Redactable Signature Schemes and Zero-knowledge Proofs: A comparative examination for applications in Decentralized Digital Identity Systems

Redactable Signature Schemes and Zero-Knowledge Proofs are two radically different approaches to enable privacy. This paper analyses their merits and drawbacks when applied to decentralized identity system. Redactable Signatures, though competitively quick and compact, are not as expressive as zero-knowledge proofs and do not provide the same level of privacy. On the other hand, zero-knowledge proofs can be much faster but some protocols require a trusted set-up. We conclude that given the benefits and drawbacks, redactable signatures are more appropriate at an earlier stage and zero-knowledge proofs are more appropriate at a later stage for decentralized identity systemsComment: 9 Pages, Trustworthy digital identity international conference 202

The Landscape of Pointcheval-Sanders Signatures: Mapping to Polynomial-Based Signatures and Beyond

Pointcheval-Sanders (PS) signatures are well-studied in the literature and have found use within e.g. threshold credential schemes and redactable anonymous credential schemes. The present work leverages a mapping between PS signatures and a related class of polynomial-based signatures to construct multiple new signature/credential schemes. Specifically, new protocols for multi-message signatures, sequential aggregate signatures, signatures for message commitments, redactable signatures, and unlinkable redactable signatures are presented. A redactable anonymous credential scheme is also constructed. All original protocols employ constant-sized secret keys rather than linear-sized (in the number of messages/attributes). Security properties of the new protocols are analysed and a general discussion of security properties for both PS signatures and the new schemes is provided

CDEdit: A Highly Applicable Redactable Blockchain with Controllable Editing Privilege and Diversified Editing Types

Redactable blockchains allow modifiers or voting committees with modification privileges to edit the data on the chain. Trapdoor holders in chameleon-based hash redactable blockchains can quickly compute hash collisions for arbitrary data, and without breaking the link of the hash-chain. However, chameleon-based hash redactable blockchain schemes have difficulty solving the problem of multi-level editing requests and competing for modification privileges. In this paper, we propose CDEdit, a highly applicable redactable blockchain with controllable editing privilege and diversified editing types. The proposed scheme increases the cost of invalid or malicious requests by paying the deposit on each edit request. At the same time, the editing privilege is subdivided into request, modification, and verification privileges, and the modification privilege token is distributed efficiently to prevent the abuse of the modification privilege and collusion attacks. We use chameleon hashes with ephemeral trapdoor (CHET) and ciphertext policy attribute-based encryption (CP-ABE) to implement two editing types of transaction-level and block-level, and present a practical instantiation and security analysis. Finally, the implementation and evaluation show that our scheme only costs low-performance overhead and is suitable for multi-level editing requests and modification privilege competition scenarios.Comment: 11 pages, 6 figure

Policy-Based Redactable Signatures

In this work we make progress towards solving an open problem posed by Bilzhause et. al, to give constructions of redactable signature schemes that allow the signer to limit the possible redactions performed by a third party. A separate, but related notion, called controlled disclosure allows a redactor to limit future redactions. We look at two types of data, sets and linear data (data organized as a sequence). In the case of sets, we limit redactions using a policy modeled by a monotone circuit or any circuit depending on the size of the universe the set is drawn from. In the case of linear data, we give a linear construction from vector commitments that limits redactions using a policy modeled as a monotone circuit. Our constructions have the attractive feature that they are built using only blackbox techniques

Composable & Modular Anonymous Credentials: Definitions and Practical Constructions

It takes time for theoretical advances to get used in practical schemes. Anonymous credential schemes are no exception. For instance, existing schemes suited for real-world use lack formal, composable definitions, partly because they do not support straight-line extraction and rely on random oracles for their security arguments. To address this gap, we propose unlinkable redactable signatures (URS), a new building block for privacy-enhancing protocols, which we use to construct the first efficient UC-secure anonymous credential system that supports multiple issuers, selective disclosure of attributes, and pseudonyms. Our scheme is one of the first such systems for which both the size of a credential and its presentation proof are independent of the number of attributes issued in a credential. Moreover, our new credential scheme does not rely on random oracles. As an important intermediary step, we address the problem of building a functionality for a complex credential system that can cover many different features. Namely, we design a core building block for a single issuer that supports credential issuance and presentation with respect to pseudonyms and then show how to construct a full-fledged credential system with multiple issuers in a modular way. We expect this flexible definitional approach to be of independent interest

Efficient Redactable Signature and Application to Anonymous Credentials

Let us assume that Alice has received a constant-size signature on a set of messages $\{m_i\}_{i=1}^n$ from some organization. Depending on the situation, Alice might need to disclose, prove relations about or hide some of these messages. Ideally, the complexity of the corresponding protocols should not depend on the hidden messages. In particular, if Alice wants to disclose only $k$ messages, then the authenticity of the latter should be verifiable in at most $O(k)$ operations. Many solutions were proposed over the past decades, but they only provide a partial answer to this problem. In particular, we note that they suffer either from the need to prove knowledge of the hidden elements or from the inability to prove that the latter satisfy some relations. In this paper, we propose a very efficient constant-size redactable signature scheme that addresses all the problems above. Signatures can indeed be redacted to remain valid only on a subset of $k$ messages included in $\{m_i\}_{i=1}^n$. The resulting redacted signature consists of 4 elements and can be verified with essentially $k$ exponentiations. Different shows of the same signature can moreover be made unlinkable leading to a very efficient anonymous credentials system

Signer-Anonymous Designated-Verifier Redactable Signatures for Cloud-Based Data Sharing

Redactable signature schemes allow to black out predefined parts of a signed message without affecting the validity of the signature, and are therefore an important building block in privacy-enhancing cryptography. However, a second look shows, that for many practical applications, they cannot be used in their vanilla form. On the one hand, already the identity of the signer may often reveal sensitive information to the receiver of a redacted message; on the other hand, if data leaks or is sold, everyone getting hold of (redacted versions of) a signed message will be convinced of its authenticity. We overcome these issues by providing a definitional framework and practically efficient instantiations of so called signer-anonymous designated-verifier redactable signatures (AD-RS). As a byproduct we also obtain the first group redactable signatures, which may be of independent interest. AD-RS are motivated by a real world use-case in the field of health care and complement existing health information sharing platforms with additional important privacy features. Moreover, our results are not limited to the proposed application, but can also be directly applied to various other contexts such as notary authorities or e-government services

Improving Revocation for Group Signature with Redactable Signature

Group signature is a major cryptographic tool allowing anonymous access to a service. However, in practice, access to a service is usually granted for some periods of time, which implies that the signing rights must be deactivated the rest of the time. This requirement thus calls for complex forms of revocation, reminiscent of the concept of time-bound keys. However, schemes satisfying this concept are rare and only allow revocation with limited granularity. That is, signing keys are associated with an expiry time and become definitively useless once the latter has passed. In this paper, we revisit the notion of group signatures with time-bound keys with several contributions. Firstly, we extend this notion to allow high granularity revocation: a member\u27s signing key can in particular be deactivated at some moments and then be automatically reinstated. Secondly, we show that this complex property is actually simple to achieve using redactable signature. In particular, we consider in this context a recent redactable signature scheme from PKC 20 that we improve by dramatically reducing the size of the public key. The resulting construction is of independent interest

PAP: A Privacy-Preserving Authentication Scheme with Anonymous Payment for V2G Networks

Vehicle-to-grid (V2G) networks, as an emerging smart grid paradigm, can be integrated with renewable energy resources to provide power services and manage electricity demands. When accessing electricity services, an electric vehicle(EV) typically provides authentication or/and payment information containing identifying data to a service provider, which raises privacy concerns as malicious entities might trace EV activity or exploit personal information. Although numerous anonymous authentication and payment schemes have been presented for V2G networks, no such privacy-preserving scheme supports authentication and payment simultaneously. Therefore, this paper is the first to present a privacy-preserving authentication scheme with anonymous payment for V2G networks (PAP, for short). In addition, this scheme also supports accountability and revocability, which are practical features to prevent malicious behavior; minimal attribute disclosure, which maximizes the privacy of EV when responding to the service provider\u27s flexible access policies; payment binding, which guarantees the accountability in the payment phase; user-controlled linkability, which enables EV to decide whether different authentication sessions are linkable for continuous services. On the performance side, we implement PAP with the pairing cryptography library, then evaluate it on different hardware platforms, showing that it is essential for V2G applications