A Traceable Ring Signature Scheme based on Coding Theory

Traceable ring signatures are a variant of ring signatures which allows the identity of a user to be revealed, when it signs two different messages with respect to the same group of users. It has applications in e-voting and in cryptocurrencies, such as the well-known Monero. We propose the first traceable ring signature scheme whose security is based on the hardness of the Syndrome Decoding problem, a problem in coding theory which is conjectured to be unsolvable by both classical and quantum algorithms. To construct the scheme, we use a variant of Stern\u27s protocol and, by applying the Fiat-Shamir transform to it in an ingenious way, we obtain a ring signature that allows traceability. We prove that the resulting protocol has the standard security properties for traceable ring signatures in the random oracle model: tag-linkability, anonymity and exculpability. As far as we know, this is the first proposal for a traceable ring signature scheme in the post-quantum setting

A Code-Based Group Signature Scheme

International audienceIn this work we propose the first code-based group signature. As it will be described below, its security is based on a relaxation of the model of Bel-lare, Shi and Zhang [3] (BSZ model) verifying the properties of anonymity, traceability and non-frameability. Furthermore, it has numerous advantages over all existing post-quantum constructions and even competes (in terms of properties) with pairing based constructions: it allows to dynamically add new members and signature and public key sizes are constant with respect to the number of group members. Last but not least, our scheme can be extended into a traceable signature according to the definition of Kiayias, Tsiounis and Yung [19] (KTY model) and handles membership revocation. The main idea of our scheme consists in building a collision of two syndromes associated to two different matrices: a random one which enables to build a random syndrome from a chosen small weight vector; and a trapdoor matrix for the syndrome decoding problem, which permits to find a small weight preimage of the previous random syndrome. These two small weight vectors will constitute the group member's secret signing key whose knowledge will be proved thanks to a variation of Stern's authentication protocol. For applications , we consider the case of the code-based CFS signature scheme [11] of Courtois, Finiasz and Sendrier

Analysis of code-based digital signature schemes

Digital signatures are in high demand because they allow authentication and non-repudiation. Existing digital signature systems, such as digital signature algorithm (DSA), elliptic curve digital signature algorithm (ECDSA), and others, are based on number theory problems such as discrete logarithmic problems and integer factorization problems. These recently used digital signatures are not secure with quantum computers. To protect against quantum computer attacks, many researchers propose digital signature schemes based on error-correcting codes such as linear, Goppa, polar, and so on. We studied 16 distinct papers based on various error-correcting codes and analyzed their various features such as signing and verification efficiency, signature size, public key size, and security against multiple attacks

Provably Secure Group Signature Schemes from Code-Based Assumptions

We solve an open question in code-based cryptography by introducing two provably secure group signature schemes from code-based assumptions. Our basic scheme satisfies the CPA-anonymity and traceability requirements in the random oracle model, assuming the hardness of the McEliece problem, the Learning Parity with Noise problem, and a variant of the Syndrome Decoding problem. The construction produces smaller key and signature sizes than the previous group signature schemes from lattices, as long as the cardinality of the underlying group does not exceed $2^{24}$, which is roughly comparable to the current population of the Netherlands. We develop the basic scheme further to achieve the strongest anonymity notion, i.e., CCA-anonymity, with a small overhead in terms of efficiency. The feasibility of two proposed schemes is supported by implementation results. Our two schemes are the first in their respective classes of provably secure groups signature schemes. Additionally, the techniques introduced in this work might be of independent interest. These are a new verifiable encryption protocol for the randomized McEliece encryption and a novel approach to design formal security reductions from the Syndrome Decoding problem.Comment: Full extension of an earlier work published in the proceedings of ASIACRYPT 201

A law-abiding peer-to-peer network for free-software distribution

... for worldwide distribution of freely redistributable software packages. The GDN takes a novel, optimistic approach to stop the illegal distribution of copyrighted and illicit material via the network. Instead of having moderators check the software archives at upload time, illegal content is removed and its uploader's access to the network permanently revoked only when the content is discovered. An important feature of the GDN is that the objects containing the software can run on untrustworthy servers. A first version of the GDN has been implemented and has been running since October 2000 across four European sites

Stronger security notions for decentralized traceable attribute-based signatures and more efficient constructions

We revisit the notion of Decentralized Traceable Attribute-Based Signatures (DTABS) introduced by El Kaafarani et al. (CT-RSA 2014) and improve the state-of-the-art in three dimensions: Firstly, we provide a new stronger security model which circumvents some shortcomings in existing models. Our model minimizes the trust placed in attribute authorities and hence provides, among other things, a stronger definition for non-frameability. In addition, our model captures the notion of tracing soundness which is important for many applications of the primitive. Secondly, we provide a generic construction that is secure w.r.t. our strong security model and show two example instantiations in the standard model which are more efficient than existing constructions (secure under weaker security definitions). Finally, we dispense with the need for the expensive zero-knowledge proofs required for proving tracing correctness by the tracing authority. As a result, tracing a signature in our constructions is significantly more efficient than existing constructions, both in terms of the size of the tracing proof and the computational cost required to generate and verify it. For instance, verifying tracing correctness in our constructions requires only 4 pairings compared to 34 pairings in the most efficient existing construction

Formalizing group blind signatures and practical constructions without random oracles

Group blind signatures combine anonymity properties of both group signatures and blind signatures and offer privacy for both the message to be signed and the signer. The primitive has been introduced with only informal definitions for its required security properties. In this paper, we offer two main contributions: first, we provide foundations for the primitive and present formal security definitions. In the process, we identify and address some subtle issues which were not considered by previous constructions and (informal) security definitions. Our second main contribution is a generic construction that yields practical schemes with a round-optimal signing protocol and constant-size signatures. Our constructions permit dynamic and concurrent enrollment of new members and satisfy strong security requirements. To the best of our knowledge, our schemes are the first provably secure constructions in the standard model. In addition, we introduce some new building blocks which may be of independent interest. © 2013 Springer-Verlag

Stronger Notions and a More Efficient Construction of Threshold Ring Signatures

We consider threshold ring signatures (introduced by Bresson et al. [BSS02], where any t signers can sign a message while anonymizing themselves within a larger (size-n) group. The signature proves that t members of the group signed, without revealing anything else about their identities. Our contributions in this paper are two-fold. First, we strengthen existing definitions of threshold ring signatures in a natural way; we demand that a signer cannot be de-anonymized even by their fellow signers. This is crucial, since in applications where a signer\u27s anonymity is important, we do not want anonymity to be compromised by a single insider. Our definitions demand non-interactive signing, which is important for anonymity, since truly anonymous interaction is difficult or impossible in many scenarios. Second, we give the first rigorous construction of a threshold ring signature with size independent of n, the number of users in the larger group. Instead, our signatures have size linear in t, the number of signers. This is also a very important contribution; signers should not have to choose between achieving their desired degree of anonymity (possibly very large n) and their need for communication efficiency