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 Thus, an out-of-date witness can be easily made current. Vice versa, a verifier with an out-of-date accumulator value can still verify a current membership witness.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

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

A new Privacy Preserving and Scalable Revocation Method for Self Sovereign Identity - The Perfect Revocation Method does not exist yet

Digital Identities are playing an essential role in our digital lives. Today, most Digital Identities are based on central architectures. Central Digital Identity providers control and know our data and thereby our Identity. Self Sovereign Identities are based on decentralized data storage and data exchange architecture, where the user is in sole control of his data and identity. Most of the issued credentials need the possibility of revocation. For a centrally managed Digital Identity system, revocation is not a problem. In decentral architectures, revocation is more challenging. Revocation can be done with different methods e.g. list based, cryptographic accumulators and with credential updates. A revocation method must be privacy preserving and must scale. This paper gives an overview of the available revocation methods, including a survey to define requirements, assess revocation groups against the requirements, highlights shortcomings of the methods and introduces a new revocation method called Linked Validity Verifiable Credentials

A Decentralized Dynamic PKI based on Blockchain

The central role of the certificate authority (CA) in traditional public key infrastructure (PKI) makes it fragile and prone to compromises and operational failures. Maintaining CAs and revocation lists is demanding especially in loosely-connected and large systems. Log-based PKIs have been proposed as a remedy but they do not solve the problem effectively. We provide a general model and a solution for decentralized and dynamic PKI based on a blockchain and web of trust model where the traditional CA and digital certificates are removed and instead, everything is registered on the blockchain. Registration, revocation, and update of public keys are based on a consensus mechanism between a certain number of entities that are already part of the system. Any node which is part of the system can be an auditor and initiate the revocation procedure once it finds out malicious activities. Revocation lists are no longer required as any node can efficiently verify the public keys through witnesses

Zero-Knowledge Proof-of-Identity: Sybil-Resistant, Anonymous Authentication on Permissionless Blockchains and Incentive Compatible, Strictly Dominant Cryptocurrencies

Zero-Knowledge Proof-of-Identity from trusted public certificates (e.g., national identity cards and/or ePassports; eSIM) is introduced here to permissionless blockchains in order to remove the inefficiencies of Sybil-resistant mechanisms such as Proof-of-Work (i.e., high energy and environmental costs) and Proof-of-Stake (i.e., capital hoarding and lower transaction volume). The proposed solution effectively limits the number of mining nodes a single individual would be able to run while keeping membership open to everyone, circumventing the impossibility of full decentralization and the blockchain scalability trilemma when instantiated on a blockchain with a consensus protocol based on the cryptographic random selection of nodes. Resistance to collusion is also considered. Solving one of the most pressing problems in blockchains, a zk-PoI cryptocurrency is proved to have the following advantageous properties: - an incentive-compatible protocol for the issuing of cryptocurrency rewards based on a unique Nash equilibrium - strict domination of mining over all other PoW/PoS cryptocurrencies, thus the zk-PoI cryptocurrency becoming the preferred choice by miners is proved to be a Nash equilibrium and the Evolutionarily Stable Strategy - PoW/PoS cryptocurrencies are condemned to pay the Price of Crypto-Anarchy, redeemed by the optimal efficiency of zk-PoI as it implements the social optimum - the circulation of a zk-PoI cryptocurrency Pareto dominates other PoW/PoS cryptocurrencies - the network effects arising from the social networks inherent to national identity cards and ePassports dominate PoW/PoS cryptocurrencies - the lower costs of its infrastructure imply the existence of a unique equilibrium where it dominates other forms of paymentComment: 2.1: Proof-of-Personhood Considered Harmful (and Illegal); 4.1.5: Absence of Active Authentication; 4.2.6: Absence of Active Authentication; 4.2.7: Removing Single-Points of Failure; 4.3.2: Combining with Non-Zero-Knowledge Authentication; 4.4: Circumventing the Impossibility of Full Decentralizatio

Merkle Tree Ladder Mode: Reducing the Size Impact of NIST PQC Signature Algorithms in Practice

We introduce the Merkle Tree Ladder (MTL) mode of operation for signature schemes. MTL mode signs messages using an underlying signature scheme in such a way that the resulting signatures are condensable: a set of MTL mode signatures can be conveyed from a signer to a verifier in fewer bits than if the MTL mode signatures were sent individually. In MTL mode, the signer sends a shorter condensed signature for each message of interest and occasionally provides a longer reference value that helps the verifier process the condensed signatures. We show that in a practical scenario involving random access to an initial series of 10,000 signatures that expands gradually over time, MTL mode can reduce the size impact of the NIST PQC signature algorithms, which have signature sizes of 666 to 7856 bytes with example parameter sets, to a condensed signature size of 472 bytes per message. Even adding the overhead of the reference values, MTL mode signatures still reduce the overall signature size impact under a range of operational assumptions. Because MTL mode itself is quantum-safe, the mode can support long-term cryptographic resiliency in applications where signature size impact is a concern without limiting cryptographic diversity only to algorithms whose signatures are naturally short

Accumulators with Applications to Anonymity-Preserving Revocation

Membership revocation is essential for cryptographic applications, from traditional PKIs to group signatures and anonymous credentials. Of the various solutions for the revocation problem that have been explored, dynamic accumulators are one of the most promising. We propose Braavos, a new, RSA-based, dynamic accumulator. It has optimal communication complexity and, when combined with efficient zero-knowledge proofs, provides an ideal solution for anonymous revocation. For the construction of Braavos we use a modular approach: we show how to build an accumulator with better functionality and security from accumulators with fewer features and weaker security guarantees. We then describe an anonymous revocation component (ARC) that can be instantiated using any dynamic accumulator. ARC can be added to any anonymous system, such as anonymous credentials or group signatures, in order to equip it with a revocation functionality. Finally, we implement ARC with Braavos and plug it into Idemix, the leading implementation of anonymous credentials. This work resolves, for the first time, the problem of practical revocation for anonymous credential systems

Design, modeling, and simulation of secure X.509 certificate revocation

TLS communication over the internet has risen rapidly in the last seven years (2015--2022), and there were over 156M active SSL certificates in 2022. The state-of-the-art Public Key Infrastructure (PKI), encompassing protocols, computational resources, and digital certificates, has evolved for 24 years to become the de-facto choice for encrypted communication over the Internet even on newer platforms such as mobile devices and Internet-of-Things (IoT) (despite being low powered with computational constraints). However, certificate revocation is one sub-protocol in TLS communication that fails to meet the rising scalability demands and remains open to exploitation. In this dissertation, the standard for X.509 revocation is systematically reviewed and critically evaluated to identify its limitations and assess their impact on internet security. Because of fragmented revocation information and limited scalability, even the latest version of the X.509 revocation standard is susceptible to Man-in-the-Middle (MiTM) attacks. Blockchain technology can provide a decentralized and peer-to-peer distributed ledger to enable a unified, tamper-proof platform for X.509 certificate authorities to collaborate securely in a trustless environment. To understand blockchain technology\u27s capabilities and limitations in distributing X.509 revocation information, different blockchain platforms are explored and compared in terms of scalability, degree of decentralization, and cost of operation. Moreover, the unification of the revocation lists leads to a massive expansion in the number of revoked certificates to query by a verifying client thus increasing the latency during revocation lookup. And, to minimize revocation-status lookup times, cryptographic constructions and approximate set-membership data structures are prototyped and analyzed. The key contributions of this dissertation are twofold: 1) the novel design of a secure and robust system for distributing X.509 certificate revocation information; and, 2) the prototype, experimentation, and optimization of cascading XOR filter, fuse filter, and cuckoo filter for quick lookup with zero false positives (and zero false negatives). The Secure Certificate Revocation as a Peer Service (SCRaaPS) is designed using the Lightweight Mining consensus algorithm-based Scrybe blockchain protocol to store and distribute certificate revocation lists. And, the cascading fuse filter (demonstrating the highest space efficiency and fastest build time) is applied to minimize the revocation lookup time with zero false positives

Efficient Asynchronous Accumulators for Distributed PKI

Abstract. Cryptographic accumulators are a tool for compact set rep-resentation 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 ac-cumulator 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 times in the number of subsequent element addi-tions. 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

Studies in authentication

This thesis presents advances in several areas of authentication. First, we consider cryptographic accumulators, which are compact digital objects representing arbitrarily large sets. They support efficient proofs of membership (or, alternatively, of non-membership). We give the first definition of cryptographic accumulators in the UC framework, and construct two new accumulators: one uniquely suited for use in a revokable anonymous credential scheme, and one uniquely suited for use in a distributed system such as a blockchain-based PKI. Next, we consider multi-designated verifier signatures (MDVS). An MDVS is a special kind of signature that can only be verified by parties explicitly specified by the signer; more than that, even if those designated verifiers wanted to prove to an external party (e.g. an adversary) that a certain message was signed by the signer, they should be unable to do so. This is crucial in contexts where off-the-record communication is desirable; the sender may not want to be provably linked to a possibly sensitive message, but still want the intended recipients to be able to verify the authenticity of the message. Existing literature defines and builds limited notions of MDVS, where the off-the-record property only holds when it is conceivable that all verifiers collude. We strengthen this property to support any subset of colluding verifiers, and give two constructions of our stronger notion of MDVS: one from functional encryption, and one from standard primitives (but with a slightly larger signature size). Finally, we consider fuzzy password authenticated key exchange (Fuzzy PAKE). PAKEs are protocols which enable two parties holding the same password (that is, the same potentially low-entropy, non-uniform string) to agree on a (high-entropy, uniform) secret key in a way that resists man-in-the-middle attacks and offline dictionary attacks on the password. We define Fuzzy PAKE, a special kind of PAKE where the passwords used for authentication may contain some errors. We provide the first efficient and general solutions to this problem that enable, for example, key agreement based on commonly used biometrics such as iris scans