This paper shows a generic and simple conversion from weak asymmetric and symmetric encryption schemes into an asymmetric encryption scheme which is secure in a very strong sense- indistinguishability against adaptive chosen-ciphertext attacks in the random oracle model. In particular, this conversion can be applied efficiently to an asymmetric encryption scheme that provides a large enough coin space and, for every message, many enough variants of the encryption, like the ElGamal encryption scheme.

Secure integration of asymmetric and symmetric encryption schemes

We provide a zero-knowledge argument for arithmetic circuit satisfiability with a communication complexity that grows logarithmically in the size of the circuit. The round complexity is also logarithmic and for an arithmetic circuit with fan-in 2 gates the computation of the prover and verifier is linear in the size of the circuit. The soundness of our argument relies solely on the well-established discrete logarithm assumption in prime order groups.

At the heart of our new argument system is an efficient zero-knowledge argument of knowledge of openings of two Pedersen multicommitments satisfying an inner product relation, which is of independent interest. The inner product argument requires logarithmic communication, logarithmic interaction and linear computation for both the prover and the verifier.

We also develop a scheme to commit to a polynomial and later reveal the evaluation at an arbitrary point, in a verifiable manner. This is used to build an optimized version of the constant round square root complexity argument of Groth CRYPTO 2009, which reduces both communication and round complexity.

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Efficient Zero-Knowledge Arguments for Arithmetic Circuits in the Discrete Log Setting

The need of secure big data storage service is more desirable than ever to date. The basic requirement of the service is to guarantee the confidentiality of the data. However, the anonymity of the service clients, one of the most essential aspects of privacy, should be considered simultaneously. Moreover, the service also should provide practical and fine-grained encrypted data sharing such that a data owner is allowed to share a ciphertext of data among others under some specified conditions. This paper, for the first time, proposes a privacy-preserving ciphertext multi-sharing mechanism to achieve the above properties. It combines the merits of proxy re-encryption with anonymous technique in which a ciphertext can be securely and conditionally shared multiple times without leaking both the knowledge of underlying message and the identity information of ciphertext senders/recipients. Furthermore, this paper shows that the new primitive is secure against chosen-ciphertext attacks in the standard model.

/pdf/privacy-preserving-ciphertext-multi-sharing-control-for-big-1w1qk9dugu.pdf

Privacy-Preserving Ciphertext Multi-Sharing Control for Big Data Storage

Proxy re-encryption (PRE) is a highly useful cryptographic primitive whereby Alice and Bob can endow a proxy with the capacity to change ciphertext recipients from Alice to Bob, without the proxy itself being able to decrypt, thereby providing delegation of decryption authority. Key-private PRE (KP-PRE) specifies an additional level of confidentiality, requiring pseudo-random proxy keys that leak no information on the identity of the delegators and delegatees.

In this paper, we propose a CPA-secure PK-PRE scheme in the standard model (which we then transform into a CCA-secure scheme in the random oracle model). Both schemes enjoy highly desirable properties such as uni-directionality and multi-hop delegation.

Unlike (the few) prior constructions of PRE and KP-PRE that typically rely on bilinear maps under ad hoc assumptions, security of our construction is based on the hardness of the standard Learning-With-Errors (LWE) problem, itself reducible from worst-case lattice hard problems that are conjectured immune to quantum cryptanalysis, or "post-quantum".

Of independent interest, we further examine the practical hardness of the LWE assumption, using Kannan's exhaustive search algorithm coupling with pruning techniques. This leads to state-of-the-art parameters not only for our scheme, but also for a number of other primitives based on LWE published the literature.

Key-Private Proxy Re-encryption under LWE

Proxy re-encryption (PRE) realizes delegation of decryption rights, enabling a proxy holding a re-encryption key to convert a ciphertext originally intended for Alice into an encryption of the same message for Bob, and cannot learn anything about the encrypted plaintext. PRE is a very useful primitive, having many applications in distributed file systems, outsourced filtering of encrypted spam, access control over network storage, confidential email, digital right management, and so on. In CT-RSA2012, Hanaoka et al. proposed a chosen-ciphertext (CCA) security definition for PRE, and claimed that it is stronger than all the previous works. Their definition is a somewhat strengthened variant of the replayable-CCA one, however, it does not fully capture the CCA security notion. In this paper, we present a full CCA security definition which is extended from theirs. We then propose the first PRE scheme with this security in the standard model (i.e. without the random oracle idealization). Our scheme is efficient and relies on mild complexity assumptions in bilinear groups.

Yuan Zhou

Papers

Achieving key privacy without losing CCA security in proxy re-encryption