Functional Encryption for Computational Hiding in Prime Order Groups via Pair Encodings

Lewko and Waters introduced the computational hiding technique in Crypto\u2712. In their technique, two computational assumptions that achieve selective and co-selective security proofs lead to adaptive security of an encryption scheme. Later, pair encoding framework was introduced by Attrapadung in Eurocrypt\u2714. The pair encoding framework generalises the computational hiding technique for functional encryption (FE). It has been used to achieve a number of new FE schemes such as FE for regular languages and unbounded attribute based encryption allowing multi-use of attributes. Nevertheless, the generalised construction of Attrapadung\u27s pair encoding for those schemes is adaptively secure only in composite order groups, which leads to efficiency loss. It remains a challenging task to explore constructions in prime order groups for gaining efficiency improvement, which leaves the research gap in the existing literature. In this work, we aim to address this drawback by proposing a new generalised construction for pair encodings in prime order groups. Our construction will lead to a number of new FE schemes in prime order groups, which have been previously introduced only in composite order groups by Attrapadung

Functional Encryption for Computational Hiding in Prime Order Groups via Pair Encodings

Lewko and Waters introduced the computational hiding technique in Crypto\u2712. In their technique, two computational assumptions that achieve selective and co-selective security proofs lead to adaptive security of an encryption scheme. Later, pair encoding framework was introduced by Attrapadung in Eurocrypt\u2714. The pair encoding framework generalises the computational hiding technique for functional encryption (FE). It has been used to achieve a number of new FE schemes such as FE for regular languages and unbounded attribute based encryption allowing multi-use of attributes. Nevertheless, the generalised construction of Attrapadung\u27s pair encoding for those schemes is adaptively secure only in composite order groups, which leads to efficiency loss. It remains a challenging task to explore constructions in prime order groups for gaining efficiency improvement, which leaves the research gap in the existing literature. In this work, we aim to address this drawback by proposing a new generalised construction for pair encodings in prime order groups. Our construction will lead to a number of new FE schemes in prime order groups, which have been previously introduced only in composite order groups by Attrapadung

Hard isogeny problems over RSA moduli and groups with infeasible inversion

We initiate the study of computational problems on elliptic curve isogeny graphs defined over RSA moduli. We conjecture that several variants of the neighbor-search problem over these graphs are hard, and provide a comprehensive list of cryptanalytic attempts on these problems. Moreover, based on the hardness of these problems, we provide a construction of groups with infeasible inversion, where the underlying groups are the ideal class groups of imaginary quadratic orders. Recall that in a group with infeasible inversion, computing the inverse of a group element is required to be hard, while performing the group operation is easy. Motivated by the potential cryptographic application of building a directed transitive signature scheme, the search for a group with infeasible inversion was initiated in the theses of Hohenberger and Molnar (2003). Later it was also shown to provide a broadcast encryption scheme by Irrer et al. (2004). However, to date the only case of a group with infeasible inversion is implied by the much stronger primitive of self-bilinear map constructed by Yamakawa et al. (2014) based on the hardness of factoring and indistinguishability obfuscation (iO). Our construction gives a candidate without using iO.Comment: Significant revision of the article previously titled "A Candidate Group with Infeasible Inversion" (arXiv:1810.00022v1). Cleared up the constructions by giving toy examples, added "The Parallelogram Attack" (Sec 5.3.2). 54 pages, 8 figure

Multilinear Maps from Obfuscation

International audienceWe provide constructions of multilinear groups equipped with natural hard problems from in-distinguishability obfuscation, homomorphic encryption, and NIZKs. This complements known results on the constructions of indistinguishability obfuscators from multilinear maps in the reverse direction. We provide two distinct, but closely related constructions and show that multilinear analogues of the DDH assumption hold for them. Our first construction is symmetric and comes with a κ-linear map e : G κ −→ G T for prime-order groups G and G T. To establish the hardness of the κ-linear DDH problem, we rely on the existence of a base group for which the (κ − 1)-strong DDH assumption holds. Our second construction is for the asymmetric setting, where e : G 1 × · · · × G κ −→ G T for a collection of κ + 1 prime-order groups G i and G T , and relies only on the standard DDH assumption in its base group. In both constructions the linearity κ can be set to any arbitrary but a priori fixed polynomial value in the security parameter. We rely on a number of powerful tools in our constructions: (probabilistic) indistinguishability obfuscation, dual-mode NIZK proof systems (with perfect soundness, witness indistinguishability and zero knowledge), and additively homomorphic encryption for the group Z + N. At a high level, we enable " bootstrapping " multilinear assumptions from their simpler counterparts in standard cryptographic groups, and show the equivalence of IO and multilinear maps under the existence of the aforementioned primitives

Dual System Encryption Framework in Prime-Order Groups

We propose a new generic framework for achieving fully secure attribute based encryption (ABE) in prime-order bilinear groups. It is generic in the sense that it can be applied to ABE for arbitrary predicate. All previously available frameworks that are generic in this sense are given only in composite-order bilinear groups. These consist of the frameworks proposed by Wee (TCC\u2714) and Attrapadung (Eurocrypt\u2714). Both frameworks provide abstractions of dual-system encryption techniques introduced by Waters (Crypto\u2709). Our framework can be considered as a prime-order version of Attrapadung\u27s framework and works in a similar manner: it relies on a main component called pair encodings, and it generically compiles any secure pair encoding scheme for a predicate in consideration to a fully secure ABE scheme for that predicate. One feature of our new compiler is that although the resulting ABE schemes will be newly defined in prime-order groups, we require essentially the same security notions of pair encodings as before. Beside the security of pair encodings, our framework assumes only the Matrix Diffie-Hellman assumption, introduced by Escala et al. (Crypto\u2713), which is a weak assumption that includes the Decisional Linear assumption as a special case. As for its applications, we can plug in available pair encoding schemes and automatically obtain the first fully secure ABE realizations in prime-order groups for predicates of which only fully secure schemes in composite-order groups were known. These include ABE for regular languages, ABE for monotone span programs (and hence Boolean formulae) with short ciphertexts or keys, and completely unbounded ABE for monotone span programs. As a side result, we establish the first generic implication from ABE for monotone span programs to ABE for branching programs. Consequently, we obtain fully-secure ABE for branching programs in some new variants, namely, unbounded, short-ciphertext, and short-key variants. Previous ABE schemes for branching programs are bounded and require linear-size ciphertexts and keys

Indistinguishability Obfuscation from DDH-like Assumptions on Constant-Degree Graded Encodings

All constructions of general purpose indistinguishability obfuscation (IO) rely on either meta-assumptions that encapsulate an exponential family of assumptions (e.g., Pass, Seth and Telang, CRYPTO 2014 and Lin, EUROCRYPT 2016), or polynomial families of assumptions on graded encoding schemes with a high polynomial degree/multilinearity (e.g., Gentry, Lewko, Sahai and Waters, FOCS 2014). We present a new construction of IO, with a security reduction based on two assumptions: (a) a DDH-like assumption — called the joint-SXDH assumption — on constant degree graded en- codings, and (b) the existence of polynomial-stretch pseudorandom generators (PRG) in NC0. Our assumption on graded encodings is simple, has constant size, and does not require handling composite-order rings. This narrows the gap between the mathematical objects that exist (bilinear maps, from elliptic curve groups) and ones that suffice to construct general purpose indistinguishability obfuscation

A Study of Pair Encodings: Predicate Encryption in Prime Order Groups

Pair encodings and predicate encodings, recently introduced by Attrapadung (Eurocrypt 2014) and Wee (TCC 2014) respectively, greatly simplify the process of designing and analyzing predicate and attribute-based encryption schemes. However, they are still somewhat limited in that they are restricted to composite order groups, and the information theoretic properties are not sufficient to argue about many of the schemes. Here we focus on pair encodings, as the more general of the two. We first study the structure of these objects, then propose a new relaxed but still information theoretic security property. Next we show a generic construction for predicate encryption in prime order groups from our new property; it results in either semi-adaptive or full security depending on the encoding, and gives security under SXDH or DLIN. Finally, we demonstrate the range of our new property by using it to design the first semi-adaptively secure CP-ABE scheme with constant size ciphertexts

Advances in Functional Encryption

Functional encryption is a novel paradigm for public-key encryption that enables both fine-grained access control and selective computation on encrypted data, as is necessary to protect big, complex data in the cloud. In this thesis, I provide a brief introduction to functional encryption, and an overview of my contributions to the area

Multilinear Maps in Cryptography

Multilineare Abbildungen spielen in der modernen Kryptographie eine immer bedeutendere Rolle. In dieser Arbeit wird auf die Konstruktion, Anwendung und Verbesserung von multilinearen Abbildungen eingegangen

Dual System Encryption via Predicate Encodings

We introduce the notion of predicate encodings, an information-theoretic primitive reminiscent of linear secret-sharing that in addition, satisfies a novel notion of reusability. Using this notion, we obtain a unifying framework for adaptively-secure public-index predicate encryption schemes for a large class of predicates. Our framework relies on Waters’ dual system encryption methodology (Crypto ’09), and encompass the identity-based encryption scheme of Lewko and Waters (TCC ’10), and the attribute-based encryption scheme of Lewko et al. (Eurocrypt ’10). In addition, we obtain several concrete improvements over prior works. Our work offers a novel interpretation of dual system encryption as a methodology for amplifying a one-time private-key primitive (i.e. predicate encodings) into a many-time public-key primitive (i.e. predicate encryption)