There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

From the Publisher:
A valuable reference for the novice as well as for the expert who needs a wider scope of coverage within the area of cryptography, this book provides easy and rapid access of information and includes more than 200 algorithms and protocols; more than 200 tables and figures; more than 1,000 numbered definitions, facts, examples, notes, and remarks; and over 1,250 significant references, including brief comments on each paper.

Handbook of Applied Cryptography

The term Federated Learning was coined as recently as 2016 to describe a machine learning setting where multiple entities collaborate in solving a machine learning problem, under the coordination of a central server or service provider. Each client’s raw data is stored locally and not exchanged or transferred; instead, focused updates intended for immediate aggregation are used to achieve the learning objective. Since then, the topic has gathered much interest across many different disciplines and the realization that solving many of these interdisciplinary problems likely requires not just machine learning but techniques from distributed optimization, cryptography, security, differential privacy, fairness, compressed sensing, systems, information theory, statistics, and more. This monograph has contributions from leading experts across the disciplines, who describe the latest state-of-the art from their perspective. These contributions have been carefully curated into a comprehensive treatment that enables the reader to understand the work that has been done and get pointers to where effort is required to solve many of the problems before Federated Learning can become a reality in practical systems. Researchers working in the area of distributed systems will find this monograph an enlightening read that may inspire them to work on the many challenging issues that are outlined. This monograph will get the reader up to speed quickly and easily on what is likely to become an increasingly important topic: Federated Learning.

/pdf/advances-and-open-problems-in-federated-learning-3t0jgb90ia.pdf

Advances and open problems in federated learning

We present Tor, a circuit-based low-latency anonymous communication service. This second-generation Onion Routing system addresses limitations in the original design by adding perfect forward secrecy, congestion control, directory servers, integrity checking, configurable exit policies, and a practical design for location-hidden services via rendezvous points. Tor works on the real-world Internet, requires no special privileges or kernel modifications, requires little synchronization or coordination between nodes, and provides a reasonable tradeoff between anonymity, usability, and efficiency. We briefly describe our experiences with an international network of more than 30 nodes. We close with a list of open problems in anonymous communication.

/pdf/tor-the-second-generation-onion-router-2qfrg0ppqb.pdf

Tor: the second-generation onion router

In the Hamadryas baboon, males are substantially larger than females. A troop of baboons is subdivided into a number of ‘one-male groups’, consisting of one adult male and one or more females with their young. The male prevents any of ‘his’ females from moving too far from him. Kummer (1971) performed the following experiment. Two males, A and B, previously unknown to each other, were placed in a large enclosure. Male A was free to move about the enclosure, but male B was shut in a small cage, from which he could observe A but not interfere. A female, unknown to both males, was then placed in the enclosure. Within 20 minutes male A had persuaded the female to accept his ownership. Male B was then released into the open enclosure. Instead of challenging male A , B avoided any contact, accepting A’s ownership.

Evolution and the Theory of Games

We introduce a new cryptographic technique that we call universal re-encryption. A conventional cryptosystem that permits re-encryption, such as ElGamal, does so only for a player with knowledge of the public key corresponding to a given ciphertext. In contrast, universal re-encryption can be done without knowledge of public keys. We propose an asymmetric cryptosystem with universal re-encryption that is half as efficient as standard ElGamal in terms of computation and storage.

/pdf/universal-re-encryption-for-mixnets-1icf0rnwrn.pdf

Universal Re-encryption for Mixnets

Tor is vulnerable to network congestion and performance problems due to bulk data transfers. A large fraction of the available network capacity is consumed by a small percentage of Tor users, resulting in severe service degradation for the majority. Bulk users continuously drain relays of excess bandwidth, creating new network bottlenecks and exacerbating the effects of existing ones. While this problem may currently be attributed to rational users utilizing the network, it may also be exploited by a relatively low-resource adversary using similar techniques to contribute to a network denial of service (DoS) attack. Degraded service discourages the use of Tor, affecting both Tor's client diversity and anonymity. 
 
Equipped with mechanisms from communication networks, we design and implement three Tor-specific algorithms that throttle bulk transfers to reduce network congestion and increase network responsiveness. Unlike existing techniques, our algorithms adapt to network dynamics using only information local to a relay. We experiment with full-network deployments of our algorithms under a range of light to heavy network loads. We find that throttling results in significant improvements to web client performance while mitigating the negative effects of bulk transfers. We also analyze how throttling affects anonymity and compare the security of our algorithms under adversarial attack. We find that throttling reduces information leakage compared to unthrottled Tor while improving anonymity against realistic adversaries.

Throttling Tor Bandwidth Parasites.

This paper describes various types of commitment functions that maintain a secret for a predictable time delay or until a moderate and predictable amount of computation has occurred. The properties we set out for such functions are based on their usefulness for various applications, such as publicly verifiable lotteries, rather than for cryptologic investigation of the functions. In these lotteries, winners are chosen fairly using only internal information. Since all this information may be published (even before the lottery closes), anyone can do the calculation and therefore verify that the winner was chosen correctly. Since the calculation uses a delaying or similar function, neither ticket purchasers nor the lottery organizer can take advantage of this information. We describe several such lotteries and the security requirements they satisfy, assuming that functions with the properties we state are used.

Temporarily hidden bit commitment and lottery applications

The previous talk was about trying to get entropy, and I’m going to talk about ignoring entropy. It’s natural to talk about anonymity in a workshop about brief encounters and security protocols, I would say anonymity is the quintessence of brief encounter, you only have a brief encounter if in fact you are anonymous, so you need to be anonymous to guarantee that it is brief, otherwise it is at best pseudonymous, because you’re preserving state from one instance of communication to another.

/pdf/why-i-m-not-an-entropist-2clh8b5swx.pdf

Why I’m Not an Entropist

The paper presents applications for the weak protection of secrets in which weakness is not just acceptable but desirable. For one application, two versions of a lottery scheme are presented in which the result of the lottery is determined by the ticket numbers purchased, but no one can control the outcome or determine what it is until after the lottery closes. This is because the outcome is kept secret in a way that is breakable after a predictable amount of time and/or computation. Another presented application is a variant on fair exchange protocols that requires no trusted third party at all.

Weakly secret bit commitment: applications to lotteries and fair exchange

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