Lattice-Based Group Signatures: Achieving Full Dynamicity (and Deniability) with Ease

In this work, we provide the first lattice-based group signature that offers full dynamicity (i.e., users have the flexibility in joining and leaving the group), and thus, resolve a prominent open problem posed by previous works. Moreover, we achieve this non-trivial feat in a relatively simple manner. Starting with Libert et al.'s fully static construction (Eurocrypt 2016) - which is arguably the most efficient lattice-based group signature to date, we introduce simple-but-insightful tweaks that allow to upgrade it directly into the fully dynamic setting. More startlingly, our scheme even produces slightly shorter signatures than the former, thanks to an adaptation of a technique proposed by Ling et al. (PKC 2013), allowing to prove inequalities in zero-knowledge. Our design approach consists of upgrading Libert et al.'s static construction (EUROCRYPT 2016) - which is arguably the most efficient lattice-based group signature to date - into the fully dynamic setting. Somewhat surprisingly, our scheme produces slightly shorter signatures than the former, thanks to a new technique for proving inequality in zero-knowledge without relying on any inequality check. The scheme satisfies the strong security requirements of Bootle et al.'s model (ACNS 2016), under the Short Integer Solution (SIS) and the Learning With Errors (LWE) assumptions. Furthermore, we demonstrate how to equip the obtained group signature scheme with the deniability functionality in a simple way. This attractive functionality, put forward by Ishida et al. (CANS 2016), enables the tracing authority to provide an evidence that a given user is not the owner of a signature in question. In the process, we design a zero-knowledge protocol for proving that a given LWE ciphertext does not decrypt to a particular message

Foundations of Fully Dynamic Group Signatures

Group signatures allow members of a group to anonymously sign on behalf of the group. Membership is administered by a designated group manager. The group manager can also reveal the identity of a signer if and when needed to enforce accountability and deter abuse. For group signatures to be applicable in practice, they need to support fully dynamic groups, i.e., users may join and leave at any time. Existing security definitions for fully dynamic group signatures are informal, have shortcomings, and are mutually incompatible. We fill the gap by providing a formal rigorous security model for fully dynamic group signatures. Our model is general and is not tailored toward a specific design paradigm and can therefore, as we show, be used to argue about the security of different existing constructions following different design paradigms. Our definitions are stringent and when possible incorporate protection against maliciously chosen keys. We consider both the case where the group management and tracing signatures are administered by the same authority, i.e., a single group manager, and also the case where those roles are administered by two separate authorities, i.e., a group manager and an opening authority. We also show that a specialization of our model captures existing models for static and partially dynamic schemes. In the process, we identify a subtle gap in the security achieved by group signatures using revocation lists. We show that in such schemes new members achieve a slightly weaker notion of traceability. The flexibility of our security model allows to capture such relaxation of traceability

Fully Anonymous Group Signature with Verifier-Local Revocation

Group signature with verifier-local revocation (VLR-GS) is a special type of revocable group sig- nature which enables a user to sign messages without referring to information regarding revoked users. Although there have been several proposals of VLR-GS schemes since the first scheme proposed by Boneh and Shacham [CCS 2004], all of these schemes only achieve a security notion called selfless anonymity, which is strictly weaker than the de facto standard security notion, full anonymity. Thus, for more than a decade, it has been an open problem whether a fully anonymous VLR-GS scheme can be achieved. In this paper, we give an affirmative answer to this problem. Concretely, we show the construction of a fully anonymous VLR-GS scheme from a digital signature scheme, a key-private public key encryption scheme, and a non-interactive zero-knowledge proof system. Also, we show that backward unlinkability, which ensures that even after a user is revoked, signatures produced by the user before the revocation remain anonymous, can be realized without additional building blocks. Although the size of group public key and signing key depend on the number of time periods, finally, we show that the size of these keys can be reduced by employing an identity-based encryption scheme

SoK: Privacy-Preserving Signatures

Modern security systems depend fundamentally on the ability of users to authenticate their communications to other parties in a network. Unfortunately, cryptographic authentication can substantially undermine the privacy of users. One possible solution to this problem is to use privacy-preserving cryptographic authentication. These protocols allow users to authenticate their communications without revealing their identity to the verifier. In the non-interactive setting, the most common protocols include blind, ring, and group signatures, each of which has been the subject of enormous research in the security and cryptography literature. These primitives are now being deployed at scale in major applications, including Intel\u27s SGX software attestation framework. The depth of the research literature and the prospect of large-scale deployment motivate us to systematize our understanding of the research in this area. This work provides an overview of these techniques, focusing on applications and efficiency

Location Privacy in VANETs: Improved Chaff-Based CMIX and Privacy-Preserving End-to-End Communication

VANETs communication systems are technologies and defined policies that can be formed to enable ITS applications to provide road traffic efficacy, warning about such issues as environmental dangers, journey circumstances, and in the provision of infotainment that considerably enhance transportation safety and quality. The entities in VANETs, generally vehicles, form part of a massive network known as the Internet of Vehicles (IoV). The deployment of large-scale VANETs systems is impossible without ensuring that such systems are themselves are safe and secure, protecting the privacy of their users. There is a risk that cars might be hacked, or their sensors become defective, causing inaccurate information to be sent across the network. Consequently, the activities and credentials of participating vehicles should be held responsible and quickly broadcast throughout a vast VANETs, considering the accountability in the system. The openness of wireless communication means that an observer can eavesdrop on vehicular communication and gain access or otherwise deduce users' sensitive information, and perhaps profile vehicles based on numerous factors such as tracing their travels and the identification of their home/work locations. In order to protect the system from malicious or compromised entities, as well as to preserve user privacy, the goal is to achieve communication security, i.e., keep users' identities hidden from both the outside world and the security infrastructure and service providers. Being held accountable while still maintaining one's privacy is a difficult balancing act. This thesis explores novel solution paths to the above challenges by investigating the impact of low-density messaging to improve the security of vehicle communications and accomplish unlinkability in VANETs. This is achieved by proposing an improved chaff-based CMIX protocol that uses fake messages to increase density to mitigate tracking in this scenario. Recently, Christian \etall \cite{vaas2018nowhere} proposed a Chaff-based CMIX scheme that sends fake messages under the presumption low-density conditions to enhance vehicle privacy and confuse attackers. To accomplish full unlinkability, we first show the following security and privacy vulnerabilities in the Christian \etall scheme: linkability attacks outside the CMIX may occur due to deterministic data-sharing during the authentication phase (e.g., duplicate certificates for each communication). Adversaries may inject fake certificates, which breaks Cuckoo Filters' (CFs) updates authenticity, and the injection may be deniable. CMIX symmetric key leakage outside the coverage may occur. We propose a VPKI-based protocol to mitigate these issues. First, we use a modified version of Wang \etall's \cite{wang2019practical} scheme to provide mutual authentication without revealing the real identity. To this end, a vehicle's messages are signed with a different pseudo-identity “certificate”. Furthermore, the density is increased via the sending of fake messages during low traffic periods to provide unlinkability outside the mix-zone. Second, unlike Christian \etall's scheme, we use the Adaptive Cuckoo Filter (ACF) instead of CF to overcome the effects of false positives on the whole filter. Moreover, to prevent any alteration of the ACFs, only RUSs distribute the updates, and they sign the new fingerprints. Third, mutual authentication prevents any leakage from the mix zones' symmetric keys by generating a fresh one for each communication through a Diffie–Hellman key exchange. As a second main contribution of this thesis, we focus on the V2V communication without the interference of a Trusted Third Party (TTP)s in case this has been corrupted, destroyed, or is out of range. This thesis presents a new and efficient end-to-end anonymous key exchange protocol based on Yang \etall's \cite{yang2015self} self-blindable signatures. In our protocol, vehicles first privately blind their own private certificates for each communication outside the mix-zone and then compute an anonymous shared key based on zero-knowledge proof of knowledge (PoK). The efficiency comes from the fact that once the signatures are verified, the ephemeral values in the PoK are also used to compute a shared key through an authenticated Diffie-Hellman key exchange protocol. Therefore, the protocol does not require any further external information to generate a shared key. Our protocol also does not require interfacing with the Roadside Units or Certificate Authorities, and hence can be securely run outside the mixed-zones. We demonstrate the security of our protocol in ideal/real simulation paradigms. Hence, our protocol achieves secure authentication, forward unlinkability, and accountability. Furthermore, the performance analysis shows that our protocol is more efficient in terms of computational and communications overheads compared to existing schemes.Kuwait Cultural Offic

Efficient signature verification and key revocation using identity based cryptography

Cryptography deals with the development and evaluation of procedures for securing digital information. It is essential whenever multiple entities want to communicate safely. One task of cryptography concerns digital signatures and the verification of a signer’s legitimacy requires trustworthy authentication and authorization. This is achieved by deploying cryptographic keys. When dynamic membership behavior and identity theft come into play, revocation of keys has to be addressed. Additionally, in use cases with limited networking, computational, or storage resources, efficiency is a key requirement for any solution. In this work we present a solution for signature verification and key revocation in constraned environments, e.g., in the Internet of Things (IoT). Where other mechanisms generate expensive overheads, we achieve revocation through a single multicast message without significant computational or storage overhead. Exploiting Identity Based Cryptography (IBC) complements the approach with efficient creation and verification of signatures. Our solution offers a framework for transforming a suitable signature scheme to a so-called Key Updatable Signature Scheme (KUSS) in three steps. Each step defines mathematical conditions for transformation and precise security notions. Thereby, the framework allows a novel combination of efficient Identity Based Signature (IBS) schemes with revocation mechanisms originally designed for confidentiality in group communications. Practical applicability of our framework is demonstrated by transforming four well-established IBS schemes based on Elliptic Curve Cryptography (ECC). The security of the resulting group Identity Based Signature (gIBS) schemes is carefully analyzed with techniques of Provable Security. We design and implement a testbed for evaluating these kind of cryptographic schemes on different computing- and networking hardware, typical for constrained environments. Measurements on this testbed provide evidence that the transformations are practicable and efficient. The revocation complexity in turn is significantly reduced compared to existing solutions. Some of our new schemes even outperform the signing process of the widely used Elliptic Curve Digital Signature Algorithm (ECDSA). The presented transformations allow future application on schemes beyond IBS or ECC. This includes use cases dealing with Post-Quantum Cryptography, where the revocation efficiency is similarly relevant. Our work provides the basis for such solutions currently under investigation.Die Kryptographie ist ein Instrument der Informationssicherheit und beschäftigt sich mit der Entwicklung und Evaluierung von Algorithmen zur Sicherung digitaler Werte. Sie ist für die sichere Kommunikation zwischen mehreren Entitäten unerlässlich. Ein Bestandteil sind digitale Signaturen, für deren Erstellung man kryptographische Schlüssel benötigt. Bei der Verifikation muss zusätzlich die Authentizität und die Autorisierung des Unterzeichners gewährleistet werden. Dafür müssen Schlüssel vertrauensvoll verteilt und verwaltet werden. Wenn sie in Kommunikationssystemen mit häufig wechselnden Teilnehmern zum Einsatz kommen, müssen die Schlüssel auch widerruflich sein. In Anwendungsfällen mit eingeschränkter Netz-, Rechen- und Speicherkapazität ist die Effizienz ein wichtiges Kriterium. Diese Arbeit liefert ein Rahmenwerk, mit dem Schlüssel effizient widerrufen und Signaturen effizient verifiziert werden können. Dabei fokussieren wir uns auf Szenarien aus dem Bereich des Internets der Dinge (IoT, Internet of Things). Im Gegensatz zu anderen Lösungen ermöglicht unser Ansatz den Widerruf von Schlüsseln mit einer einzelnen Nachricht innerhalb einer Kommunikationsgruppe. Dabei fällt nur geringer zusätzlicher Rechen- oder Speicheraufwand an. Ferner vervollständigt die Verwendung von Identitätsbasierter Kryptographie (IBC, Identity Based Cryptography) unsere Lösung mit effizienter Erstellung und Verifikation der Signaturen. Hierfür liefert die Arbeit eine dreistufige mathematische Transformation von geeigneten Signaturverfahren zu sogenannten Key Updatable Signature Schemes (KUSS). Neben einer präzisen Definition der Sicherheitsziele werden für jeden Schritt mathematische Vorbedingungen zur Transformation festgelegt. Dies ermöglicht die innovative Kombination von Identitätsbasierten Signaturen (IBS, Identity Based Signature) mit effizienten und sicheren Mechanismen zum Schlüsselaustausch, die ursprünglich für vertrauliche Gruppenkommunikation entwickelt wurden. Wir zeigen die erfolgreiche Anwendung der Transformationen auf vier etablierten IBSVerfahren. Die ausschließliche Verwendung von Verfahren auf Basis der Elliptic Curve Cryptography (ECC) erlaubt es, den geringen Kapazitäten der Zielgeräte gerecht zu werden. Eine Analyse aller vier sogenannten group Identity Based Signature (gIBS) Verfahren mit Techniken aus dem Forschungsgebiet der Beweisbaren Sicherheit zeigt, dass die zuvor definierten Sicherheitsziele erreicht werden. Zur praktischen Evaluierung unserer und ähnlicher kryptographischer Verfahren wird in dieser Arbeit eine Testumgebung entwickelt und mit IoT-typischen Rechen- und Netzmodulen bestückt. Hierdurch zeigt sich sowohl die praktische Anwendbarkeit der Transformationen als auch eine deutliche Reduktion der Komplexität gegenüber anderen Lösungsansätzen. Einige der von uns vorgeschlagenen Verfahren unterbieten gar die Laufzeiten des meistgenutzten Elliptic Curve Digital Signature Algorithm (ECDSA) bei der Erstellung der Signaturen. Die Systematik der Lösung erlaubt prinzipiell auch die Transformation von Verfahren jenseits von IBS und ECC. Dadurch können auch Anwendungsfälle aus dem Bereich der Post-Quanten-Kryptographie von unseren Ergebnissen profitieren. Die vorliegende Arbeit liefert die nötigen Grundlagen für solche Erweiterungen, die aktuell diskutiert und entwickelt werden

Efficient Zero-Knowledge Proofs and their Applications

A zero-knowledge proof is a fundamental cryptographic primitive that enables the verification of statements without revealing unnecessary information. Zero-knowledge proofs are a key component of many cryptographic protocols and, often, one of their main efficiency bottlenecks. In recent years there have been great advances in improving the efficiency of zero-knowledge proofs, bring them closer to wide deployability. In this thesis we make another step towards the construction of computationally-efficient zero-knowledge proofs. Specifically, we construct efficient zero-knowledge proofs for the satisfiability of arithmetic circuits for which the computational cost of the prover is only a constant factor more expensive than direct evaluation of the circuit. We also construct efficient zero-knowledge proofs to check the correct execution of (Tiny)RAM programs. In this case the computational cost for the prover is a superconstant factor larger than executing the program directly. Our proofs also support efficient verification and small proof sizes. For security, they rely on symmetric primitives and could potentially withstand attacks from quantum computers. On a different research direction, we look at group signatures, a fundamental primitive which relies on zero-knowledge proofs. A group signature enables users to sign anonymously on behalf of a group of users. In case of dispute a Manager can identify the author of a signature and potentially banish the user from the group. In this thesis we address the fundamental question of defining the security of fully dynamic group signatures, for which the users can join and leave at any time. Differently from other restricted settings, this case has been largely overlooked in the past. Our security model is general, does not implicitly assume existing design paradigms and captures the security of existing models for more restricted settings

Zero Knowledge Protocols and Applications

The historical goal of cryptography is to securely transmit or store a message in an insecure medium. In that era, before public key cryptography, we had two kinds of people: those who had the correct key, and those who did not. Nowadays however, we live in a complex world with equally complex goals and requirements: securely passing a note from Alice to Bob is not enough. We want Alice to use her smartphone to vote for Carol, without Bob the tallier, or anyone else learning her vote; we also want guarantees that Alice’s ballot contains a single, valid vote and we want guarantees that Bob will tally the ballots properly. This is in fact made possible because of zero knowledge protocols. This thesis presents research performed in the area of zero knowledge protocols across the following threads: we relax the assumptions necessary for the Damgard, Fazio and ˚ Nicolosi (DFN) transformation, a technique which enables one to collapse a number of three round protocols into a single message. This approach is motivated by showing how it could be used as part of a voting scheme. Then we move onto a protocol that lets us prove that a given computation (modeled as an arithmetic circuit) was performed correctly. It improves upon the state of the art in the area by significantly reducing the communication cost. A second strand of research concerns multi-user signatures, which enable a signer to sign with respect to a set of users. We give new definitions for important primitives in the area as well as efficient instantiations using zero knowledge protocols. Finally, we present two possible answers to the question posed by voting receipts. One is to maximise privacy by building a voting system that provides receipt-freeness automatically. The other is to use them to enable conventual and privacy preserving vote copying

Anonymity and trust in the electronic world

Privacy has never been an explicit goal of authorization mechanisms. The traditional approach to authorisation relies on strong authentication of a stable identity using long term credentials. Audit is then linked to authorization via the same identity. Such an approach compels users to enter into a trust relationship with large parts of the system infrastructure, including entities in remote domains. In this dissertation we advance the view that this type of compulsive trust relationship is unnecessary and can have undesirable consequences. We examine in some detail the consequences which such undesirable trust relationships can have on individual privacy, and investigate the extent to which taking a unified approach to trust and anonymity can actually provide useful leverage to address threats to privacy without compromising the principal goals of authentication and audit. We conclude that many applications would benefit from mechanisms which enabled them to make authorization decisions without using long-term credentials. We next propose specific mechanisms to achieve this, introducing a novel notion of a short-lived electronic identity, which we call a surrogate. This approach allows a localisation of trust and entities are not compelled to transitively trust other entities in remote domains. In particular, resolution of stable identities needs only ever to be done locally to the entity named. Our surrogates allow delegation, enable role-based access control policies to be enforced across multiple domains, and permit the use of non-anonymous payment mechanisms, all without compromising the privacy of a user. The localisation of trust resulting from the approach proposed in this dissertation also has the potential to allow clients to control the risks to which they are exposed by bearing the cost of relevant countermeasures themselves, rather than forcing clients to trust the system infrastructure to protect them and to bear an equal share of the cost of all countermeasures whether or not effective for them. This consideration means that our surrogate-based approach and mechanisms are of interest even in Kerberos-like scenarios where anonymity is not a requirement, but the remote authentication mechanism is untrustworthy

Post-Quantum Era Privacy Protection for Intelligent Infrastructures

As we move into a new decade, the global world of Intelligent Infrastructure (II) services integrated into the Internet of Things (IoT) are at the forefront of technological advancements. With billions of connected devices spanning continents through interconnected networks, security and privacy protection techniques for the emerging II services become a paramount concern. In this paper, an up-to-date privacy method mapping and relevant use cases are surveyed for II services. Particularly, we emphasize on post-quantum cryptography techniques that may (or must when quantum computers become a reality) be used in the future through concrete products, pilots, and projects. The topics presented in this paper are of utmost importance as (1) several recent regulations such as Europe's General Data Protection Regulation (GDPR) have given privacy a significant place in digital society, and (2) the increase of IoT/II applications and digital services with growing data collection capabilities are introducing new threats and risks on citizens' privacy. This in-depth survey begins with an overview of security and privacy threats in IoT/IIs. Next, we summarize some selected Privacy-Enhancing Technologies (PETs) suitable for privacy-concerned II services, and then map recent PET schemes based on post-quantum cryptographic primitives which are capable of withstanding quantum computing attacks. This paper also overviews how PETs can be deployed in practical use cases in the scope of IoT/IIs, and maps some current projects, pilots, and products that deal with PETs. A practical case study on the Internet of Vehicles (IoV) is presented to demonstrate how PETs can be applied in reality. Finally, we discuss the main challenges with respect to current PETs and highlight some future directions for developing their post-quantum counterparts