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

We propose a new general primitive called lossy trapdoor functions (lossy TDFs), and realize it under a variety of different number theoretic assumptions, including hardness of the decisional Diffie-Hellman (DDH) problem and the worst-case hardness of standard lattice problems. Using lossy TDFs, we develop a new approach for constructing many important cryptographic primitives, including standard trapdoor functions, CCA-secure cryptosystems, collisionresistant hash functions, and more. All of our constructions are simple, efficient, and black-box. Taken all together, these results resolve some long-standing open problems in cryptography. They give the first known (injective) trapdoor functions based on problems not directly related to integer factorization, and provide the first known CCA-secure cryptosystem based solely on worst-case lattice assumptions.

Abstract This paper presents a novel framework for the generic construction of hybrid encryptionschemes which produces more efficient schemes than the ones known before. A previous

Abstract We put forward a new paradigm for building hybrid encryption schemes from constrainedchosen-ciphertext secure (CCCA) key-encapsulation mechanisms (KEMs) plus authenticated

We describe a practical identity-based encryption scheme that is secure in the standard model against chosen-ciphertext attacks. Our construction applies “direct chosen-ciphertext techniques ” to Waters ’ chosen-plaintext secure scheme and is not based on hierarchical identity-based encryption. Furthermore, we give an improved concrete security analysis for Waters ’ scheme. As a result, one can instantiate the scheme in smaller groups, resulting in efficiency improvements. 1

We initiate the study of one-wayness under correlated products. We are interested in identifying necessary and sufficient conditions for a function f and a distribution on inputs (x1,..., xk), so that the function (f(x1),..., f(xk)) is one-way. The main motivation of this study is the construction of public-key encryption schemes that are secure against chosen-ciphertext attacks (CCA). We show that any collection of injective trapdoor functions that is secure under very natural correlated products can be used to construct a CCA-secure public-key encryption scheme. The construction is simple, black-box, and admits a direct proof of security. We provide evidence that security under correlated products is achievable by demonstrating that any collection of lossy trapdoor functions, a powerful primitive introduced by Peikert and Waters (STOC ’08), yields a collection of injective trapdoor functions that is secure under the above mentioned natural correlated products. Although we eventually base security under correlated products on lossy trapdoor functions, we argue that the former notion is potentially weaker as a general assumption. Specifically, there is no fully-black-box construction of lossy trapdoor functions from trapdoor functions that are secure under correlated products.

We describe a CCA-secure public-key encryption scheme, in the Cramer-Shoup paradigm, based on the Linear assumption of Boneh, Boyen, and Shacham. Through a comparison to the Kiltz tag-encryption scheme from TCC 2006, our scheme gives evidence that the Cramer-Shoup paradigm yields CCA encryption with shorter ciphertexts than the Canetti-Halevi-Katz paradigm. We present a generalization of the Linear assumption into a family of progressively weaker assumptions and show how to instantiate our Linear Cramer-Shoup encryption using the progressively weaker members of this family.

We propose a new computational problem called the twin Diffie-Hellman problem. This problem is closely related to the usual (computational) Diffie-Hellman problem and can be used in many of the same cryptographic constructions that are based on the Diffie-Hellman problem. Moreover, the twin Diffie-Hellman problem is at least as hard as the ordinary Diffie-Hellman problem. However, we are able to show that the twin Diffie-Hellman problem remains hard, even in the presence of a decision oracle that recognizes solutions to the problem — this is a feature not enjoyed by the ordinary Diffie-Hellman problem. In particular, we show how to build a certain “trapdoor test ” that allows us to effectively answer such decision oracle queries without knowing any of the corresponding discrete logarithms. Our new techniques have many applications. As one such application, we present a new variant of ElGamal encryption with very short ciphertexts, and with a very simple and tight security proof, in the random oracle model, under the assumption that the ordinary Diffie-Hellman problem is hard. We present several other applications as well, including: a new variant of Diffie and Hellman’s non-interactive key exchange protocol; a new variant of Cramer-Shoup encryption, with a very simple proof in the standard model; a new variant of Boneh-Franklin identity-based encryption, with very short ciphertexts; a more robust version of a password-authenticated key exchange protocol of Abdalla and Pointcheval. 1

Abstract. We present a non-interactive chosen ciphertext secure threshold encryption system. The proof of security is set in the standard model and does not use random oracles. Our construction uses the recent identity based encryption system of Boneh and Boyen and the chosen ciphertext secure construction of Canetti, Halevi, and Katz.

We propose a practical key encapsulation mechanism with a simple and intuitive design concept. Security against chosen-ciphertext attacks can be proved in the standard model under a new assumption, the Gap Hashed Diffie-Hellman (GHDH) assumption. The security reduction is tight and simple. Secure key encapsulation, combined with an appropriately secure symmetric encryption scheme, yields a hybrid public-key encryption scheme which is secure against chosen-ciphertext attacks. The implied encryption scheme is very efficient: compared to the previously most efficient scheme by Kurosawa and Desmedt [Crypto 2004] it has 128 bits shorter ciphertexts, between 25-50% shorter public/secret keys, and it is slightly more efficient in terms of encryption/decryption speed. Furthermore, our scheme enjoys (the option of) public verifiability of the ciphertexts and it inherits all practical advantages of secure hybrid encryption. Our results extend to key encapsulation mechanisms based on the class of Gap Hashed Multi-Diffie-Hellman (GHMDH) assumptions which is a natural generalization of GHDH.