Prochlo: Strong Privacy for Analytics in the Crowd

The large-scale monitoring of computer users' software activities has become commonplace, e.g., for application telemetry, error reporting, or demographic profiling. This paper describes a principled systems architecture---Encode, Shuffle, Analyze (ESA)---for performing such monitoring with high utility while also protecting user privacy. The ESA design, and its Prochlo implementation, are informed by our practical experiences with an existing, large deployment of privacy-preserving software monitoring. (cont.; see the paper

CONSTRUCTION OF EFFICIENT AUTHENTICATION SCHEMES USING TRAPDOOR HASH FUNCTIONS

In large-scale distributed systems, where adversarial attacks can have widespread impact, authentication provides protection from threats involving impersonation of entities and tampering of data. Practical solutions to authentication problems in distributed systems must meet specific constraints of the target system, and provide a reasonable balance between security and cost. The goal of this dissertation is to address the problem of building practical and efficient authentication mechanisms to secure distributed applications. This dissertation presents techniques to construct efficient digital signature schemes using trapdoor hash functions for various distributed applications. Trapdoor hash functions are collision-resistant hash functions associated with a secret trapdoor key that allows the key-holder to find collisions between hashes of different messages. The main contributions of this dissertation are as follows: 1. A common problem with conventional trapdoor hash functions is that revealing a collision producing message pair allows an entity to compute additional collisions without knowledge of the trapdoor key. To overcome this problem, we design an efficient trapdoor hash function that prevents all entities except the trapdoor key-holder from computing collisions regardless of whether collision producing message pairs are revealed by the key-holder. 2. We design a technique to construct efficient proxy signatures using trapdoor hash functions to authenticate and authorize agents acting on behalf of users in agent-based computing systems. Our technique provides agent authentication, assurance of agreement between delegator and agent, security without relying on secure communication channels and control over an agent’s capabilities. 3. We develop a trapdoor hash-based signature amortization technique for authenticating real-time, delay-sensitive streams. Our technique provides independent verifiability of blocks comprising a stream, minimizes sender-side and receiver-side delays, minimizes communication overhead, and avoids transmission of redundant information. 4. We demonstrate the practical efficacy of our trapdoor hash-based techniques for signature amortization and proxy signature construction by presenting discrete log-based instantiations of the generic techniques that are efficient to compute, and produce short signatures. Our detailed performance analyses demonstrate that the proposed schemes outperform existing schemes in computation cost and signature size. We also present proofs for security of the proposed discrete-log based instantiations against forgery attacks under the discrete-log assumption

Authentication Protocols and Privacy Protection

Tato dizertační práce se zabývá kryptografickými prostředky pro autentizaci. Hlavním tématem však nejsou klasické autentizační protokoly, které nabízejí pouze ověření identity, ale tzv. atributové autentizační systémy, pomocí kterých mohou uživatelé prokazovat svoje osobní atributy. Tyto atributy pak mohou představovat jakékoliv osobní informace, např. věk, národnost či místo narození. Atributy mohou být prokazovány anonymně a s podporou mnoha funkcí na ochranu digitální identity. Mezi takové funkce patří např. nespojitelnost autentizačních relací, nesledovatelnost, možnost výběru prokazovaných atributů či efektivní revokace. Atributové autentizační systémy jsou již nyní považovány za nástupce současných systémů v oficiálních strategických plánech USA (NSTIC) či EU (ENISA). Část požadovaných funkcí je již podporována existujícími kryptografickými koncepty jako jsou U-Prove či idemix. V současné době však není známý systém, který by poskytoval všechny potřebné funkce na ochranu digitální identity a zároveň byl prakticky implementovatelný na zařízeních, jako jsou čipové karty. Mezi klíčové slabiny současných systémů patří především chybějící nespojitelnost relací a absence revokace. Není tak možné efektivně zneplatnit zaniklé uživatele, ztracené či ukradené autentizační karty či karty škodlivých uživatelů. Z těchto důvodů je v této práci navrženo kryptografické schéma, které řeší slabiny nalezené při analýze existujících řešení. Výsledné schéma, jehož návrh je založen na ověřených primitivech, jako jsou $\Sigma$-protokoly pro důkazy znalostí, kryptografické závazky či ověřitelné šifrování, pak podporuje všechny požadované vlastnosti pro ochranu soukromí a digitální identity. Zároveň je však návrh snadno implementovatelný v prostředí smart-karet. Tato práce obsahuje plný kryptografický návrh systému, formální ověření klíčových vlastností, matematický model schématu v programu Mathematica pro ověření funkčnosti a výsledky experimentální implementace v prostředí .NET smart-karet. I přesto, že navrhovaný systém obsahuje podporu všech funkcí na ochranu soukromí, včetně těch, které chybí u existujících systémů, jeho výpočetní složitost zůstává stejná či nižší, doba ověření uživatele je tedy kratší než u existujících systémů. Výsledkem je schéma, které může velmi znatelně zvýšit ochranu soukromí uživatelů při jejich ověřování, především při využití v elektronických dokladech, přístupových systémech či Internetových službách.This dissertation thesis deals with the cryptographic constructions for user authentication. Rather than classical authentication protocols which allow only the identity verification, the attribute authentication systems are the main topic of this thesis. The attribute authentication systems allow users to give proofs about the possession of personal attributes. These attributes can represent any personal information, for example age, nationality or birthplace. The attribute ownership can be proven anonymously and with the support of many features for digital identity protection. These features include, e.g., the unlinkability of verification sessions, untraceability, selective disclosure of attributes or efficient revocation. Currently, the attribute authentication systems are considered to be the successors of existing authentication systems by the official strategies of USA (NSTIC) and EU (ENISA). The necessary features are partially provided by existing cryptographic concepts like U-Prove and idemix. But at this moment, there is no system providing all privacy-enhancing features which is implementable on computationally restricted devices like smart-cards. Among all weaknesses of existing systems, the missing unlinkability of verification sessions and the absence of practical revocation are the most critical ones. Without these features, it is currently impossible to invalidate expired users, lost or stolen authentication cards and cards of malicious users. Therefore, a new cryptographic scheme is proposed in this thesis to fix the weaknesses of existing schemes. The resulting scheme, which is based on established primitives like $\Sigma$-protocols for proofs of knowledge, cryptographic commitments and verifiable encryption, supports all privacy-enhancing features. At the same time, the scheme is easily implementable on smart-cards. This thesis includes the full cryptographic specification, the formal verification of key properties, the mathematical model for functional verification in Mathematica software and the experimental implementation on .NET smart-cards. Although the scheme supports all privacy-enhancing features which are missing in related work, the computational complexity is the same or lower, thus the time of verification is shorter than in existing systems. With all these features and properties, the resulting scheme can significantly improve the privacy of users during their verification, especially when used in electronic ID systems, access systems or Internet services.

Bringing data minimization to digital wallets at scale with general-purpose zero-knowledge proofs

Today, digital identity management for individuals is either inconvenient and error-prone or creates undesirable lock-in effects and violates privacy and security expectations. These shortcomings inhibit the digital transformation in general and seem particularly concerning in the context of novel applications such as access control for decentralized autonomous organizations and identification in the Metaverse. Decentralized or self-sovereign identity (SSI) aims to offer a solution to this dilemma by empowering individuals to manage their digital identity through machine-verifiable attestations stored in a "digital wallet" application on their edge devices. However, when presented to a relying party, these attestations typically reveal more attributes than required and allow tracking end users' activities. Several academic works and practical solutions exist to reduce or avoid such excessive information disclosure, from simple selective disclosure to data-minimizing anonymous credentials based on zero-knowledge proofs (ZKPs). We first demonstrate that the SSI solutions that are currently built with anonymous credentials still lack essential features such as scalable revocation, certificate chaining, and integration with secure elements. We then argue that general-purpose ZKPs in the form of zk-SNARKs can appropriately address these pressing challenges. We describe our implementation and conduct performance tests on different edge devices to illustrate that the performance of zk-SNARK-based anonymous credentials is already practical. We also discuss further advantages that general-purpose ZKPs can easily provide for digital wallets, for instance, to create "designated verifier presentations" that facilitate new design options for digital identity infrastructures that previously were not accessible because of the threat of man-in-the-middle attacks

Formal Foundations for Anonymous Communication

Mit jeder Online-Tätigkeit hinterlassen wir digitale Fußspuren. Unternehmen und Regierungen nutzen die privaten Informationen, die von den riesigen Datenmengen der Online-Spuren abgeleitet werden können, um ihre Nutzer und Büger zu manipulieren. Als Gegenmaßnahme wurden anonyme Kommunikationsnetze vorgeschlagen. Diesen fehlen jedoch umfassende formale Grundlagen und folglich ist der Vergleich zwischen verschiedenen Ansätzen nur sehr eingeschränkt möglich. Mit einer gemeinsamen Grundlage zwischen allen Forschern und Entwicklern von anonymen Kommunikationsnetzen können Missverständnisse vermieden werden und die dringend benötigte Entwicklung von den Netzen wird beschleunigt. Mit Vergleichbarkeit zwischen den Lösungen, können die für den jeweiligen Anwendungsfall optimalen Netze besser identifiziert und damit die Entwicklungsanstrengungen gezielter auf Projekte verteilt werden. Weiterhin ermöglichen formale Grundlagen und Vergleichbarkeit ein tieferes Verständnis für die Grenzen und Effekte der eingesetzten Techniken zu erlangen. Diese Arbeit liefert zuerst neue Erkenntnisse zu generellen Formalisierungen für anonyme Kommunikation, bevor sie sich dann auf die praktisch am meisten verbreitete Technik konzentriert: Onion Routing und Mix Netzwerke. Als erstes wird die Vergleichbarkeit zwischen Privatsphärezielen sichergestellt, indem sie formal definiert und miteinander verglichen werden. Dabei enteht eine umfangreiche Hierarchie von eindeutigen Privatsphärezielen. Als zweites werden vorgeschlagene Netzwerke analysiert, um deren Grundbausteine zu identifizieren und deren Schutz als Auswirkung in der Hierarchy zu untersuchen. Diese Grunlagen erlauben Konflikte und Schwachstellen in existierenden Arbeiten zu entdecken und aufzuklären. Genauer zeigt sich damit, dass basierend of derselben informalen Definition verschieden stark schützende formale Versionen entstanden sind. Weiterhin werden in dieser Arbeit die Notions genutzt um existierende Unmöglichkeitsresultate für anonyme Kommunikation zu vergleichen. Dabei wird nicht nur die erste vollständige Sicht auf alle bekannten Schranken für anonyme Kommunikationsnetze gegeben, sondern mit einem tiefgründigen Ansatz werden die existierenden Schranken auch gestärkt und zu praktischen, dem Stand der Kunst entsprechenden Netzen in Bezug gesetzt. Letztlich konnten durch die generellen Betrachtungen von vorgeschlagenen Netzwerken und ihren Grundbausteinen, insbesondere auch Angriffe auf die vorherrschende Klasse von anonymen Kommunikationsnetzen gefunden werden: auf Onion Routing und Mix-Netzwerke. Davon motiviert wurden als zweiter Teil dieser Arbeit die formalen Grundlagen und praktisch eingesetzten Lösungen for Onion Routing und Mix-Netzwerke untersucht. Dabei wurde festgestellt, dass die bereits erwähnten Angriffe teilweise auf eine fehlerhafte, aber weit verbreitete Beweisstrategie für solche Netze zurückzuführen sind und es wurde eine sichere Beweisstrategie als deren Ersatz vorgeschlagen. Weiterhin wurde die neue Strategie für ein vorgeschlagenes, aber bisher nicht weiter verwendetes Paketformat eingesetzt und dieses als sicher bewiesen. Dieses Paketformat unterstützt allerdings keine Rückantworten, was höchstwahrscheinlich der Grund ist, aus dem sich aktuelle Netze auf ein unsicheres Paketformat verlassen. Deshalb wurde im Rahmen dieser Arbeit eine konzeptuelle, sichere Lösung für Onion Routing mit Rückantworten entworfen. Als weitere verwandte Beiträge, zeigt die Arbeit Beziehungen von Teilen der generellen Ergebnisse für anonyme Kommunikationsnetze zu ähnlichen, aber bisher hauptsächlich getrennt betrachteten Forschungsbereichen, wie Privatsphäre auf der Bitübertragungsschicht, Kontaktnachverfolgung und privatsphäre-schützenden, digitalen Bezahlsystemen

Practice-Oriented Privacy in Cryptography

While formal cryptographic schemes can provide strong privacy guarantees, heuristic schemes that prioritize efficiency over formal rigor are often deployed in practice, which can result in privacy loss. Academic schemes that do receive rigorous attention often lack concrete efficiency or are difficult to implement. This creates tension between practice and research, leading to deployed privacy-preserving systems that are not backed by strong cryptographic guarantees. To address this tension between practice and research, we propose a practice-oriented privacy approach, which focuses on designing systems with formal privacy models that can effectively map to real-world use cases. This approach includes analyzing existing privacy-preserving systems to measure their privacy guarantees and how they are used. Furthermore, it explores solutions in the literature and analyzes gaps in their models to design augmented systems that apply more clearly to practice. We focus on two settings of privacy-preserving payments and communications. First, we introduce BlockSci, a software platform that can be used to perform analyses on the privacy and usage of blockchains. Specifically, we assess the privacy of the Dash cryptocurrency and analyze the velocity of cryptocurrencies, finding that Dash’s PrivateSend may still be vulnerable to clustering attacks and that a significant fraction of transactions on Bitcoin are “self-churn” transactions. Next, we build a technique for reducing bandwidth in mixing cryptocurrencies, which suffer from a practical limitation: the size of the transaction growing linearly with the size of the anonymity set. Our proposed technique efficiently samples cover traffic from a finite and public set of known values, while deriving a compact description of the resulting transaction set. We show how this technique can be integrated with various currencies and different cover sampling distributions. Finally, we look at the problem of establishing secure communication channels without access to a trusted public key infrastructure. We construct a scheme that uses network latency and reverse turing tests to detect the presence of eavesdroppers, prove our construction secure, and implement it on top of an existing communication protocol. This line of work bridges the gap between theoretical cryptographic research and real-world deployments to bring better privacy-preserving schemes to end users

Envisioning the Future of Cyber Security in Post-Quantum Era: A Survey on PQ Standardization, Applications, Challenges and Opportunities

The rise of quantum computers exposes vulnerabilities in current public key cryptographic protocols, necessitating the development of secure post-quantum (PQ) schemes. Hence, we conduct a comprehensive study on various PQ approaches, covering the constructional design, structural vulnerabilities, and offer security assessments, implementation evaluations, and a particular focus on side-channel attacks. We analyze global standardization processes, evaluate their metrics in relation to real-world applications, and primarily focus on standardized PQ schemes, selected additional signature competition candidates, and PQ-secure cutting-edge schemes beyond standardization. Finally, we present visions and potential future directions for a seamless transition to the PQ era

Security of Ubiquitous Computing Systems

The chapters in this open access book arise out of the EU Cost Action project Cryptacus, the objective of which was to improve and adapt existent cryptanalysis methodologies and tools to the ubiquitous computing framework. The cryptanalysis implemented lies along four axes: cryptographic models, cryptanalysis of building blocks, hardware and software security engineering, and security assessment of real-world systems. The authors are top-class researchers in security and cryptography, and the contributions are of value to researchers and practitioners in these domains. This book is open access under a CC BY license