨w,v⟩ = 0. This construction is inspired by the unbounded IPFE of Tomida and Takashima (ASIACRYPT 2018) and the unbounded zero inner product encryption of Okamoto and Takashima (ASIACRYPT 2012). The UZP-IPFE stands secure against general attackers capable of decrypting the challenge ciphertext. Concretely, it provides full attribute-hiding security in the indistinguishability-based semi-adaptive model under the standard symmetric external Diffie-Hellman assumption. • non-zero predicate IPFE. We present the first unbounded non-zero predicate IPFE (UNP-IPFE) that successfully recovers ⟨x, y⟩ if ⟨w, v⟩ ≠ 0. We generically transform an unbounded quadratic FE (UQFE) scheme to weak attribute-hiding UNP-IPFE in both public and secret key settings. Interestingly, our secret key simulation secure UNP-IPFE has succinct secret keys and is constructed from a novel succinct UQFE that we build in the random oracle model. We leave the problem of constructing a succinct public key UNP-IPFE or UQFE in the standard model as an important open problem

    Toothpicks: More Efficient Fork-Free Two-Round Multi-Signatures

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    Tightly secure cryptographic schemes can be implemented with standardized parameters, while still having a sufficiently high security level backed up by their analysis. In a recent work, Pan and Wagner (Eurocrypt 2023) presented the first tightly secure two-round multi-signature scheme without pairings, called Chopsticks. While this is an interesting first theoretical step, Chopsticks is much less efficient than its non-tight counterparts. In this work, we close this gap by proposing a new tightly secure two-round multi-signature scheme that is as efficient as non-tight schemes. Our scheme is based on the DDH assumption without pairings. Compared to Chopsticks, we reduce the signature size by more than a factor of 3 and the communication complexity by more than a factor of 2. Technically, we achieve this as follows: (1) We develop a new pseudorandom path technique, as opposed to the pseudorandom matching technique in Chopsticks. (2) We construct a more efficient commitment scheme with suitable properties, which is an important primitive in both our scheme and Chopsticks. Surprisingly, we observe that the commitment scheme does not have to be binding, enabling our efficient construction

    Post Quantum Fuzzy Stealth Signatures and Applications

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    Private payments in blockchain-based cryptocurrencies have been a topic of research, both academic and industrial, ever since the advent of Bitcoin. Stealth address payments were proposed as a solution to improve payment privacy for users and are, in fact, deployed in several major cryptocurrencies today. The mechanism lets users receive payments so that none of these payments are linkable to each other or the recipient. Currently known stealth address mechanisms either (1) are insecure in certain reasonable adversarial models, (2) are inefficient in practice or (3) are incompatible with many existing currencies. In this work, we formalize the underlying cryptographic abstraction of this mechanism, namely, stealth signatures with formal game-based definitions. We show a surprising application of our notions to passwordless authentication defined in the Fast IDentity Online (FIDO) standard. We then present SPIRIT, the first efficient post-quantum secure stealth signature construction based on the NIST standardized signature and key-encapsulation schemes, Dilithium and Kyber. The basic form of SPIRIT is only secure in a weak security model, but we provide an efficiency-preserving and generic transform, which boosts the security of SPIRIT to guarantee the strongest security notion defined in this work. Compared to state-of-the-art, there is an approximately 800x improvement in the signature size while keeping signing and verification as efficient as 0.2 ms. We extend SPIRIT with a fuzzy tracking functionality where recipients can outsource the tracking of incoming transactions to a tracking server, satisfying an anonymity notion similar to that of fuzzy message detection (FMD) recently introduced in [CCS 2021]. We also extend SPIRIT with a new fuzzy tracking framework called scalable fuzzy tracking that we introduce in this work. This new framework can be considered as a dual of FMD, in that it reduces the tracking server\u27s computational workload to sublinear in the number of users, as opposed to linear in FMD. Experimental results show that, for millions of users, the server only needs 3.4 ms to filter each incoming message which is a significant improvement upon the state-of-the-art
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