Revisiting Cryptographic Accumulators, Additional Properties and Relations to other Primitives

Cryptographic accumulators allow to accumulate a finite set of values into a single succinct accumulator. For every accumulated value, one can efficiently compute a witness, which certifies its membership in the accumulator. However, it is computationally infeasible to find a witness for any non-accumulated value. Since their introduction, various accumulator schemes for numerous practical applications and with different features have been proposed. Unfortunately, to date there is no unifying model capturing all existing features. Such a model can turn out to be valuable as it allows to use accumulators in a black-box fashion. To this end, we propose a unified formal model for (randomized) cryptographic accumulators which covers static and dynamic accumulators, their universal features and includes the notions of undeniability and indistinguishability. Additionally, we provide an exhaustive classification of all existing schemes. In doing so, it turns out that most accumulators are distinguishable. Fortunately, a simple, light-weight generic transformation allows to make many existing dynamic accumulator schemes indistinguishable. As this transformation, however, comes at the cost of reduced collision freeness, we additionally propose the first indistinguishable scheme that does not suffer from this shortcoming. Finally, we employ our unified model for presenting a black-box construction of commitments from indistinguishable accumulators as well as a black-box construction of indistinguishable, undeniable universal accumulators from zero-knowledge sets. Latter yields the first universal accumulator construction that provides indistinguishability

Certificate Transparency with Enhancements and Short Proofs

Browsers can detect malicious websites that are provisioned with forged or fake TLS/SSL certificates. However, they are not so good at detecting malicious websites if they are provisioned with mistakenly issued certificates or certificates that have been issued by a compromised certificate authority. Google proposed certificate transparency which is an open framework to monitor and audit certificates in real time. Thereafter, a few other certificate transparency schemes have been proposed which can even handle revocation. All currently known constructions use Merkle hash trees and have proof size logarithmic in the number of certificates/domain owners. We present a new certificate transparency scheme with short (constant size) proofs. Our construction makes use of dynamic bilinear-map accumulators. The scheme has many desirable properties like efficient revocation, low verification cost and update costs comparable to the existing schemes. We provide proofs of security and evaluate the performance of our scheme.Comment: A preliminary version of the paper was published in ACISP 201

Certificate Transparency with Enhancements and Short Proofs

Browsers can detect malicious websites that are provisioned with forged or fake TLS/SSL certificates. However, they are not so good at detecting malicious websites if they are provisioned with mistakenly issued certificates or certificates that have been issued by a compromised certificate authority. Google proposed certificate transparency which is an open framework to monitor and audit certificates in real time. Thereafter, a few other certificate transparency schemes have been proposed which can even handle revocation. All currently known constructions use Merkle hash trees and have proof size logarithmic in the number of certificates/domain owners. We present a new certificate transparency scheme with short (constant size) proofs. Our construction makes use of dynamic bilinear-map accumulators. The scheme has many desirable properties like efficient revocation, low verification cost and update costs comparable to the existing schemes. We provide proofs of security and evaluate the performance of our scheme.Comment: A preliminary version of the paper was published in ACISP 201

An Overview of Cryptographic Accumulators

This paper is a primer on cryptographic accumulators and how to apply them practically. A cryptographic accumulator is a space- and time-efficient data structure used for set-membership tests. Since it is possible to represent any computational problem where the answer is yes or no as a set-membership problem, cryptographic accumulators are invaluable data structures in computer science and engineering. But, to the best of our knowledge, there is neither a concise survey comparing and contrasting various types of accumulators nor a guide for how to apply the most appropriate one for a given application. Therefore, we address that gap by describing cryptographic accumulators while presenting their fundamental and so-called optional properties. We discuss the effects of each property on the given accumulator's performance in terms of space and time complexity, as well as communication overhead.Comment: Note: This is an extended version of a paper published In Proceedings of the 7th International Conference on Information Systems Security and Privacy (ICISSP 2021), pages 661-66

Revisiting Privacy-aware Blockchain Public Key Infrastructure

Privacy-aware Blockchain Public Key Infrastructure (PB- PKI) is a recent proposal by Louise Axon (2017) to create a privacy-preserving Public Key Infrastructure on the Blockchain. However, PB-PKI suffers from operational problems. We found that the most important change, i.e., the key update process proposed in PB-PKI for privacy is broken. Other issues include authenticating a user during key update and ensuring proper key revocation. In this paper, we provide solutions to the problems of PB-PKI. We suggest generating fresh keys during key update. Furthermore, we use ring signatures for authenticating the user requesting key updates and use Asynchronous accumulators to handle the deletion of revoked keys. We show that the approach is feasible and implement a proof of concept

Improved Extension Protocols for Byzantine Broadcast and Agreement

Byzantine broadcast (BB) and Byzantine agreement (BA) are two most fundamental problems and essential building blocks in distributed computing, and improving their efficiency is of interest to both theoreticians and practitioners. In this paper, we study extension protocols of BB and BA, i.e., protocols that solve BB/BA with long inputs of l bits using lower costs than l single-bit instances. We present new protocols with improved communication complexity in almost all settings: authenticated BA/BB with t < n/2, authenticated BB with t < (1-?)n, unauthenticated BA/BB with t < n/3, and asynchronous reliable broadcast and BA with t < n/3. The new protocols are advantageous and significant in several aspects. First, they achieve the best-possible communication complexity of ?(nl) for wider ranges of input sizes compared to prior results. Second, the authenticated extension protocols achieve optimal communication complexity given the current best available BB/BA protocols for short messages. Third, to the best of our knowledge, our asynchronous and authenticated protocols in the setting are the first extension protocols in that setting

An Investigation of Flow-based Algorithms for Sybil Defense

abstract: Distributed systems are prone to attacks, called Sybil attacks, wherein an adversary may generate an unbounded number of bogus identities to gain control over the system. In this thesis, an algorithm, DownhillFlow, for mitigating such attacks is presented and tested experimentally. The trust rankings produced by the algorithm are significantly better than those of the distributed SybilGuard protocol and only slightly worse than those of the best-known Sybil defense algorithm, ACL. The results obtained for ACL are consistent with those obtained in previous studies. The running times of the algorithms are also tested and two results are obtained: first, DownhillFlow’s running time is found to be significantly faster than any existing algorithm including ACL, terminating in slightly over one second on the 300,000-node DBLP graph. This allows it to be used in settings such as dynamic networks as-is with no additional functionality needed. Second, when ACL is configured such that it matches DownhillFlow’s speed, it fails to recognize large portions of the input graphs and its accuracy among the portion of the graphs it does recognize becomes lower than that of DownhillFlow.Dissertation/ThesisMasters Thesis Computer Science 201

Post-Quantum ID-based Ring Signatures from Symmetric-key Primitives

Ring signatures and ID-based cryptography are considered promising in terms of application. A ring signature authenticates messages while the author of the message remains anonymous. ID-based cryptographic primitives suppress the need for certificates in public key infrastructures (PKI). In this work, we propose a generic construction for post-quantum ID-based ring signatures (IDRS) based on symmetric-key primitives from which we derive the first two constructions of IDRS. The first construction named PicRS utilizes the Picnic digital signature to ensure its security while the second construction XRS is motivated by the stateful digital signature XMSS instead of Picnic, allowing a signature size reduction. Both constructions have a competitive signature size when compared with state-of-the-art lattice-based IDRS. XRS can achieve a competitive signature size of 889KB for a ring of 4096 users while the fully stateless PicRS achieves a signature size of 1.900MB for a ring of 4096 users. In contrast, the shortest lattice-based IDRS achieves a signature size of 335MB for the same ring size

Post-Quantum EPID Signatures from Symmetric Primitives

EPID signatures are used extensively in real-world systems for hardware enclave attestation. As such, there is a strong interest in making these schemes post-quantum secure. In this paper we initiate the study of EPID signature schemes built only from symmetric primitives, such as hash functions and PRFs. We present two constructions in the random oracle model. The first is a scheme satisfying the EPID signature syntax and security definitions needed for private hardware attestation used in Intel’s SGX. The second achieves significantly shorter signatures for many applications, including the use case of remote hardware attestation. While our EPID signatures for attestation are longer than standard post-quantum signatures, they are short enough for applications where the data being signed is large, such as analytics on large private data sets, or streaming media to a trusted display. We evaluate several instantiations of our schemes so that the costs and benefits of these constructions are clear. Along the way we also give improvements to the zero-knowledge Merkle inclusion proofs of Derler et al. (2017)

Aggregating and thresholdizing hash-based signatures using STARKs

This work presents an approach for compressing hash-based signatures using STARKs (Ben-Sasson et. al.\u2718). We focus on constructing a hash-based t-of-n threshold signature scheme, as well as an aggregate signature scheme. In both constructions, an aggregator collects individual one-time hash-based signatures and outputs a STARK proof attesting that the signatures are valid and meet the required thresholds. This proof then serves the role of the aggregate or threshold signature. We demonstrate the concrete performance of such constructions, having implemented the algebraic intermediate representations (AIR) for them, along with an experimental evaluation over our implementation of the STARK protocol. We find that, even when we aggregate thousands of signatures, the final aggregated size ranges between 100KB and 200KB. This makes our schemes attractive when there exist at least $50$ one-or-few-times hash-based signatures -- such as in the blockchain setting. We also observe that for STARK-based signature aggregation, the size of individual signatures is less important than the number of hash invocations and the complexity of the signature verification algorithm. This implies that simple hash-based signature variants (e.g. Lamport, HORST, BPQS) are well-suited for aggregation, as their large individual signatures serve only as witnesses to the ZKP circuit and are not needed for aggregate signature verification. Our constructions are directly applicable as scalable solutions for post-quantum secure blockchains which typically employ blocks of hundreds or thousands of signed transactions. Moreover, stateful hash-based one-or-few-times signatures are already used in some PQ-ready blockchains, as address reuse is typically discouraged for privacy reasons