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We propose a fully functional identity-based encryption scheme (IBE). The scheme has chosen ciphertext security in the random oracle model assuming an elliptic curve variant of the computational Diffie-Hellman problem. Our system is based on the Weil pairing. We give precise definitions for secure identity based encryption schemes and give several applications for such systems.

/pdf/identity-based-encryption-from-the-weil-pairing-sfqdyadlbb.pdf

Identity-Based Encryption from the Weil Pairing

We propose a fully functional identity-based encryption (IBE) scheme. The scheme has chosen ciphertext security in the random oracle model assuming a variant of the computational Diffie--Hellman problem. Our system is based on bilinear maps between groups. The Weil pairing on elliptic curves is an example of such a map. We give precise definitions for secure IBE schemes and give several applications for such systems.

/pdf/identity-based-encryption-from-the-weil-pairing-1x4254f2zc.pdf

As more sensitive data is shared and stored by third-party sites on the Internet, there will be a need to encrypt data stored at these sites. One drawback of encrypting data, is that it can be selectively shared only at a coarse-grained level (i.e., giving another party your private key). We develop a new cryptosystem for fine-grained sharing of encrypted data that we call Key-Policy Attribute-Based Encryption (KP-ABE). In our cryptosystem, ciphertexts are labeled with sets of attributes and private keys are associated with access structures that control which ciphertexts a user is able to decrypt. We demonstrate the applicability of our construction to sharing of audit-log information and broadcast encryption. Our construction supports delegation of private keys which subsumesHierarchical Identity-Based Encryption (HIBE).

/pdf/attribute-based-encryption-for-fine-grained-access-control-3fjqefx4um.pdf

Attribute-based encryption for fine-grained access control of encrypted data

We propose a novel paradigm for defining security of cryptographic protocols, called universally composable security. The salient property of universally composable definitions of security is that they guarantee security even when a secure protocol is composed of an arbitrary set of protocols, or more generally when the protocol is used as a component of an arbitrary system. This is an essential property for maintaining security of cryptographic protocols in complex and unpredictable environments such as the Internet. In particular, universally composable definitions guarantee security even when an unbounded number of protocol instances are executed concurrently in an adversarially controlled manner, they guarantee non-malleability with respect to arbitrary protocols, and more. We show how to formulate universally composable definitions of security for practically any cryptographic task. Furthermore, we demonstrate that practically any such definition can be realized using known techniques, as long as only a minority of the participants are corrupted. We then proceed to formulate universally composable definitions of a wide array of cryptographic tasks, including authenticated and secure communication, key-exchange, public-key encryption, signature, commitment, oblivious transfer, zero knowledge and more. We also make initial steps towards studying the realizability of the proposed definitions in various settings.

/pdf/universally-composable-security-a-new-paradigm-for-3kyhnjofve.pdf

Universally composable security: a new paradigm for cryptographic protocols

We study the problem of searching on data that is encrypted using a public key system. Consider user Bob who sends email to user Alice encrypted under Alice’s public key. An email gateway wants to test whether the email contains the keyword “urgent” so that it could route the email accordingly. Alice, on the other hand does not wish to give the gateway the ability to decrypt all her messages. We define and construct a mechanism that enables Alice to provide a key to the gateway that enables the gateway to test whether the word “urgent” is a keyword in the email without learning anything else about the email. We refer to this mechanism as Public Key Encryption with keyword Search. As another example, consider a mail server that stores various messages publicly encrypted for Alice by others. Using our mechanism Alice can send the mail server a key that will enable the server to identify all messages containing some specific keyword, but learn nothing else. We define the concept of public key encryption with keyword search and give several constructions.

https://link.springer.com/content/pdf/10.1007%2F978-3-540-24676-3_30.pdf

Public Key Encryption with Keyword Search

Non-Interactive Zero Knowledge (NIZK), introduced by Blum, Feldman, and Micali in 1988, is a fundamental cryptographic primitive which has attracted considerable attention in the last decade and has been used throughout modern cryptography in several essential ways. For example, NIZK plays a central role in building provably secure public-key cryptosystems based on general complexity-theoretic assumptions that achieve security against chosen ciphertext attacks. In essence, in a multi-party setting, given a fixed common random string of polynomial size which is visible to all parties, NIZK allows an arbitrary polynomial number of Provers to send messages to polynomially many Verifiers, where each message constitutes an NIZK proof for an arbitrary polynomial-size NP statement.

In this paper, we take a closer look at NIZK in the multi-party setting. First, we consider non-malleable NIZK, and generalizing and substantially strengthening the results of Sahai, we give the first construction of NIZK which remains non-malleable after polynomially-many NIZK proofs. Second, we turn to the definition of standard NIZK itself, and propose a strengthening of it. In particular, one of the concerns in the technical definition of NIZK (as well as non-malleable NIZK) is that the so-called "simulator" of the Zero-Knowledge property is allowed to pick a different "common random string" from the one that Provers must actually use to prove NIZK statements in real executions. In this paper, we propose a new definition for NIZK that eliminates this shortcoming, and where Provers and the simulator use the same common random string. Furthermore, we show that both standard and non-malleable NIZK (as well as NIZK Proofs of Knowledge) can be constructed achieving this stronger definition. We call such NIZK Robust NIZK and show how to achieve it. Our results also yields the simplest known public-key encryption scheme based on general assumptions secure against adaptive chosen-ciphertext attack (CCA2).

/pdf/robust-non-interactive-zero-knowledge-191f2952w1.pdf

Robust Non-interactive Zero Knowledge

A commitment protocol is a fundamental cryptographic primitive used as a basic building block throughoutmodern cryptography. In STOC 1991, Dolev Dwork and Naor showed that in many settings the implementation of this fundamental primitive requires a strong non-malleability property in order not to be susceptible to a certain class of attacks. In this paper, assuming that a common random string is available to all players, we show how to implement nonmalleable commitment without any interaction and based on any one-way function. In contrast, all previous solutions required either logarithmically many rounds of interaction or strong algebraic

Non-interactive and non-malleable commitment

A Single-Database Private Information Retrieval (PIR) is a protocol that allows a user to privately retrieve from a database an entry with as small as possible communication complexity. We call a PIR protocol non-trivial if its total communication is strictly less than the size of the database. Non-trivial PIR is an important cryptographic primitive with many applications. Thus, understanding which assumptions are necessary for implementing such a primitive is an important task, although (so far) not a well-understood one. In this paper we show that any non-trivial PIR implies Oblivious Transfer, a far better understood primitive. Our result not only significantly clarifies our understanding of any non-trivial PIR protocol, but also yields the following consequences: - Any non-trivial PIR is complete for all two-party and multiparty secure computations. - There exists a communication-efficient reduction from any PIR protocol to a 1-out-of-n Oblivious Transfer protocol (also called SPIR). - There is strong evidence that the assumption of the existence of a one-way function is necessary but not sufficient for any non-trivial PIR protocol.

/pdf/single-database-private-information-retrieval-implies-g0wryzkbsz.pdf

Giovanni Di Crescenzo

Papers

Public Key Encryption with Keyword Search

Robust Non-interactive Zero Knowledge

Non-interactive and non-malleable commitment

Single database private information retrieval implies oblivious transfer

Public Key Encryption with keyword Search.