arXiv subjects

Yvo Desmedt

Publications and source records attributed to Yvo Desmedt.

14 recordsLinked to original sources

arXiv2026

Oblivis: A Framework for Delegated and Efficient Oblivious Transfer

As database deployments shift toward cloud platforms and edge devices, thin clients need to securely retrieve sensitive records without leaking their query intent or metadata to the proxies that mediate access. Oblivious Transfer (OT) is a core tool for private retrieval, yet existing OTs assume direct client-database interaction and lack support for delegated querying or lightweight clients. We present Oblivis, a modular framework of new OT protocols that enable delegated, privacy-preserving query execution. Oblivis allows clients to retrieve database records without direct access, protects against leakage to both databases and proxies, and is designed with practical efficiency in mind. Its components include: (1) Delegated-Query OT, which permits secure outsourcing of query generation; (2) Multi-Receiver OT for merged, cloud-hosted databases; (3) a compiler producing constant-size responses suitable for thin clients; and (4) Supersonic OT, a proxy-based, informationtheoretic, and highly efficient 1-out-of-2 OT. The protocols are formally defined and proven secure in the simulation-based paradigm, under non-colluding assumption. We implement and empirically evaluate Supersonic OT. It achieves at least a 92x speedup over a highly efficient 1-out-of-2 OT, and a 2.6x-106x speedup over a standard OT extension across 200-100,000 invocations. Our implementation further shows that Supersonic OT remains efficient even on constrained hardware, e.g., it completes an end-to-end transfer in 1.36 ms on a Raspberry Pi 4.

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arXiv2024

Supersonic OT: Fast Unconditionally Secure Oblivious Transfer

Oblivious Transfer (OT) is a fundamental cryptographic protocol with applications in secure Multi-Party Computation, Federated Learning, and Private Set Intersection. With the advent of quantum computing, it is crucial to develop unconditionally secure core primitives like OT to ensure their continued security in the post-quantum era. Despite over four decades since OT's introduction, the literature has predominantly relied on computational assumptions, except in cases using unconventional methods like noisy channels or a fully trusted party. Introducing "Supersonic OT", a highly efficient and unconditionally secure OT scheme that avoids public-key-based primitives, we offer an alternative to traditional approaches. Supersonic OT enables a receiver to obtain a response of size O(1). Its simple (yet non-trivial) design facilitates easy security analysis and implementation. The protocol employs a basic secret-sharing scheme, controlled swaps, the one-time pad, and a third-party helper who may be corrupted by a semi-honest adversary. Our implementation and runtime analysis indicate that a single instance of Supersonic OT completes in 0.35 milliseconds, making it up to 2000 times faster than the state-of-the-art base OT.

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arXiv2024

Delegated-Query Oblivious Transfer and its Practical Applications

Databases play a pivotal role in the contemporary World Wide Web and the world of cloud computing. Unfortunately, numerous privacy violations have recently garnered attention in the news. To enhance database privacy, we consider Oblivious Transfer (OT), an elegant cryptographic technology. Our observation reveals that existing research in this domain primarily concentrates on theoretical cryptographic applications, overlooking various practical aspects: - OTs assume parties have direct access to databases. Our "1-out-of-2 Delegated-Query OT" enables parties to privately query a database, without direct access. - With the rise of cloud computing, physically separated databases may no longer remain so. Our "1-out-of-2 Delegated-Query Multi-Receiver OT" protects privacy in such evolving scenarios. - Research often ignores the limitations of thin clients, e.g., Internet of Things devices. To address this, we propose a compiler that transforms any 1-out-of-n OT into a thin client version.

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arXiv2021

Extremal Set Theory and LWE Based Access Structure Hiding Verifiable Secret Sharing with Malicious-Majority and Free Verification

Secret sharing allows distributing a secret among several parties such that only authorized subsets, specified by an access structure, can reconstruct the secret. Sehrawat and Desmedt (COCOON 2020) introduced hidden access structures, that remain secret until some authorized subset of parties collaborate. However, their scheme assumes semi-honest parties and supports only restricted access structures. We address these shortcomings by constructing an access structure hiding verifiable secret sharing scheme that supports all monotone access structures. It is the first secret sharing scheme to support cheater identification and share verifiability in malicious-majority settings. The verification procedure of our scheme incurs no communication overhead. As the building blocks of our scheme, we introduce and construct: (i) a set-system with $> \exp\left(c\frac{2(\log h)^2}{(\log\log h)}\right)+2\exp\left(c\frac{(\log h)^2}{(\log\log h)}\right)$ subsets of a set of $h$ elements. Our set-system, $\mathcal{H}$, is defined over $\mathbb{Z}_m$, where $m$ is a non-prime-power. The size of each set in $\mathcal{H}$ is divisible by $m$ but the sizes of their pairwise intersections are not, unless one set is a subset of another, (ii) a new variant of the learning with errors (LWE) problem, called PRIM-LWE, wherein the secret matrix is sampled such that its determinant is a generator of $\mathbb{Z}_q^*$, where $q$ is the LWE modulus. The security of our scheme relies on the hardness of the LWE problem, and its share size is $$(1+ o(1)) \dfrac{2^{\ell}}{\sqrt{π\ell/2}}(2 q^{\varrho + 0.5} + \sqrt{q} + \mathrmΘ(h)),$$ where $\varrho \leq 1$ is a constant and $\ell$ is the total number of parties. We also provide directions for future work to reduce the share size to \[\leq \dfrac{1}{3} \left( (1+ o(1)) \dfrac{2^{\ell}}{\sqrt{π\ell/2}}(2 q^{\varrho + 0.5} + 2\sqrt{q}) \right).\]

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arXiv2021

Access Structure Hiding Secret Sharing from Novel Set Systems and Vector Families

Secret sharing provides a means to distribute shares of a secret such that any authorized subset of shares, specified by an access structure, can be pooled together to recompute the secret. The standard secret sharing model requires public access structures, which violates privacy and facilitates the adversary by revealing high-value targets. In this paper, we address this shortcoming by introducing \emph{hidden access structures}, which remain secret until some authorized subset of parties collaborate. The central piece of this work is the construction of a set-system $\mathcal{H}$ with strictly greater than $\exp\left(c \dfrac{1.5 (\log h)^2}{\log \log h}\right)$ subsets of a set of $h$ elements. Our set-system $\mathcal{H}$ is defined over $\mathbb{Z}_m$, where $m$ is a non-prime-power, such that the size of each set in $\mathcal{H}$ is divisible by $m$ but the sizes of their pairwise intersections are not divisible by $m$, unless one set is a subset of another. We derive a vector family $\mathcal{V}$ from $\mathcal{H}$ such that superset-subset relationships in $\mathcal{H}$ are represented by inner products in $\mathcal{V}$. We use $\mathcal{V}$ to "encode" the access structures and thereby develop the first \emph{access structure hiding} secret sharing scheme. For a setting with $\ell$ parties, our scheme supports $2^{\binom{\ell}{\ell/2+1}}$ out of the $2^{2^{\ell - O(\log \ell)}}$ total monotone access structures, and its maximum share size for any access structures is $(1+ o(1)) \dfrac{2^{\ell+1}}{\sqrt{π\ell/2}}$. The scheme assumes semi-honest polynomial-time parties, and its security relies on the Generalized Diffie-Hellman assumption.

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arXiv2020

Bi-Homomorphic Lattice-Based PRFs and Unidirectional Updatable Encryption

We define a pseudorandom function (PRF) $F: \mathcal{K} \times \mathcal{X} \rightarrow \mathcal{Y}$ to be bi-homomorphic when it is fully Key homomorphic and partially Input Homomorphic (KIH), i.e., given $F(k_1, x_1)$ and $F(k_2, x_2)$, there is an efficient algorithm to compute $F(k_1 \oplus k_2, x_1 \ominus x_2)$, where $\oplus$ and $\ominus$ are (binary) group operations. The homomorphism on the input is restricted to a fixed subset of the input bits, i.e., $\ominus$ operates on some pre-decided $m$-out-of-$n$ bits, where $|x_1| = |x_2| = n$, and the remaining $n-m$ bits are identical in both inputs. In addition, the output length, $\ell$, of the operator $\ominus$ is not fixed and is defined as $n \leq \ell \leq 2n$, hence leading to Homomorphically induced Variable input Length (HVL) as $n \leq |x_1 \ominus x_2| \leq 2n$. We present a learning with errors (LWE) based construction for a HVL-KIH-PRF family. Our construction is inspired by the key homomorphic PRF construction due to Banerjee and Peikert (Crypto 2014). An updatable encryption scheme allows rotations of the encryption key, i.e., moving existing ciphertexts from old to new key. These updates are carried out via \emph{update tokens}, which can be used by an untrusted party since the update procedure does not involve decryption of the ciphertext. We use our novel PRF family to construct an updatable encryption scheme, named QPC-UE-UU, which is quantum-safe, post-compromise secure and supports unidirectional ciphertext updates, i.e., the update tokens can be used to perform ciphertext updates but they cannot be used to undo already completed updates. Our PRF family also leads to the first left/right key homomorphic constrained-PRF family with HVL.

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arXiv2019

Realistic versus Rational Secret Sharing

The study of Rational Secret Sharing initiated by Halpern and Teague regards the reconstruction of the secret in secret sharing as a game. It was shown that participants (parties) may refuse to reveal their shares and so the reconstruction may fail. Moreover, a refusal to reveal the share may be a dominant strategy of a party. In this paper we consider secret sharing as a sub-action or subgame of a larger action/game where the secret opens a possibility of consumption of a certain common good. We claim that utilities of participants will be dependent on the nature of this common good. In particular, Halpern and Teague scenario corresponds to a rivalrous and excludable common good. We consider the case when this common good is non-rivalrous and non-excludable and find many natural Nash equilibria. We list several applications of secret sharing to demonstrate our claim and give corresponding scenarios. In such circumstances the secret sharing scheme facilitates a power sharing agreement in the society. We also state that non-reconstruction may be beneficial for this society and give several examples.

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arXiv2016

Function-Based Access Control (FBAC): From Access Control Matrix to Access Control Tensor

Security researchers have stated that the core concept behind current implementations of access control predates the Internet. These assertions are made to pinpoint that there is a foundational gap in this field, and one should consider revisiting the concepts from the ground up. Moreover, Insider threats, which are an increasing threat vector against organizations are also associated with the failure of access control. Access control models derived from access control matrix encompass three sets of entities, Subjects, Objects and Operations. Typically, objects are considered to be files and operations are regarded as Read, Write, and Execute. This implies an `open sesame' approach when granting access to data, i.e. once access is granted, there is no restriction on command executions. Inspired by Functional Encryption, we propose applying access authorizations at a much finer granularity, but instead of an ad-hoc or computationally hard cryptographic approach, we postulate a foundational transformation to access control. From an abstract viewpoint, we suggest storing access authorizations as a three-dimensional tensor, which we call Access Control Tensor (ACT). In Function-based Access Control (FBAC), applications do not give blind folded execution right and can only invoke commands that have been authorized for data segments. In other words, one might be authorized to use a certain command on one object, while being forbidden to use exactly the same command on another object. The theoretical foundations of FBAC are presented along with Policy, Enforcement and Implementation (PEI) requirements of it. A critical analysis of the advantages of deploying FBAC, how it will result in developing a new generation of applications, and compatibility with existing models and systems is also included. Finally, a proof of concept implementation of FBAC is presented.

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arXiv2015

Making Code Voting Secure against Insider Threats using Unconditionally Secure MIX Schemes and Human PSMT Protocols

Code voting was introduced by Chaum as a solution for using a possibly infected-by-malware device to cast a vote in an electronic voting application. Chaum's work on code voting assumed voting codes are physically delivered to voters using the mail system, implicitly requiring to trust the mail system. This is not necessarily a valid assumption to make - especially if the mail system cannot be trusted. When conspiring with the recipient of the cast ballots, privacy is broken. It is clear to the public that when it comes to privacy, computers and "secure" communication over the Internet cannot fully be trusted. This emphasizes the importance of using: (1) Unconditional security for secure network communication. (2) Reduce reliance on untrusted computers. In this paper we explore how to remove the mail system trust assumption in code voting. We use PSMT protocols (SCN 2012) where with the help of visual aids, humans can carry out $\mod 10$ addition correctly with a 99\% degree of accuracy. We introduce an unconditionally secure MIX based on the combinatorics of set systems. Given that end users of our proposed voting scheme construction are humans we \emph{cannot use} classical Secure Multi Party Computation protocols. Our solutions are for both single and multi-seat elections achieving: \begin{enumerate}[i)] \item An anonymous and perfectly secure communication network secure against a $t$-bounded passive adversary used to deliver voting, \item The end step of the protocol can be handled by a human to evade the threat of malware. \end{enumerate} We do not focus on active adversaries.

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arXiv2012

Edge-Colored Graphs with Applications To Homogeneous Faults

In this paper, we use the concept of colored edge graphs to model homogeneous faults in networks. We then use this model to study the minimum connectivity (and design) requirements of networks for being robust against homogeneous faults within certain thresholds. In particular, necessary and sufficient conditions for most interesting cases are obtained. For example, we will study the following cases: (1) the number of colors (or the number of non-homogeneous network device types) is one more than the homogeneous fault threshold; (2) there is only one homogeneous fault (i.e., only one color could fail); and (3) the number of non-homogeneous network device types is less than five.

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arXiv2012

Your Facebook Deactivated Friend or a Cloaked Spy (Extended Abstract)

With over 750 million active users, Facebook is the most famous social networking website. One particular aspect of Facebook widely discussed in the news and heavily researched in academic circles is the privacy of its users. In this paper we introduce a zero day privacy loophole in Facebook. We call this the deactivated friend attack. The concept of the attack is very similar to cloaking in Star Trek while its seriousness could be estimated from the fact that once the attacker is a friend of the victim, it is highly probable the attacker has indefinite access to the victims private information in a cloaked way. We demonstrate the impact of the attack by showing the ease of gaining trust of Facebook users and being befriended online. With targeted friend requests we were able to add over 4300 users and maintain access to their Facebook profile information for at least 261 days. No user was able to unfriend us during this time due to cloaking and short de-cloaking sessions. The short de-cloaking sessions were enough to get updates about the victims. We also provide several solutions for the loophole, which range from mitigation to a permanent solution

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arXiv2011

Preliminary Analysis of Google+'s Privacy

In this paper we provide a preliminary analysis of Google+ privacy. We identified that Google+ shares photo metadata with users who can access the photograph and discuss its potential impact on privacy. We also identified that Google+ encourages the provision of other names including maiden name, which may help criminals performing identity theft. We show that Facebook lists are a superset of Google+ circles, both functionally and logically, even though Google+ provides a better user interface. Finally we compare the use of encryption and depth of privacy control in Google+ versus in Facebook.

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arXiv2010

Equilibria of Plurality Voting with Abstentions

In the traditional voting manipulation literature, it is assumed that a group of manipulators jointly misrepresent their preferences to get a certain candidate elected, while the remaining voters are truthful. In this paper, we depart from this assumption, and consider the setting where all voters are strategic. In this case, the election can be viewed as a game, and the election outcomes correspond to Nash equilibria of this game. We use this framework to analyze two variants of Plurality voting, namely, simultaneous voting, where all voters submit their ballots at the same time, and sequential voting, where the voters express their preferences one by one. For simultaneous voting, we characterize the preference profiles that admit a pure Nash equilibrium, but show that it is computationally hard to check if a given profile fits our criterion. For sequential voting, we provide a complete analysis of the setting with two candidates, and show that for three or more candidates the equilibria of sequential voting may behave in a counterintuitive manner.

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arXiv2002

Perfectly Secure Message Transmission Revisited

Achieving secure communications in networks has been one of the most important problems in information technology. Dolev, Dwork, Waarts, and Yung have studied secure message transmission in one-way or two-way channels. They only consider the case when all channels are two-way or all channels are one-way. Goldreich, Goldwasser, and Linial, Franklin and Yung, Franklin and Wright, and Wang and Desmedt have studied secure communication and secure computation in multi-recipient (multicast) models. In a ``multicast channel'' (such as Ethernet), one processor can send the same message--simultaneously and privately--to a fixed subset of processors. In this paper, we shall study necessary and sufficient conditions for achieving secure communications against active adversaries in mixed one-way and two-way channels. We also discuss multicast channels and neighbor network channels.

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