Chosen-Ciphertext Secure Identity-Based Encryption in the Standard Model with short Ciphertexts

We describe a practical identity-based encryption scheme that is secure in the standard model against chosen-ciphertext (CCA2) attacks. Security is based on an assumption comparable to (but slightly stronger than) Bilinear Decisonal Diffie-Hellman (BDDH). A comparison shows that our construction outperforms all known identity-based encryption schemes in the standard model and its performance is even comparable with the one from the random-oracle based Boneh/Franklin IBE scheme. Our proposed IBE scheme has furthermore the property that it fulfills some notion of ``redundancy-freeness , i.e. the encryption algorithm is not only a probabilistic injection but also a surjection. As a consequence the ciphertext overhead is nearly optimal: to encrypt $k$ bit messages for $k$ bit identities and with $k$ bit randomness we get $3k$ bit ciphertexts to guarantee (roughly) $k$ bits of security

URDP: General Framework for Direct CCA2 Security from any Lattice-Based PKE Scheme

Design efficient lattice-based cryptosystem secure against adaptive chosen ciphertext attack (IND-CCA2) is a challenge problem. To the date, full CCA2-security of all proposed lattice-based PKE schemes achieved by using a generic transformations such as either strongly unforgeable one-time signature schemes (SU-OT-SS), or a message authentication code (MAC) and weak form of commitment. The drawback of these schemes is that encryption requires "separate encryption". Therefore, the resulting encryption scheme is not sufficiently efficient to be used in practice and it is inappropriate for many applications such as small ubiquitous computing devices with limited resources such as smart cards, active RFID tags, wireless sensor networks and other embedded devices. In this work, for the first time, we introduce an efficient universal random data padding (URDP) scheme, and show how it can be used to construct a "direct" CCA2-secure encryption scheme from "any" worst-case hardness problems in (ideal) lattice in the standard model, resolving a problem that has remained open till date. This novel approach is a "black-box" construction and leads to the elimination of separate encryption, as it avoids using general transformation from CPA-secure scheme to a CCA2-secure one. IND-CCA2 security of this scheme can be tightly reduced in the standard model to the assumption that the underlying primitive is an one-way trapdoor function.Comment: arXiv admin note: text overlap with arXiv:1302.0347, arXiv:1211.6984; and with arXiv:1205.5224 by other author

Bounded Collusion ABE for TMs from IBE

We give an attribute-based encryption system for Turing Machines that is provably secure assuming only the existence of identity-based encryption (IBE) for large identity spaces. Currently, IBE is known to be realizable from most mainstream number theoretic assumptions that imply public key cryptography including factoring, the search Diffie-Hellman assumption, and the Learning with Errors assumption. Our core construction provides security against an attacker that makes a single key query for a machine $T$ before declaring a challenge string $w^*$ that is associated with the challenge ciphertext. We build our construction by leveraging a Garbled RAM construction of Gentry, Halevi, Raykova, and Wichs; however, to prove security we need to introduce a new notion of security called iterated simulation security. We then show how to transform our core construction into one that is secure for an a-priori bounded number $q = q(\lambda)$ of key queries that can occur either before or after the challenge ciphertext. We do this by first showing how one can use a special type of non-committing encryption to transform a system that is secure only if a single key is chosen before the challenge ciphertext is declared into one where the single key can be requested either before or after the challenge ciphertext. We give a simple construction of this non-committing encryption from public key encryption in the Random Oracle Model. Next, one can apply standard combinatorial techniques to lift from single-key adaptive security to $q$-key adaptive security

Security of a biometric identity-based encryption scheme

Biometric identity-based encryption (Bio-IBE) is a kind of fuzzy identity-based encryption (fuzzy IBE) where a ciphertext encrypted under an identity w' can be decrypted using a secret key corresponding to the identity w which is close to w' as measured by some metric. Recently, Yang et al. proposed a constant-size Bio-IBE scheme and proved that it is secure against adaptive chosen-ciphertext attack (CCA2) in the random oracle model. Unfortunately, in this paper, we will show that their Bio-IBE scheme is even not chosen-plaintext secure. Specifically, user w using his secret key is able to decrypt any ciphertext encrypted under an identity w' even though w is not close to w'.Comment: Journal version of the paper will be appearing in International Journal of Network Securit

Server-Aided Revocable Predicate Encryption: Formalization and Lattice-Based Instantiation

Efficient user revocation is a necessary but challenging problem in many multi-user cryptosystems. Among known approaches, server-aided revocation yields a promising solution, because it allows to outsource the major workloads of system users to a computationally powerful third party, called the server, whose only requirement is to carry out the computations correctly. Such a revocation mechanism was considered in the settings of identity-based encryption and attribute-based encryption by Qin et al. (ESORICS 2015) and Cui et al. (ESORICS 2016), respectively. In this work, we consider the server-aided revocation mechanism in the more elaborate setting of predicate encryption (PE). The latter, introduced by Katz, Sahai, and Waters (EUROCRYPT 2008), provides fine-grained and role-based access to encrypted data and can be viewed as a generalization of identity-based and attribute-based encryption. Our contribution is two-fold. First, we formalize the model of server-aided revocable predicate encryption (SR-PE), with rigorous definitions and security notions. Our model can be seen as a non-trivial adaptation of Cui et al.'s work into the PE context. Second, we put forward a lattice-based instantiation of SR-PE. The scheme employs the PE scheme of Agrawal, Freeman and Vaikuntanathan (ASIACRYPT 2011) and the complete subtree method of Naor, Naor, and Lotspiech (CRYPTO 2001) as the two main ingredients, which work smoothly together thanks to a few additional techniques. Our scheme is proven secure in the standard model (in a selective manner), based on the hardness of the Learning With Errors (LWE) problem.Comment: 24 page

Efficient Conditional Proxy Re-encryption with Chosen-Ciphertext Security

Recently, a variant of proxy re-encryption, named conditional proxy re-encryption (C-PRE), has been introduced. Compared with traditional proxy re-encryption, C-PRE enables the delegator to implement fine-grained delegation of decryption rights, and thus is more useful in many applications. In this paper, based on a careful observation on the existing definitions and security notions for C-PRE, we reformalize more rigorous definition and security notions for C-PRE. We further propose a more efficient C-PRE scheme, and prove its chosenciphertext security under the decisional bilinear Diffie-Hellman (DBDH) assumption in the random oracle model. In addition, we point out that a recent C-PRE scheme fails to achieve the chosen-ciphertext security

A Type-and-Identity-based Proxy Re-Encryption Scheme and its Application in Healthcare

Proxy re-encryption is a cryptographic primitive developed to delegate the decryption right from one party (the delegator) to another (the delegatee). In a proxy re-encryption scheme, the delegator assigns a key to a proxy to re-encrypt all messages encrypted with his public key such that the re-encrypted ciphertexts can be decrypted with the delegatee’s private key. We propose a type-and-identity-based proxy re-encryption scheme based on the Boneh-Franklin Identity Based Encryption (IBE) scheme. In our scheme, the delegator can categorize messages into different types and delegate the decryption right of each type to the delegatee through a proxy. Our scheme enables the delegator to provide the proxy fine-grained re-encryption capability. As an application, we propose a fine-grained Personal Health Record (PHR) disclosure scheme for healthcare service by applying the proposed scheme