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ranging from fast and generic symmetric ciphers to compact public key and white-box constructions based on generic affine transformations combined with specially designed low degree non-linear layers. While explaining our design process we show several instructive attacks on the
weaker variants of our schemes.
(ABE) schemes for all boolean circuits. GVW show that TOR schemes can be constructed assuming the hardness of the learning-with-errors (LWE) problem.
We propose a slightly weaker variant of TOR schemes called correlation-relaxed two-to-one recoding (CR-TOR). Unlike the TOR schemes, our weaker variant does not require an encoding function to
be pseudorandom on correlated inputs. We instead replace it with an indistinguishability property that states a ciphertext is hard to decrypt without access to a certain encoding. The primary benefit of this relaxation is that it allows the construction of ABE for circuits using the TOR paradigm from a broader class of cryptographic assumptions.
We show how to construct a CR-TOR scheme from the noisy cryptographic multilinear maps of Garg, Gentry, and Halevi as well as those of Coron, Lepoint, and Tibouchi. Our framework leads to an instantiation of ABE for circuits that is conceptually different from the existing constructions.
In this paper, we partially fill this gap by showing that it is possible to deal with simple additive relations at the expense of relaxing the model of the attack. We introduce several natural relaxations of RKA-security, study the relations between these notions, and describe efficient constructions either under lattice assumptions or under general assumptions. Our results enrich the landscape of RKA security and suggest useful trade-offs between the attack model and the family of possible relations.
This problem was addressed only recently by the introduction of seekable sequential key generators (SSKG). Every instance of this cryptographic primitive produces a forward-secure sequence of symmetric (authentication) keys, but also offers an explicit fast-forward functionality. The only currently known SSKG construction replaces traditional hash chains by the iterated evaluation of a shortcut one-way permutation, a factoring-based and hence in practice not too efficient building block.
In this paper we revisit the challenge of marrying forward-secure key generation with seekability and show that symmetric primitives like PRGs, block ciphers, and hash functions suffice for obtaining secure SSKGs. Our scheme is not only considerably more efficient than the prior number-theoretic construction, but also extends the seeking functionality in a way that we believe is important in practice. Our construction is provably (forward-)secure in the standard model.
We provide a new framework of cryptographic agents that unifies various cryptographic objects and security definitions, similar to how the Universal Composition framework unifies various multi-party computation tasks like commitment, coin-tossing and zero-knowledge proofs.
Our contributions can be summarized as follows.
- Our main contribution is a new model of cryptographic computation, that unifies and extends cryptographic primitives such as Obfuscation, Functional Encryption, Fully Homomorphic Encryption, Witness encryption, Property Preserving Encryption and the like, all of which can be cleanly modeled as \'\'schemata\'\' in our framework. We provide a new indistinguishability preserving (IND-PRE) definition of security that interpolates indistinguishability and simulation style definitions, implying the former while (often) sidestepping the impossibilities of the latter.
- We present a notion of reduction from one schema to another and a powerful composition theorem with respect to IND-PRE security. This provides a modular means to build and analyze secure schemes for complicated schemata based on those for simpler schemata. Further, this provides a way to abstract out and study the relative complexity of different schemata. We show that obfuscation is a \'\'complete\'\' schema under this notion, under standard cryptographic assumptions.
- IND-PRE-security can be parameterized by the choice of the \'\'test\'\' family. For obfuscation, the strongest security definition (by considering all PPT tests) is unrealizable in general. But we identify a family of tests, called \\Delta, such that all known impossibility results, for obfuscation as well as functional encryption, are by-passed. On the other hand, for each of the example primitives we consider in this paper -- obfuscation, functional encryption, fully-homomorphic encryption and property-preserving encryption -- \\Delta-IND-PRE security for the corresponding schema implies the standard achievable security definitions in the literature.
- We provide a stricter notion of reduction that composes with respect to \\Delta-IND-PRE security.
- Based on \\Delta-IND-PRE-security we obtain a new definition for security of obfuscation, called adaptive differing-inputs obfuscation. We illustrate its power by using it for new constructions of functional encryption schemes, with and without function-hiding.
- Last but not the least, our framework can be used to model abstractions like the generic group model and the random oracle model, letting one translate a general class of constructions in these heuristic models to constructions based on standard model assumptions. We illustrate this by adapting a functional encryption scheme (for inner product predicate) that was shown secure in the generic group model to be secure based on a new standard model assumption we propose, called the generic bilinear group agents assumption.