A Secure Modified ID-Based Undeniable Signature Scheme

Verifiable Pairing and its Applications. In Chae Hoon Lim and Moti Yung, editors, Information Security Applications: 5th International Workshop, WISA 2004, Jeju Island, Korea, August 23-25, 2004, Revised Selected Papers, volume 3325 of Lecture Notes in Computer Science, pp. 170-187. (http://www.springerlink.com/index/C4QB7C13NL0EY5VN) which contains an improved and generalized result of this paper

Computing cardinalities of Q-curve reductions over finite fields

We present a specialized point-counting algorithm for a class of elliptic curves over F\_{p^2} that includes reductions of quadratic Q-curves modulo inert primes and, more generally, any elliptic curve over F\_{p^2} with a low-degree isogeny to its Galois conjugate curve. These curves have interesting cryptographic applications. Our algorithm is a variant of the Schoof--Elkies--Atkin (SEA) algorithm, but with a new, lower-degree endomorphism in place of Frobenius. While it has the same asymptotic asymptotic complexity as SEA, our algorithm is much faster in practice.Comment: To appear in the proceedings of ANTS-XII. Added acknowledgement of Drew Sutherlan

A note on the low order assumption in class groups of imaginary quadratic number fields

In this short note we analyze the low order assumption in the imaginary quadratic number fields. We show how this assumption is broken for Mersenne primes. We also provide a description on how to possible attack this assumption for other class of prime numbers leveraging some new mathematical tool coming from higher (cubic) number fields

Spot the Black Hat in a Dark Room: Parallelized Controlled Access Searchable Encryption on FPGAs

The advent of cloud computing offers clients with the opportunity to outsource storage and processing of large volumes of shared data to third party service providers, thereby enhancing overall accessibility and operational productivity. However, security concerns arising from the threat of insider and external attacks often require the data to be stored in an encrypted manner. Secure and efficient keyword searching on such large volumes of encrypted data is an important and yet one of the most challenging services to realize in practice. Even more challenging is to incorporate fine-grained client-specific access control - a commonly encountered requirement in cloud applications - in such searchable encryption solutions. Existing searchable encryption schemes in literature tend to focus on the use of specialized data structures for efficiency, and are not explicitly designed to address controlled access scenarios. In this paper, we propose a novel controlled access searchable encryption (CASE) scheme. As the name suggests, CASE inherently embeds access control in its key management process, and scales efficiently with increase in the volume of encrypted data handled by the system. We provide a concrete construction for CASE that is privacy-preserving under well-known cryptographic assumptions. We then present a prototype implementation for our proposed construction on an ensemble of Artix 7 FPGAs. The architecture for our implementation exploits the massively parallel capabilities provided by hardware, especially in the design of data structures for efficient storage and retrieval of data. The implementation requires a total of 192 FPGAs to support a document collection comprising of 100 documents with a dictionary of 1000 keywords. In addition, the hardware implementation of CASE is found to outperform its software counterpart in terms of both search efficiency and scalability. To the best of our knowledge, this is the first hardware implementation of a searchable encryption scheme to be reported in the literature

Efficient compression of SIDH public keys

Supersingular isogeny Diffie-Hellman (SIDH) is an attractive candidate for post-quantum key exchange, in large part due to its relatively small public key sizes. A recent paper by Azarderakhsh, Jao, Kalach, Koziel and Leonardi showed that the public keys defined in Jao and De Feo\u27s original SIDH scheme can be further compressed by around a factor of two, but reported that the performance penalty in utilizing this compression blew the overall SIDH runtime out by more than an order of magnitude. Given that the runtime of SIDH key exchange is currently its main drawback in relation to its lattice- and code-based post-quantum alternatives, an order of magnitude performance penalty for a factor of two improvement in bandwidth presents a trade-off that is unlikely to favor public-key compression in many scenarios. In this paper, we propose a range of new algorithms and techniques that accelerate SIDH public-key compression by more than an order of magnitude, making it roughly as fast as a round of standalone SIDH key exchange, while further reducing the size of the compressed public keys by approximately 12.5%. These improvements enable the practical use of compression, achieving public keys of only 330 bytes for the concrete parameters used to target 128 bits of quantum security and further strengthens SIDH as a promising post-quantum primitive

Fully Collusion Resistant Trace-and-Revoke Functional Encryption for Arbitrary Identities

Functional Encryption (FE) has been extensively studied in the recent years, mainly focusing on the feasibility of constructing FE for general functionalities, as well as some realizations for restricted functionalities of practical interest, such as inner-product. However, little consideration has been given to the issue of key leakage on FE. The property of FE that allows multiple users to obtain the same functional keys from the holder of the master secret key raises an important problem: if some users leak their keys or collude to create a pirated decoder, how can we identify at least one of those users, given some information about the compromised keys or the pirated decoder? Moreover, how do we disable the decryption capabilities of those users (i.e. traitors)? Two recent works have offered potential solutions to the above traitor scenario. However, the two solutions satisfy weaker notions of security and traceability, can only tolerate bounded collusions (i.e., there is an a priori bound on the number of keys the pirated decoder obtains), or can only handle a polynomially large universe of possible identities. In this paper, we study trace-and-revoke mechanism on FE and provide the first construction of trace-and-revoke FE that supports arbitrary identities, is both fully collusion resistant and fully anonymous. Our construction relies on a generic transformation from revocable predicate functional encryption with broadcast (RPFE with broadcast, which is an extension of revocable predicate encryption with broadcast proposed by Kim and J. Wu at ASIACRYPT\u272020) to trace-and-revoke FE. Since this construction admits a generic construction of trace-and-revoke inner-product FE (IPFE), we instantiate the trace-and-revoke IPFE from the well-studied Learning with Errors (LWE). This is achieved by proposing a new LWE-based attribute-based IPFE (ABIPFE) scheme to instantiate RPFE with broadcast

Privacy Preserving Cryptographic Protocols for Secure Heterogeneous Networks

Disertační práce se zabývá kryptografickými protokoly poskytující ochranu soukromí, které jsou určeny pro zabezpečení komunikačních a informačních systémů tvořících heterogenní sítě. Práce se zaměřuje především na možnosti využití nekonvenčních kryptografických prostředků, které poskytují rozšířené bezpečnostní požadavky, jako je například ochrana soukromí uživatelů komunikačního systému. V práci je stanovena výpočetní náročnost kryptografických a matematických primitiv na různých zařízeních, které se podílí na zabezpečení heterogenní sítě. Hlavní cíle práce se zaměřují na návrh pokročilých kryptografických protokolů poskytujících ochranu soukromí. V práci jsou navrženy celkově tři protokoly, které využívají skupinových podpisů založených na bilineárním párování pro zajištění ochrany soukromí uživatelů. Tyto navržené protokoly zajišťují ochranu soukromí a nepopiratelnost po celou dobu datové komunikace spolu s autentizací a integritou přenášených zpráv. Pro navýšení výkonnosti navržených protokolů je využito optimalizačních technik, např. dávkového ověřování, tak aby protokoly byly praktické i pro heterogenní sítě.The dissertation thesis deals with privacy-preserving cryptographic protocols for secure communication and information systems forming heterogeneous networks. The thesis focuses on the possibilities of using non-conventional cryptographic primitives that provide enhanced security features, such as the protection of user privacy in communication systems. In the dissertation, the performance of cryptographic and mathematic primitives on various devices that participate in the security of heterogeneous networks is evaluated. The main objectives of the thesis focus on the design of advanced privacy-preserving cryptographic protocols. There are three designed protocols which use pairing-based group signatures to ensure user privacy. These proposals ensure the protection of user privacy together with the authentication, integrity and non-repudiation of transmitted messages during communication. The protocols employ the optimization techniques such as batch verification to increase their performance and become more practical in heterogeneous networks.