477 research outputs found

    Making Existential-Unforgeable Signatures Strongly Unforgeable in the Quantum Random-Oracle Model

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    Strongly unforgeable signature schemes provide a more stringent security guarantee than the standard existential unforgeability. It requires that not only forging a signature on a new message is hard, it is infeasible as well to produce a new signature on a message for which the adversary has seen valid signatures before. Strongly unforgeable signatures are useful both in practice and as a building block in many cryptographic constructions. This work investigates a generic transformation that compiles any existential-unforgeable scheme into a strongly unforgeable one, which was proposed by Teranishi et al. and was proven in the classical random-oracle model. Our main contribution is showing that the transformation also works against quantum adversaries in the quantum random-oracle model. We develop proof techniques such as adaptively programming a quantum random-oracle in a new setting, which could be of independent interest. Applying the transformation to an existential-unforgeable signature scheme due to Cash et al., which can be shown to be quantum-secure assuming certain lattice problems are hard for quantum computers, we get an efficient quantum-secure strongly unforgeable signature scheme in the quantum random-oracle model.Comment: 15 pages, to appear in Proceedings TQC 201

    Improving the exact security of digital signature schemes

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    Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (p. 28-30).by Leonid Reyzin.S.M

    Group Signatures: Provable Security, Efficient Constructions and Anonymity from Trapdoor-Holders

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    To date, a group signature construction which is efficient, scalable, allows dynamic adversarial joins, and proven secure in a formal model has not been suggested. In this work we give the first such construction in the random oracle model. The demonstration of an efficient construction proven secure in a formal model that captures all intuitive security properties of a certain primitive is a basic goal in cryptographic design. To this end we adapt a formal model for group signatures capturing all the basic requirements that have been identified as desirable in the area and we construct an efficient scheme and prove its security. Our construction is based on the Strong-RSA assumption (as in the work of Ateniese et al.). In our system, due to the requirements of provable security in a formal model, we give novel constructions as well as innovative extensions of the underlying mathematical requirements and properties. Our task, in fact, requires the investigation of some basic number-theoretic techniques for arguing security over the group of quadratic residues modulo a composite when its factorization is known. Along the way we discover that in the basic construction, anonymity does not depend on factoring-based assumptions, which, in turn, allows the natural separation of user join management and anonymity revocation authorities. Anonymity can, in turn, be shown even against an adversary controlling the join manager

    A Survey on Homomorphic Encryption Schemes: Theory and Implementation

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    Legacy encryption systems depend on sharing a key (public or private) among the peers involved in exchanging an encrypted message. However, this approach poses privacy concerns. Especially with popular cloud services, the control over the privacy of the sensitive data is lost. Even when the keys are not shared, the encrypted material is shared with a third party that does not necessarily need to access the content. Moreover, untrusted servers, providers, and cloud operators can keep identifying elements of users long after users end the relationship with the services. Indeed, Homomorphic Encryption (HE), a special kind of encryption scheme, can address these concerns as it allows any third party to operate on the encrypted data without decrypting it in advance. Although this extremely useful feature of the HE scheme has been known for over 30 years, the first plausible and achievable Fully Homomorphic Encryption (FHE) scheme, which allows any computable function to perform on the encrypted data, was introduced by Craig Gentry in 2009. Even though this was a major achievement, different implementations so far demonstrated that FHE still needs to be improved significantly to be practical on every platform. First, we present the basics of HE and the details of the well-known Partially Homomorphic Encryption (PHE) and Somewhat Homomorphic Encryption (SWHE), which are important pillars of achieving FHE. Then, the main FHE families, which have become the base for the other follow-up FHE schemes are presented. Furthermore, the implementations and recent improvements in Gentry-type FHE schemes are also surveyed. Finally, further research directions are discussed. This survey is intended to give a clear knowledge and foundation to researchers and practitioners interested in knowing, applying, as well as extending the state of the art HE, PHE, SWHE, and FHE systems.Comment: - Updated. (October 6, 2017) - This paper is an early draft of the survey that is being submitted to ACM CSUR and has been uploaded to arXiv for feedback from stakeholder

    Doświadczenia międzynarodowe w zakresie systemu elektronicznej identyfikacji

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    Autorzy mówili modele systemów identyfikacji elektronicznej stosowane w krajach europejskich i USA. W wielu krajach państwo w ramach zapewnienia bezpieczeństwa i interoperacyjności określiło jedynie podstawowe standardy funkcjonowania dla zaangażowanych instytucji. Scharakteryzowano rynek usług zaufania w Europie, podkreślając jego duże zróżnicowanie. Najczęstszą usługą zaufania oferowaną przed przedsiębiorców, instytucji prywatnych i publicznych jest wyda-wanie kwalifikowanych certyfikatów. Natomiast najrzadziej, prawdopodobnie ze względu na poziom skomplikowania przedsięwzięcia dostawcy usług zaufania podejmują się stworzenia kwalifikowanych usług, rejestrowanych elektronicznych doręczeń. Omówiono także kwestie bezpieczeństwa w elektronicznej identyfikacji podejmowane w państwa EU.The authors have discussed the models of electronic identification systems used in the European countries and the USA. In many countries, the state has defined only the basic standards of functioning for the engaged institutions as part of safety and interoperability guarantee. The trust services market in Europe has been defined, while putting an emphasis on its diversity. The most common trust service offered by entrepreneurs or private and public institutions is the issuance of qualified certificates. On the other hand, the trust services providers very rarely undertake to create qualified electronically registered services, probably due to the complexity of the enterprise. Also the issues of electronic identification security undertaken by the EU states

    Cryptographic Hash Functions in Groups and Provable Properties

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    We consider several "provably secure" hash functions that compute simple sums in a well chosen group (G,*). Security properties of such functions provably translate in a natural way to computational problems in G that are simple to define and possibly also hard to solve. Given k disjoint lists Li of group elements, the k-sum problem asks for gi ∊ Li such that g1 * g2 *...* gk = 1G. Hardness of the problem in the respective groups follows from some "standard" assumptions used in public-key cryptology such as hardness of integer factoring, discrete logarithms, lattice reduction and syndrome decoding. We point out evidence that the k-sum problem may even be harder than the above problems. Two hash functions based on the group k-sum problem, SWIFFTX and FSB, were submitted to NIST as candidates for the future SHA-3 standard. Both submissions were supported by some sort of a security proof. We show that the assessment of security levels provided in the proposals is not related to the proofs included. The main claims on security are supported exclusively by considerations about available attacks. By introducing "second-order" bounds on bounds on security, we expose the limits of such an approach to provable security. A problem with the way security is quantified does not necessarily mean a problem with security itself. Although FSB does have a history of failures, recent versions of the two above functions have resisted cryptanalytic efforts well. This evidence, as well as the several connections to more standard problems, suggests that the k-sum problem in some groups may be considered hard on its own, and possibly lead to provable bounds on security. Complexity of the non-trivial tree algorithm is becoming a standard tool for measuring the associated hardness. We propose modifications to the multiplicative Very Smooth Hash and derive security from multiplicative k-sums in contrast to the original reductions that related to factoring or discrete logarithms. Although the original reductions remain valid, we measure security in a new, more aggressive way. This allows us to relax the parameters and hash faster. We obtain a function that is only three times slower compared to SHA-256 and is estimated to offer at least equivalent collision resistance. The speed can be doubled by the use of a special modulus, such a modified function is supported exclusively by the hardness of multiplicative k-sums modulo a power of two. Our efforts culminate in a new multiplicative k-sum function in finite fields that further generalizes the design of Very Smooth Hash. In contrast to the previous variants, the memory requirements of the new function are negligible. The fastest instance of the function expected to offer 128-bit collision resistance runs at 24 cycles per byte on an Intel Core i7 processor and approaches the 17.4 figure of SHA-256. The new functions proposed in this thesis do not provably achieve a usual security property such as preimage or collision resistance from a well-established assumption. They do however enjoy unconditional provable separation of inputs that collide. Changes in input that are small with respect to a well defined measure never lead to identical output in the compression function

    Efficient Elliptic Curve Cryptography Software Implementation on Embedded Platforms

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    Can NSEC5 be practical for DNSSEC deployments?

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    NSEC5 is proposed modification to DNSSEC that simultaneously guarantees two security properties: (1) privacy against offline zone enumeration, and (2) integrity of zone contents, even if an adversary compromises the authoritative nameserver responsible for responding to DNS queries for the zone. This paper redesigns NSEC5 to make it both practical and performant. Our NSEC5 redesign features a new fast verifiable random function (VRF) based on elliptic curve cryptography (ECC), along with a cryptographic proof of its security. This VRF is also of independent interest, as it is being standardized by the IETF and being used by several other projects. We show how to integrate NSEC5 using our ECC-based VRF into the DNSSEC protocol, leveraging precomputation to improve performance and DNS protocol-level optimizations to shorten responses. Next, we present the first full-fledged implementation of NSEC5—extending widely-used DNS software to present a nameserver and recursive resolver that support NSEC5—and evaluate their performance under aggressive DNS query loads. Our performance results indicate that our redesigned NSEC5 can be viable even for high-throughput scenarioshttps://eprint.iacr.org/2017/099.pdfFirst author draf
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