A group signature scheme based on the integer factorization and the subgroup discrete logarithm problems

8 pagesGroup signature schemes allow a user, belonging to a speci c group of users, to sign a message in an anonymous way on behalf of the group. In general, these schemes need the collaboration of a Trusted Third Party which, in case of a dispute, can reveal the identity of the real signer. A new group signature scheme is presented whose security is based on the Integer Factorization Problem (IFP) and on the Subgroup Discrete Logarithm Problem (SDLP).This work has been partially supported by the \Fundaci on Memoria D. Samuel Sol orzano Barruso" under the Project FS/7-2010.Peer reviewe

Brandt's fully private auction protocol revisited

International audienceAuctions have a long history, having been recorded as early as 500 B.C. [Kri02]. Nowadays, electronic auctions have been a great success and are increasingly used in various applications, including high performance computing [BAGS02]. Many cryptographic protocols have been proposed to address the various security requirements of these electronic transactions, in particular to ensure privacy. Brandt [Bra06] developed a protocol that computes the winner using homomorphic operations on a distributed ElGamal encryption of the bids. He claimed that it ensures full privacy of the bidders, i.e. no information apart from the winner and the winning price is leaked. We first show that this protocol – when using malleable interactive zero-knowledge proofs – is vulnerable to attacks by dishonest bidders. Such bidders can manipulate the publicly available data in a way that allows the seller to deduce all participants' bids. We provide an efficient parallelized implementation of the protocol and the attack to show its practicality. Additionally we discuss some issues with verifiability as well as attacks on non-repudiation, fairness and the privacy of individual bidders exploiting authentication problems

Privacy-by-Design Regulatory Compliance Automation in Cloud Environment

The proposed Master's thesis revolves around the development of a privacy-preserving Attribute Verifier for regulatory compliance, first designed cryptographically, and then implemented in a Cloud Environment. The Attribute Verifier makes use of the Attribute Verification Protocol and its underlying encryption scheme, composed of Decentralized Attribute-Based Encryption (DABE) combined with a Zero- Knowledge Proof (ZKP) approach. The contribution of this work was integrating a ticketing system, concerning tickets of compliance, with the existing protocol, and automating the whole workflow, simulating all the actors involved, in AWS Cloud Environment. The major goal was to improve the security and privacy of sensitive data kept in the cloud as well as to comply with Cloud Regulatory, Standards, and different Data Protection Regulations. In particular, the use case covered in this Thesis refers to the General Protection Data Regulation (GDPR), specifically the compliance with Article 32. The word "Automation" in the title refers to the achievement of having automated in AWS Cloud Environment, through code, three main security objectives: Privacy, Identity and Access Management, and Attribute-based Access Control. A goal that was pursued because, in the majority of the cases, adherence to a Regulatory still requires heavy manual effort, especially when it's about pure Data Protection Regulations, i.e. in a legal setting. And when the manual effort is not required, confidentiality can be still heavily affected, and that's where the need for a privacy-by-design solution comes from. The Attribute Verifier was developed to verify the attributes of a Prover (e.g. a company, an institution, a healthcare provider, etc.) without revealing the actual attributes or assets and to grant access to encrypted data only if the verification is successful. The proposed example, among many applicable, it's the one a National Bank attempting to demonstrate to a Verifier, i.e. the European Central Bank, compliance with Article 32 of the GDPR

Private Multi-party Matrix Multiplication and Trust Computations

This paper deals with distributed matrix multiplication. Each player owns only one row of both matrices and wishes to learn about one distinct row of the product matrix, without revealing its input to the other players. We first improve on a weighted average protocol, in order to securely compute a dot-product with a quadratic volume of communications and linear number of rounds. We also propose a protocol with five communication rounds, using a Paillier-like underlying homomorphic public key cryptosystem, which is secure in the semi-honest model or secure with high probability in the malicious adversary model. Using ProVerif, a cryptographic protocol verification tool, we are able to check the security of the protocol and provide a countermeasure for each attack found by the tool. We also give a randomization method to avoid collusion attacks. As an application, we show that this protocol enables a distributed and secure evaluation of trust relationships in a network, for a large class of trust evaluation schemes.Comment: Pierangela Samarati. SECRYPT 2016 : 13th International Conference on Security and Cryptography, Lisbonne, Portugal, 26--28 Juillet 2016. 201

Minimum disclosure proofs of knowledge

AbstractProtocols are given for allowing a “prover” to convince a “verifier” that the prover knows some verifiable secret information, without allowing the verifier to learn anything about the secret. The secret can be probabilistically or deterministically verifiable, and only one of the prover or the verifier need have constrained resources. This paper unifies and extends models and techniques previously put forward by the authors, and compares some independent related work

Theory and application of computationally independent one-way functions: Interactive proof of ability - Revisited

We introduce the concept of computationally independent pair of one-way functions (CI-OWF). We also provide two rich classes of examples of such functions based on standard assumptions. We revisit two-party interactive protocols for proving possession of computational power and existing two-flow challenge-response protocols. We analyze existing protocols for proof of computation power and propose a new two-flow protocol using CI-OWF based on square Diffie-Hellman problem

Constant-size dynamic k-times anonymous authentication

Dynamic k-times anonymous authentication (k-TAA) schemes allow members of a group to be authenticated anonymously by application providers for a bounded number of times, where application providers can independently and dynamically grant or revoke access right to members in their own group. In this paper, we construct a dynamic k-TAA scheme with space and time complexities of O(log(k)) and a variant, in which the authentication protocol only requires constant time and space complexities at the cost of O(k) -sized public key. We also describe some tradeoff issues between different system characteristics. We detail all the zero-knowledge proof-of-knowledge protocols involved and show that our construction is secure in the random oracle model under the q-strong Diffie-Hellman assumption and q-decisional Diffie-Hellman inversion assumption. We provide a proof-of-concept implementation, experiment on its performance, and show that our scheme is practical

Pseudonym systems

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (p. 50-52).by Anna Lysyanskaya.S.M