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.

Clouds, social networks and the internet have made communication, storage and computation easily available, at least in recent years. Some engineers and scientists have been arguing that such an infrastructure should have a proper backup. Networks and clouds have been down frequently, e.g., due to accidental destruction of underwater communication cables and software problems, but without having a too long lasted impact. The recent deliberate damage of 4 undersea internet cables between Asia and Europe has changed the threat model to one were “faults” are caused maliciously. Social networks too might see a dramatic change in the near future. Currently, these can be regarded as communication platforms allowing users to broadcast/multicast data. However, if social networks are publishing venues, they can “edit” content. We address the question of communication, computation and storage in a world in which the infrastructure is either under attack and/or evolving. In earlier work, we showed a proof of concept to achieve both reliable and private email even when the availability of untrusted cloud servers was not guaranteed. In this paper we survey this earlier work. We use this successful demonstration to propose a blueprint for a new communication infrastructure to address the aforementioned concerns.

The concept of covertly passing data over a communications channel has existed for hundreds of years. The advent of interconnected computer networks employing intricate layers of protocols created a new medium through which to covertly pass data. This paper explores covert channels on computer networks and examines current and possible future threats. It assesses the risks to individuals and enterprises, and proposes countermeasures to help detect and mitigate the risks brought on by these covert channels.

Nowadays Communication Security usually refers to digital communication and in particular via the Internet. We explain why the topic should be broadened to include any communication, in particular when done in person, e.g., with co-authors, colleagues, reporters, etc. Thousands of papers have been written on blockchain, and consensus. Despite this, the problem of Byzantine attack has been ignored in some important apps! One of these examples is (Outlook) Calendar. Moreover, the Byzantine attack can also be used in the political world. We explain how using it may undermine the security of nations. Finally, we observe that topics on which a lot of research has been done, such as Private Set Intersection have ignored the problem of Byzantine attacks. Although the Byzantine general problem is typically described in a peer-to-peer setting, we show that it can also occur in other scenarios.

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.

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.

Secret sharing, a well-known cryptographic technique, introduced 40 years ago as a private and reliable variant of classical storage, has now become a major cryptographic primitive with numerous real-world applications. In this paper we consider the digital forensics aspects of secret sharing. We investigate the problem of framing which occurs when a coalition is able to calculate the share of a participant who does not belong to it. In the extreme case one authorized coalition can calculate shares of another authorized coalition and use the secret in some way blaming another authorized coalition for their action. In this context seniority plays an important role. We define seniority, which comes natural in the context of hierarchical access structures. Roughly speaking, our work shows that in an ideal secret sharing scheme an authorized coalition cannot frame participants who are less senior than all members of the coalition and is able to frame a participant who is more senior than at least one pivotal member of the coalition. We show that for any monotone access structure there exists a (non-ideal) frameproof secret sharing scheme.

The study of Rational Secret Sharing initiated by Halpern and Teague regards the reconstruction of the secretin 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.

For 35 years, the cryptographic community has created the impression that anonymous communication is always possible. Chaum's dining cryptographer's solution is regarded as achieving unconditional security. Chaum's MIX approach, namely, applying a uniformly random permutation on the plaintexts to be sent, is often given as a definition for anonymity. However, people working in law enforcement know that attempts by whistle-blowers and criminals to remain at large, often fail. The content of the message may reveal the name of the sender or may leak for whom the message is really intended. To understand this issue, we give a definition of perfect anonymity. We contrast our definition with Chaum's approach and discuss the impact on Chaum's dining cryptographer's solution.