Generic Double-Authentication Preventing Signatures and a Post-Quantum Instantiation

Double-authentication preventing signatures (DAPS) are a variant of digital signatures which have received considerable attention recently (Derler et al. EuroS&P 2018, Poettering AfricaCrypt 2018). They are unforgeable signatures in the usual sense and sign messages that are composed of an address and a payload. Their distinguishing feature is the property that signing two different payloads with respect to the same address allows to publicly extract the secret signing key from two such signatures. DAPS are known in the factoring, the discrete logarithm and the lattice setting. The majority of the constructions are ad-hoc. Only recently, Derler et al. (EuroS&P 2018) presented the first generic construction that allows to extend any discrete logarithm based secure signatures scheme to DAPS. However, their scheme has the drawback that the number of potential addresses (the address space) used for signing is polynomially bounded (and in fact small) as the size of secret and the public keys of the resulting DAPS are linear in the address space. In this paper we overcome this limitation and present a generic construction of DAPS with constant size keys and signatures. Our techniques are not tailored to a specific algebraic setting and in particular allow us to construct the first DAPS without structured hardness assumptions, i.e., from symmetric key primitives, yielding a candidate for post-quantum secure DAPS

Viiteraamistik turvariskide haldamiseks plokiahela abil

Turvalise tarkvara loomiseks on olemas erinevad programmid (nt OWASP), ohumudelid (nt STRIDE), turvariskide juhtimise mudelid (nt ISSRM) ja eeskirjad (nt GDPR). Turvaohud aga arenevad pidevalt, sest traditsiooniline tehnoloogiline infrastruktuur ei rakenda turvameetmeid kavandatult. Blockchain näib leevendavat traditsiooniliste rakenduste turvaohte. Kuigi plokiahelapõhiseid rakendusi peetakse vähem haavatavateks, ei saanud need erinevate turvaohtude eest kaitsmise hõbekuuliks. Lisaks areneb plokiahela domeen pidevalt, pakkudes uusi tehnikaid ja sageli vahetatavaid disainikontseptsioone, mille tulemuseks on kontseptuaalne ebaselgus ja segadus turvaohtude tõhusal käsitlemisel. Üldiselt käsitleme traditsiooniliste rakenduste TJ-e probleemi, kasutades vastumeetmena plokiahelat ja plokiahelapõhiste rakenduste TJ-t. Alustuseks uurime, kuidas plokiahel leevendab traditsiooniliste rakenduste turvaohte, ja tulemuseks on plokiahelapõhine võrdlusmudel (PV), mis järgib TJ-e domeenimudelit. Järgmisena esitleme PV-it kontseptualiseerimisega alusontoloogiana kõrgema taseme võrdlusontoloogiat (ULRO). Pakume ULRO kahte eksemplari. Esimene eksemplar sisaldab Cordat, kui lubatud plokiahelat ja finantsjuhtumit. Teine eksemplar sisaldab lubadeta plokiahelate komponente ja tervishoiu juhtumit. Mõlemad ontoloogiaesitlused aitavad traditsiooniliste ja plokiahelapõhiste rakenduste TJ-es. Lisaks koostasime veebipõhise ontoloogia parsimise tööriista OwlParser. Kaastööde tulemusel loodi ontoloogiapõhine turberaamistik turvariskide haldamiseks plokiahela abil. Raamistik on dünaamiline, toetab TJ-e iteratiivset protsessi ja potentsiaalselt vähendab traditsiooniliste ja plokiahelapõhiste rakenduste turbeohte.Various programs (e.g., OWASP), threat models (e.g., STRIDE), security risk management models (e.g., ISSRM), and regulations (e.g., GDPR) exist to communicate and reduce the security threats to build secure software. However, security threats continuously evolve because the traditional technology infrastructure does not implement security measures by design. Blockchain is appearing to mitigate traditional applications’ security threats. Although blockchain-based applications are considered less vulnerable, they did not become the silver bullet for securing against different security threats. Moreover, the blockchain domain is constantly evolving, providing new techniques and often interchangeable design concepts, resulting in conceptual ambiguity and confusion in treating security threats effectively. Overall, we address the problem of traditional applications’ SRM using blockchain as a countermeasure and the SRM of blockchain-based applications. We start by surveying how blockchain mitigates the security threats of traditional applications, and the outcome is a blockchain-based reference model (BbRM) that adheres to the SRM domain model. Next, we present an upper-level reference ontology (ULRO) as a foundation ontology and provide two instantiations of the ULRO. The first instantiation includes Corda as a permissioned blockchain and the financial case. The second instantiation includes the permissionless blockchain components and the healthcare case. Both ontology representations help in the SRM of traditional and blockchain-based applications. Furthermore, we built a web-based ontology parsing tool, OwlParser. Contributions resulted in an ontology-based security reference framework for managing security risks using blockchain. The framework is dynamic, supports the iterative process of SRM, and potentially lessens the security threats of traditional and blockchain-based applications.https://www.ester.ee/record=b551352

Cryptography for Bitcoin and friends

Numerous cryptographic extensions to Bitcoin have been proposed since Satoshi Nakamoto introduced the revolutionary design in 2008. However, only few proposals have been adopted in Bitcoin and other prevalent cryptocurrencies, whose resistance to fundamental changes has proven to grow with their success. In this dissertation, we introduce four cryptographic techniques that advance the functionality and privacy provided by Bitcoin and similar cryptocurrencies without requiring fundamental changes in their design: First, we realize smart contracts that disincentivize parties in distributed systems from making contradicting statements by penalizing such behavior by the loss of funds in a cryptocurrency. Second, we propose CoinShuffle++, a coin mixing protocol which improves the anonymity of cryptocurrency users by combining their transactions and thereby making it harder for observers to trace those transactions. The core of CoinShuffle++ is DiceMix, a novel and efficient protocol for broadcasting messages anonymously without the help of any trusted third-party anonymity proxies and in the presence of malicious participants. Third, we combine coin mixing with the existing idea to hide payment values in homomorphic commitments to obtain the ValueShuffle protocol, which enables us to overcome major obstacles to the practical deployment of coin mixing protocols. Fourth, we show how to prepare the aforementioned homomorphic commitments for a safe transition to post-quantum cryptography.Seit seiner revolutionären Erfindung durch Satoshi Nakamoto im Jahr 2008 wurden zahlreiche kryptographische Erweiterungen für Bitcoin vorgeschlagen. Gleichwohl wurden nur wenige Vorschläge in Bitcoin und andere weit verbreitete Kryptowährungen integriert, deren Resistenz gegen tiefgreifende Veränderungen augenscheinlich mit ihrer Verbreitung wächst. In dieser Dissertation schlagen wir vier kryptographische Verfahren vor, die die Funktionalität und die Datenschutzeigenschaften von Bitcoin und ähnlichen Kryptowährungen verbessern ohne deren Funktionsweise tiefgreifend verändern zu müssen. Erstens realisieren wir Smart Contracts, die es erlauben widersprüchliche Aussagen einer Vertragspartei mit dem Verlust von Kryptogeld zu bestrafen. Zweitens schlagen wir CoinShuffle++ vor, ein Mix-Protokoll, das die Anonymität von Benutzern verbessert, indem es ihre Transaktionen kombiniert und so deren Rückverfolgung erschwert. Sein Herzstück ist DiceMix, ein neues und effizientes Protokoll zur anonymen Veröffentlichung von Nachrichten ohne vertrauenswürdige Dritte und in der Präsenz von bösartigen Teilnehmern. Drittens kombinieren wir dieses Protokoll mit der existierenden Idee, Geldbeträge in Commitments zu verbergen, und erhalten so das ValueShuffle-Protokoll, das uns ermöglicht, große Hindernisse für den praktischen Einsatz von Mix-Protokollen zu überwinden. Viertens zeigen wir, wie die dabei benutzten Commitments für einen sicheren Übergang zu Post-Quanten-Kryptographie vorbereitet werden können

Efficient FPGA Implementations of LowMC and Picnic

Post-quantum cryptography has received increased attention in recent years, in particular, due to the standardization effort by NIST. One of the second-round candidates in the NIST post-quantum standardization project is Picnic, a post-quantum secure signature scheme based on efficient zero-knowledge proofs of knowledge. In this work, we present the first FPGA implementation of Picnic. We show how to efficiently calculate LowMC, the block cipher used as a one-way function in Picnic, in hardware despite the large number of constants needed during computation. We then combine our LowMC implementation and efficient instantiations of Keccak to build the full Picnic algorithm. Additionally, we conform to recently proposed hardware interfaces for post-quantum schemes to enable easier comparisons with other designs. We provide evaluations of our Picnic implementation for both, the standalone design and a version wrapped with a PCIe interface, and compare them to the state-of-the-art software implementations of Picnic and similar hardware designs. Concretely, signing messages on our FPGA takes 0.25 ms for the L1 security level and 1.24 ms for the L5 security level, beating existing optimized software implementations by a factor of 4

Cryptographic Tools for Privacy Preservation

Data permeates every aspect of our daily life and it is the backbone of our digitalized society. Smartphones, smartwatches and many more smart devices measure, collect, modify and share data in what is known as the Internet of Things.Often, these devices don’t have enough computation power/storage space thus out-sourcing some aspects of the data management to the Cloud. Outsourcing computation/storage to a third party poses natural questions regarding the security and privacy of the shared sensitive data.Intuitively, Cryptography is a toolset of primitives/protocols of which security prop- erties are formally proven while Privacy typically captures additional social/legislative requirements that relate more to the concept of “trust” between people, “how” data is used and/or “who” has access to data. This thesis separates the concepts by introducing an abstract model that classifies data leaks into different types of breaches. Each class represents a specific requirement/goal related to cryptography, e.g. confidentiality or integrity, or related to privacy, e.g. liability, sensitive data management and more.The thesis contains cryptographic tools designed to provide privacy guarantees for different application scenarios. In more details, the thesis:(a) defines new encryption schemes that provide formal privacy guarantees such as theoretical privacy definitions like Differential Privacy (DP), or concrete privacy-oriented applications covered by existing regulations such as the European General Data Protection Regulation (GDPR);(b) proposes new tools and procedures for providing verifiable computation’s guarantees in concrete scenarios for post-quantum cryptography or generalisation of signature schemes;(c) proposes a methodology for utilising Machine Learning (ML) for analysing the effective security and privacy of a crypto-tool and, dually, proposes a secure primitive that allows computing specific ML algorithm in a privacy-preserving way;(d) provides an alternative protocol for secure communication between two parties, based on the idea of communicating in a periodically timed fashion

Post-Quantum Signal Key Agreement with SIDH

In the effort to transition cryptographic primitives and protocols to quantum-resistant alternatives, an interesting and useful challenge is found in the Signal protocol. The initial key agreement component of this protocol, called X3DH, has so far proved more subtle to replace - in part due to the unclear security model and properties the original protocol is designed for. This paper defines a formal security model for the original signal protocol, in the context of the standard eCK and CK+ type models, which we call the Signal-adapted-CK model. We then propose a secure replacement for the Signal X3DH key exchange protocol based on SIDH, and provide a proof of security in the Signal-adapted-CK model, showing our protocol satisfies all security properties of the original Signal X3DH. We call this new protocol SI-X3DH. Our protocol refutes the claim of Brendel, Fischlin, Günther, Janson, and Stebila [Selected Areas in Cryptography (2020)] that SIDH cannot be used to construct a secure X3DH replacement due to adaptive attacks. Unlike the generic constructions proposed in the literature, our protocol achieves deniability without expensive machinery such as post-quantum ring signatures. It also benefits from the efficiency of SIDH as a key-exchange protocol, compared to other post-quantum key exchange protocols such as CSIDH

Tightly-Secure Signatures from Five-Move Identification Protocols

We carry out a concrete security analysis of signature schemes obtained from five-move identification protocols via the Fiat-Shamir transform. Concretely, we obtain tightly-secure signatures based on the computational Diffie-Hellman (CDH), the short-exponent CDH, and the Factoring (FAC) assumptions. All our signature schemes have tight reductions to search problems, which is in stark contrast to all known signature schemes obtained from the classical Fiat-Shamir transform (based on three-move identification protocols), which either have a non-tight reduction to a search problem, or a tight reduction to a (potentially) stronger decisional problem. Surprisingly, our CDH-based scheme turns out to be (a slight simplification of) the Chevallier-Mames signature scheme (CRYPTO 05), thereby providing a theoretical explanation of its tight security proof via five-move identification protocols

Efficient Hybrid Exact/Relaxed Lattice Proofs and Applications to Rounding and VRFs

In this work, we study hybrid exact/relaxed zero-knowledge proofs from lattices, where the proved relation is exact in one part and relaxed in the other. Such proofs arise in important real-life applications such as those requiring verifiable PRF evaluation and have so far not received significant attention as a standalone problem. We first introduce a general framework, LANES+, for realizing such hybrid proofs efficiently by combining standard relaxed proofs of knowledge RPoK and the LANES framework (due to a series of works in Crypto\u2720, Asiacrypt\u2720, ACM CCS\u2720). The latter framework is a powerful lattice-based proof system that can prove exact linear and multiplicative relations. The advantage of LANES+ is its ability to realize hybrid proofs more efficiently by exploiting RPoK for the high-dimensional part of the secret witness while leaving a low-dimensional secret witness part for the exact proof that is proven at a significantly lower cost via LANES. Thanks to the flexibility of LANES+, other exact proof systems can also be supported. We apply our LANES+ framework to construct substantially shorter proofs of rounding, which is a central tool for verifiable deterministic lattice-based cryptography. Based on our rounding proof, we then design an efficient long-term verifiable random function (VRF), named LaV. LaV leads to the shortest VRF outputs among the proposals of standard (i.e., long-term and stateless) VRFs based on quantum-safe assumptions. Of independent interest, we also present generalized results for challenge difference invertibility, a fundamental soundness security requirement for many proof systems

LATKE: An identity-binding PAKE from lattice assumptions

In a recent work, Cremers, Naor, Paz, and Ronen (CRYPTO \u2722) point out the problem of catastrophic impersonation in balanced password authenticated key exchange protocols (PAKEs). Namely, in a balanced PAKE, when a single party is compromised, the attacker learns the password and can subsequently impersonate anyone to anyone using the same password. The authors of the work present two solutions to this issue: CHIP, an identity-binding PAKE (iPAKE), and CRISP, a strong identity-binding PAKE (siPAKE). These constructions prevent the impersonation attack by generating a secret key on setup that is inextricably tied to the party\u27s identity, and then deleting the password. Thus, upon compromise, all an attacker can immediately do is impersonate the victim. The strong variant goes further, preventing attackers from performing any precomputation before the compromise occurs. In this work we present LATKE, an iPAKE from lattice assumptions in the random oracle model. In order to achieve security and correctness, we must make changes to CHIP\u27s primitives, security models, and protocol structure