Crypto@ZeroDB

Proxy Re-Encryption (PRE) is a type of Public-Key Encryption (PKE) which provides an additional re-encryption functionality. Although PRE is inherently more complex than PKE, attack models for PRE have not been developed further than those inherited from PKE. In this paper we address this gap and define a parametric family of attack models for PRE, based on the availability of both the decryption and re-encryption oracles during the security game. This family enables the definition of a set of intermediate security notions for PRE that ranges from ``plain'' IND-CPA to ``full'' IND-CCA. We analyze some relations among these notions of security, and in particular, the separations that arise when the re-encryption oracle leaks re-encryption keys. In addition, we discuss which of these security notions represent meaningful adversarial models for PRE. Finally, we provide an example of a recent ``CCA1- secure'' scheme from PKC 2014 whose security model does not capture chosen-ciphertext attacks through re-encryption and for which we describe an attack under a more realistic security notion. This attack emphasizes the fact that PRE schemes that leak re-encryption keys cannot achieve strong security notions.

The use of alternative foundations for constructing more secure and efficient cryptographic schemes is a topic worth exploring. In the case of proxy re-encryption, the vast majority of schemes are based on number theoretic problems such as the discrete logarithm. In this paper we present NTRUReEncrypt, a new bidirectional and multihop proxy re-encryption scheme based on NTRU, a widely known lattice-based cryptosystem. We provide two versions of our scheme: the first one is based on the conventional NTRU encryption scheme and, although it lacks a security proof, remains as efficient as its predecessor; the second one is based on a variant of NTRU proposed by Stehlé and Steinfeld, which is proven CPA-secure under the hardness of the Ring-LWE problem. To the best of our knowledge, our proposals are the first proxy re-encryption schemes to be based on the NTRU primitive. In addition, we provide experimental results to show the efficiency of our proposal, as well as a comparison with previous proxy re-encryption schemes, which confirms that our first scheme outperforms the rest by an order of magnitude.

Several generic methods exist for achieving chosen-ciphertext attack (CCA)-secure public-key encryption schemes from weakly secure cryptosystems, such as the Fujisaki–Okamoto and REACT transformations. In the context of proxy re-encryption (PRE), it would be desirable to count on analogous constructions that allow PRE schemes to achieve better security notions. In this paper, we study the adaptation of these transformations to proxy re-encryption and find both negative and positive results. On the one hand, we show why it is not possible to directly integrate these transformations with weakly secure PRE schemes because of general obstacles coming from both the constructions themselves and the security models, and we identify 12 PRE schemes that exhibit these problems. On the other hand, we propose an extension of the Fujisaki–Okamoto transformation for PRE, which achieves a weak form of CCA security in the random oracle model, and we describe the sufficient conditions for applying it

Among Big Data technologies, Hadoop stands out for its capacity to store and process large-scale datasets. However, although Hadoop was not designed with security in mind, it is widely used by plenty of organizations, some of which have strong data protection requirements. Traditional access control solutions are not enough, and cryptographic solutions must be put in place to protect sensitive information. In this paper, we describe a cryptographically-enforced access control system for Hadoop, based on proxy re-encryption. Our proposed solution fits in well with the outsourcing of Big Data processing to the cloud, since information can be stored in encrypted form in external servers in the cloud and processed only if access has been delegated. Experimental results show that the overhead produced by our solution is manageable, which makes it suitable for some applications.