Efficient asynchronous accumulators for distributed PKI

Cryptographic accumulators are a tool for compact set representation and secure set membership proofs. When an element is added to a set by means of an accumulator, a membership witness is generated. This witness can later be used to prove the membership of the element. Typically, the membership witness has to be synchronized with the accumulator value, and to be updated every time another element is added to the accumulator. In this work we propose an accumulator that, unlike any prior scheme, does not require strict synchronization. In our construction a membership witness needs to be updated only a logarithmic number of times in the number of subsequent element additions. Thus, an out-of-date witness can be easily made current. Vice versa, a verifier with an out-of-date accumulator value can still verify a current membership witness. These properties make our accumulator construction uniquely suited for use in distributed applications, such as blockchain-based public key infrastructures

Cryptography with anonymity in mind

Advances in information technologies gave a rise to powerful ubiquitous com- puting devices, and digital networks have enabled new ways of fast communication, which immediately found tons of applications and resulted in large amounts of data being transmitted. For decades, cryptographic schemes and privacy-preserving protocols have been studied and researched in order to offer end users privacy of their data and implement useful functionalities at the same time, often trading security properties for cryptographic assumptions and efficiency. In this plethora of cryptographic constructions, anonymity properties play a special role, as they are important in many real-life scenarios. However, many useful cryptographic primitives lack anonymity properties or imply prohibitive costs to achieve them. In this thesis, we expand the territory of cryptographic primitives with anonymity in mind. First, we define Anonymous RAM, a generalization of a single- user Oblivious RAM to multiple mistrusted users, and present two constructions thereof with different trade-offs between assumptions and efficiency. Second, we define an encryption scheme that allows to establish chains of ciphertexts anony- mously and verify their integrity. Furthermore, the aggregatable version of the scheme allows to build a Parallel Anonymous RAM, which enhances Anonymous RAM by supporting concurrent users. Third, we show our technique for construct- ing efficient non-interactive zero-knowledge proofs for statements that consist of both algebraic and arithmetic statements. Finally, we show our framework for constructing efficient single secret leader election protocols, which have been recently identified as an important component in proof-of-stake cryptocurrencies.Fortschritte in der Informationstechnik haben leistungsstarke allgegenwärtige Rechner hervorgerufen, während uns digitale Netzwerke neue Wege für die schnelle Kommunikation ermöglicht haben. Durch die Vielzahl von Anwendungen führte dies zur Übertragung von riesigen Datenvolumen. Seit Jahrzehnten wurden bereits verschiedene kryptographische Verfahren und Technologien zum Datenschutz erforscht und analysiert. Das Ziel ist die Privatsphäre der Benutzer zu schützen und gleichzeitig nützliche Funktionalität anzubieten, was oft mit einem Kompromiss zwischen Sicherheitseigenschaften, kryptographischen Annahmen und Effizienz verbunden ist. In einer Fülle von kryptographischen Konstruktionen spielen Anonymitätseigenschaften eine besondere Rolle, da sie in vielen realistischen Szenarien sehr wichtig sind. Allerdings fehlen vielen kryptographischen Primitive Anonymitätseigenschaften oder sie stehen im Zusammenhang mit erheblichen Kosten. In dieser Dissertation erweitern wir den Bereich von kryptographischen Prim- itiven mit einem Fokus auf Anonymität. Erstens definieren wir Anonymous RAM, eine Verallgemeinerung von Einzelbenutzer-Oblivious RAM für mehrere misstraute Benutzer, und stellen dazu zwei Konstruktionen mit verschiedenen Kompromissen zwischen Annahmen und Effizienz vor. Zweitens definieren wir ein Verschlüsselungsverfahren, das es erlaubt anonym eine Verbindung zwischen Geheimtexten herzustellen und deren Integrität zu überprüfen. Darüber hinaus bietet die aggregierbare Variante von diesem Verfahren an, Parallel Anonymous RAM zu bauen. Dieses verbessert Anonymous RAM, indem es mehrere Benutzer in einer parallelen Ausführung unterstützen kann. Drittens zeigen wir eine Meth- ode für das Konstruieren effizienter Zero-Knowledge-Protokolle, die gleichzeitig aus algebraischen und arithmetischen Teilen bestehen. Zuletzt zeigen wir ein Framework für das Konstruieren effizienter Single-Leader-Election-Protokolle, was kürzlich als ein wichtiger Bestandteil in den Proof-of-Stake Kryptowährungen erkannt worden ist

Efficient and Verifiable Algorithms for Secure Outsourcing of Cryptographic Computations

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.Reducing computational cost of cryptographic computations for resource-constrained devices is an active research area. One of the practical solutions is to securely outsource the computations to an external and more powerful cloud server. Modular exponentiations are the most expensive computation from the cryptographic point of view. Therefore, outsourcing modular exponentiations to a single, external and potentially untrusted cloud server while ensuring the security and privacy provide an efficient solution. In this paper, we propose new efficient outsourcing algorithms for modular exponentiations using only one untrusted cloud server. These algorithms cover public-base & private-exponent, private-base & public-exponent, private-base & privateexponent, and more generally private-base & private-exponents simultaneous modular exponentiations. Our algorithms are the most efficient solutions utilizing only one single untrusted server with best checkability probabilities. Furthermore, unlike existing schemes, which have fixed checkability probability, our algorithms provide adjustable predetermined checkability parameters. Finally, we apply our algorithms to outsource Oblivious Transfer Protocols and Blind Signatures which are expensive primitives in modern cryptography

Threshold Partially-Oblivious PRFs with Applications to Key Management

An Oblivious PRF (OPRF) is a protocol between a server holding a key to a PRF and a user holding an input. At the end of the interaction, the user learns the output of the OPRF on its input and nothing else. The server learns nothing, including nothing about the user\u27s input or the function\u27s output. OPRFs have found many applications in multiple areas of cryptography. Everspaugh et al. (Usenix 2015) introduced Partially Oblivious PRF (pOPRF) in which the OPRF accepts an additional non-secret input that can be chosen by the server itself, and showed applications in the setting of password hardening protocols. We further investigate pOPRFs showing new constructions, including distributed multi-server schemes, and new applications. We build simple pOPRFs from regular OPRFs, in particular obtaining very efficient DH-based pOPRFs, and provide (n,t)-threshold implementation of such schemes. We apply these schemes to build Oblivious Key Management Systems (KMS) as a much more secure alternative to traditional wrapping-based KMS. The new system hides keys and object identifiers from the KMS, offers unconditional security for key transport, enables forward security, provides key verifiability, reduces storage, and more. Further, we show how to provide all these features in a distributed threshold implementation that additionally protects the service against server compromise. Finally, we extend the scheme to a threshold Oblivious KMS with updatable encryption so that upon the periodic change of OPRF keys by the server, an efficient update procedure allows a client of the KMS service to non-interactively update all its encrypted data to be decryptable only by the new key. Our techniques improve on the efficiency and security of several recent works on updatable encryption from Crypto and Eurocrypt. We report on an implementation of the above schemes and their performance, showing their practicality and readiness for use in real-world systems. In particular, our pOPRF constructions achieve speeds of over an order of magnitude relative to previous pOPRF schemes

Efficient and secure ranked multi-keyword search on encrypted cloud data

Information search and document retrieval from a remote database (e.g. cloud server) requires submitting the search terms to the database holder. However, the search terms may contain sensitive information that must be kept secret from the database holder. Moreover, the privacy concerns apply to the relevant documents retrieved by the user in the later stage since they may also contain sensitive data and reveal information about sensitive search terms. A related protocol, Private Information Retrieval (PIR), provides useful cryptographic tools to hide the queried search terms and the data retrieved from the database while returning most relevant documents to the user. In this paper, we propose a practical privacy-preserving ranked keyword search scheme based on PIR that allows multi-keyword queries with ranking capability. The proposed scheme increases the security of the keyword search scheme while still satisfying efficient computation and communication requirements. To the best of our knowledge the majority of previous works are not efficient for assumed scenario where documents are large files. Our scheme outperforms the most efficient proposals in literature in terms of time complexity by several orders of magnitude

Structure-Preserving Smooth Projective Hashing

International audienceSmooth projective hashing has proven to be an extremely useful primitive, in particular when used in conjunction with commitments to provide implicit decommitment. This has lead to applications proven secure in the UC framework, even in presence of an adversary which can do adaptive corruptions, like for example Password Authenticated Key Exchange (PAKE), and 1-out-of-m Oblivious Transfer (OT). However such solutions still lack in efficiency, since they heavily scale on the underlying message length. Structure-preserving cryptography aims at providing elegant and efficient schemes based on classical assumptions and standard group operations on group elements. Recent trend focuses on constructions of structure- preserving signatures, which require message, signature and verification keys to lie in the base group, while the verification equations only consist of pairing-product equations. Classical constructions of Smooth Projective Hash Function suffer from the same limitation as classical signatures: at least one part of the computation (messages for signature, witnesses for SPHF) is a scalar. In this work, we introduce and instantiate the concept of Structure- Preserving Smooth Projective Hash Function, and give as applications more efficient instantiations for one-round PAKE and three-round OT, and information retrieval thanks to Anonymous Credentials, all UC- secure against adaptive adversaries