PPAA: Peer-to-Peer Anonymous Authentication (Extended Version)

In the pursuit of authentication schemes that balance user privacy and accountability, numerous anonymous credential systems have been constructed. However, existing systems assume a client-server architecture in which only the clients, but not the servers, care about their privacy. In peer-to-peer (P2P) systems where both clients and servers are peer users with privacy concerns, no existing system correctly strikes that balance between privacy and accountability. In this paper, we provide this missing piece: a credential system in which peers are {\em pseudonymous} to one another (that is, two who interact more than once can recognize each other via pseudonyms) but are otherwise anonymous and unlinkable across different peers. Such a credential system finds applications in, e.g., Vehicular Ad-hoc Networks (VANets) and P2P networks. We formalize the security requirements of our proposed credential system, provide a construction for it, and prove the security of our construction. Our solution is efficient: its complexities are independent of the number of users in the system

Critical Perspectives on Provable Security: Fifteen Years of Another Look Papers

We give an overview of our critiques of “proofs” of security and a guide to our papers on the subject that have appeared over the past decade and a half. We also provide numerous additional examples and a few updates and errata

Lift-and-Shift: Obtaining Simulation Extractable Subversion and Updatable SNARKs Generically

Zero-knowledge proofs and in particular succinct non-interactive zero-knowledge proofs (so called zk-SNARKs) are getting increasingly used in real-world applications, with cryptocurrencies being the prime example. Simulation extractability (SE) is a strong security notion of zk-SNARKs which informally ensures non-malleability of proofs. This property is acknowledged as being highly important by leading companies in this field such as Zcash and supported by various attacks against the malleability of cryptographic primitives in the past. Another problematic issue for the practical use of zk-SNARKs is the requirement of a fully trusted setup, as especially for large-scale decentralized applications finding a trusted party that runs the setup is practically impossible. Quite recently, the study of approaches to relax or even remove the trust in the setup procedure, and in particular subversion as well as updatable zk-SNARKs (with latter being the most promising approach), has been initiated and received considerable attention since then. Unfortunately, so far SE-SNARKs with aforementioned properties are only constructed in an ad-hoc manner and no generic techniques are available. In this paper we are interested in such generic techniques and therefore firstly revisit the only available lifting technique due to Kosba et al. (called COCO) to generically obtain SE-SNARKs. By exploring the design space of many recently proposed SNARK- and STARK-friendly symmetric-key primitives we thereby achieve significant improvements in the prover computation and proof size. Unfortunately, the COCO framework as well as our improved version (called OCOCO) is not compatible with updatable SNARKs. Consequently, we propose a novel generic lifting transformation called Lamassu. It is built using different underlying ideas compared to COCO (and OCOCO). In contrast to COCO it only requires key-homomorphic signatures (which allow to shift keys) covering well studied schemes such as Schnorr or ECDSA. This makes Lamassu highly interesting, as by using the novel concept of so called updatable signatures, which we introduce in this paper, we can prove that Lamassu preserves the subversion and in particular updatable properties of the underlying zk-SNARK. This makes Lamassu the first technique to also generically obtain SE subversion and updatable SNARKs. As its performance compares favorably to OCOCO, Lamassu is an attractive alternative that in contrast to OCOCO is only based on well established cryptographic assumptions

Survey on securing data storage in the cloud

Cloud Computing has become a well-known primitive nowadays; many researchers and companies are embracing this fascinating technology with feverish haste. In the meantime, security and privacy challenges are brought forward while the number of cloud storage user increases expeditiously. In this work, we conduct an in-depth survey on recent research activities of cloud storage security in association with cloud computing. After an overview of the cloud storage system and its security problem, we focus on the key security requirement triad, i.e., data integrity, data confidentiality, and availability. For each of the three security objectives, we discuss the new unique challenges faced by the cloud storage services, summarize key issues discussed in the current literature, examine, and compare the existing and emerging approaches proposed to meet those new challenges, and point out possible extensions and futuristic research opportunities. The goal of our paper is to provide a state-of-the-art knowledge to new researchers who would like to join this exciting new field

On the Portability of Generalized Schnorr Proofs

All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately

XTR and Tori

At the turn of the century, 80-bit security was the standard. When considering discrete-log based cryptosystems, it could be achieved using either subgroups of 1024-bit finite fields or using (hyper)elliptic curves. The latter would allow more compact and efficient arithmetic, until Lenstra and Verheul invented XTR. Here XTR stands for \u27ECSTR\u27, itself an abbreviation for Efficient and Compact Subgroup Trace Representation. XTR exploits algebraic properties of the cyclotomic subgroup of sixth degree extension fields, allowing representation only a third of their regular size, making finite field DLP-based systems competitive with elliptic curve ones. Subsequent developments, such as the move to 128-bit security and improvements in finite field DLP, rendered the original XTR and closely related torus-based cryptosystems no longer competitive with elliptic curves. Yet, some of the techniques related to XTR are still relevant for certain pairing-based cryptosystems. This chapter describes the past and the present of XTR and other methods for efficient and compact subgroup arithmetic

(Convertible) Undeniable Signatures without Random Oracles

We propose a convertible undeniable signature scheme without random oracles. Our construction is based on Waters\u27 and Kurosawa and Heng\u27s schemes that were proposed in Eurocrypt 2005. The security of our scheme is based on the CDH and the decision linear assumption. Comparing only the part of undeniable signatures, our scheme uses more standard assumptions than the existing undeniable signatures without random oracles due to Laguillamie and Vergnaud

SoK: How (not) to Design and Implement Post-Quantum Cryptography

Post-quantum cryptography has known a Cambrian explosion in the last decade. What started as a very theoretical and mathematical area has now evolved into a sprawling research field, complete with side-channel resistant embedded implementations, large scale deployment tests and standardization efforts. This study systematizes the current state of knowledge on post-quantum cryptography. Compared to existing studies, we adopt a transversal point of view and center our study around three areas: (i) paradigms, (ii) implementation, (iii) deployment. Our point of view allows to cast almost all classical and post-quantum schemes into just a few paradigms. We highlight trends, common methodologies, and pitfalls to look for and recurrent challenges