Delegatable Anonymous Credentials

We construct an efficient delegatable anonymous credential system. Users can anonymously and unlinkably obtain credentials from any authority, delegate their credentials to other users, and prove possession of a credential $L$ levels away from the given authority. The size of the proof (and time to compute it) is $O(Lk)$, where $k$ is the security parameter. The only other construction of delegatable anonymous credentials (Chase and Lysyanskaya, Crypto 2006) relies on general non-interactive proofs for NP-complete languages of size $k \Omega(2^{L})$. We revise the entire approach to constructing anonymous credentials and identify \emph{randomizable} zero-knowledge proof of knowledge systems as the key building block. We formally define the notion of randomizable non-interactive zero-knowledge proofs, and give the first construction by showing how to appropriately rerandomize Groth and Sahai (Eurocrypt 2008) proofs. We show that such proof systems, in combination with an appropriate authentication scheme and a few other protocols, allow us to construct delegatable anonymous credentials. Finally, we instantiate these building blocks under appropriate assumptions about groups with bilinear maps

Delegatable Anonymous Credentials from Mercurial Signatures

In a delegatable anonymous credential system, participants may use their credentials anonymously as well as anonymously delegate them to other participants. Such systems are more usable than traditional anonymous credential systems because a popular credential issuer can delegate some of its responsibilities without compromising users\u27 privacy. They also provide stronger privacy guarantees than traditional anonymous credential systems because the identities of credential issuers are hidden. The identity of a credential issuer may convey information about a user\u27s identity even when all other information about the user is concealed. The only previously known constructions of delegatable anonymous credentials were prohibitively inefficient. They were based on non-interactive zero-knowledge (NIZK) proofs. In this paper, we provide a simple construction of delegatable anonymous credentials and prove its security in the generic group model. Our construction is direct, not based on NIZK proofs, and is therefore considerably more efficient. In fact, in our construction, only five group elements are needed per link to represent an anonymous credential chain. Our main building block is a new type of signature scheme, a mercurial signature, which allows a signature $\sigma$ on a message $M$ under public key $\mathsf{pk}$ to be transformed into a signature $\sigma\u27$ on an equivalent but unlinkable message $M\u27$ under an equivalent but unlinkable public key $\mathsf{pk}\u27$

Practical Delegatable Attribute-Based Anonymous Credentials with Chainable Revocation

Delegatable Anonymous Credentials (DAC) are an enhanced Anonymous Credentials (AC) system that allows credential owners to use credentials anonymously, as well as anonymously delegate them to other users. In this work, we introduce a new concept called Delegatable Attribute-based Anonymous Credentials with Chainable Revocation (DAAC-CR), which extends the functionality of DAC by allowing 1) fine-grained attribute delegation, 2) issuers to restrict the delegation capabilities of the delegated users at a fine-grained level, including the depth of delegation and the sets of delegable attributes, and 3) chainable revocation, meaning if a credential within the delegation chain is revoked, all subsequent credentials derived from it are also invalid. We provide a practical DAAC-CR instance based on a novel primitive that we identify as structure-preserving signatures on equivalence classes on vector commitments (SPSEQ-VC). This primitive may be of independent interest, and we detail an efficient construction. Compared to traditional DAC systems that rely on non-interactive zero-knowledge (NIZK) proofs, the credential size in our DAAC-CR instance is constant, independent of the length of delegation chain and the number of attributes. We formally prove the security of our scheme in the generic group model and demonstrate its practicality through performance benchmarks

Multi-Issuer Anonymous Credentials Without a Root Authority

The rise of blockchain technology has boosted interest in privacy-enhancing technologies, in particular, anonymous transaction authentication. Permissionless blockchains realize transaction anonymity through one-time pseudonyms, whereas permissioned blockchains leverage anonymous credentials. Earlier solutions of anonymous credentials assume a single issuer; as a result, they hide the identity of users but still reveal the identity of the issuer. A countermeasure is delegatable credentials, which support multiple issuers as long as a root authority exists. Assuming a root authority however, is unsuitable for blockchain technology and decentralized applications. This paper introduces a solution for anonymous credentials that guarantees user anonymity, even without a root authority. The proposed solution is secure in the universal composability framework and allows users to produce anonymous signatures that are logarithmic in the number of issuers and constant in the number of user attributes

Practical Delegatable Anonymous Credentials From Equivalence Class Signatures

Anonymous credentials systems (ACs) are a powerful cryptographic tool for privacy-preserving applications and provide strong user privacy guarantees for authentication and access control. ACs allow users to prove possession of attributes encoded in a credential without revealing any information beyond them. A delegatable AC (DAC) system is an enhanced AC system that allows the owners of credentials to delegate the obtained credential to other users. This allows to model hierarchies as usually encountered within public-key infrastructures (PKIs). DACs also provide stronger privacy guarantees than traditional AC systems since the identities of issuers and delegators are also hidden. A credential issuer\u27s identity may convey information about a user\u27s identity even when all other information about the user is protected. We present a novel delegatable anonymous credential scheme that supports attributes, provides anonymity for delegations, allows the delegators to restrict further delegations, and also comes with an efficient construction. In particular, our DAC credentials do not grow with delegations, i.e., are of constant size. Our approach builds on a new primitive that we call structure-preserving signatures on equivalence classes on updatable commitments (SPSEQ-UC). The high-level idea is to use a special signature scheme that can sign vectors of set commitments which can be extended by additional set commitments. Signatures additionally include a user\u27s public key, which can be switched. This allows us to efficiently realize delegation in the DAC. Similar to conventional SPSEQ signatures, the signatures and messages can be publicly randomized and thus allow unlinkable showings in the DAC system. We present further optimizations such as cross-set commitment aggregation that, in combination, enable selective, efficient showings in the DAC without using costly zero-knowledge proofs. We present an efficient instantiation that is proven to be secure in the generic group model and finally demonstrate the practical efficiency of our DAC by presenting performance benchmarks based on an implementation

Delegatable Attribute-based Anonymous Credentials from Dynamically Malleable Signatures

In this paper, we introduce the notion of delegatable attribute-based anonymous credentials (DAAC). Such systems offer fine-grained anonymous access control and they give the credential holder the ability to issue more restricted credentials to other users. In our model, credentials are parameterized with attributes that (1) express what the credential holder himself has been certified and (2) define which attributes he may issue to others. Furthermore, we present a practical construction of DAAC. For this construction, we deviate from the usual approach of embedding a certificate chain in the credential. Instead, we introduce a novel approach for which we identify a new primitive we call dynamically malleable signatures (DMS) as the main ingredient. This primitive may be of independent interest. We also give a first instantiation of DMS with efficient protocols

Mercurial Signatures for Variable-Length Messages

Mercurial signatures are a useful building block for privacy-preserving schemes, such as anonymous credentials, delegatable anonymous credentials, and related applications. They allow a signature $\sigma$ on a message $m$ under a public key $\mathsf{pk}$ to be transformed into a signature $\sigma\u27$ on an equivalent message $m\u27$ under an equivalent public key $\mathsf{pk}\u27$ for an appropriate notion of equivalence. For example, $\mathsf{pk}$ and $\mathsf{pk}\u27$ may be unlinkable pseudonyms of the same user, and $m$ and $m\u27$ may be unlinkable pseudonyms of a user to whom some capability is delegated. The only previously known construction of mercurial signatures suffers a severe limitation: in order to sign messages of length $n$, the signer\u27s public key must also be of length $n$. In this paper, we eliminate this restriction and provide a signing protocol that admits messages of any length. This significantly improves the applicability of mercurial signatures to chains of anonymous credentials

Delegatable Functional Signatures

We introduce delegatable functional signatures (DFS) which support the delegation of signing capabilities to another party, called the evaluator, with respect to a functionality F. In a DFS, the signer of a message can choose an evaluator, specify how the evaluator can modify the signature without voiding its validity, allow additional input and decide how the evaluator can further delegate its capabilities. The main contribution of this paper is twofold. First, we propose DFS, a novel cryptographic primitive that unifies several seemingly different signature primitives, including functional signatures as defined by Boyle, Goldwasser, and Ivan (eprint 2013/401), sanitizable signatures, identity based signatures, and blind signatures. To achieve this unification, we present several definitions of unforgeability and privacy. Finding appropriate and meaningful definitions in this context is challenging due to the natural mealleability of DFS and due to the multi-party setting that may involve malicious keys. Second, we present a complete characterization of the instantiability of DFS under common assumptions, like the existence of one-way functions. Here, we present both positive and negative results. On the positive side we show that DFS not achieving our notion of privacy can be constructed from one-way functions. Furthermore, we show that unforgerable and private DFS can be constructed from doubly enhanced trapdoor permutations. On the negative side we show that the previous result is optimal regarding its underlying assumptions presenting an impossibility result for unforgeable private DFS from one-way permutations