New Security Definitions, Constructions and Applications of Proxy Re-Encryption

La externalización de la gestión de la información es una práctica cada vez más común, siendo la computación en la nube (en inglés, cloud computing) el paradigma más representativo. Sin embargo, este enfoque genera también preocupación con respecto a la seguridad y privacidad debido a la inherente pérdida del control sobre los datos. Las soluciones tradicionales, principalmente basadas en la aplicación de políticas y estrategias de control de acceso, solo reducen el problema a una cuestión de confianza, que puede romperse fácilmente por los proveedores de servicio, tanto de forma accidental como intencionada. Por lo tanto, proteger la información externalizada, y al mismo tiempo, reducir la confianza que es necesario establecer con los proveedores de servicio, se convierte en un objetivo inmediato. Las soluciones basadas en criptografía son un mecanismo crucial de cara a este fin. Esta tesis está dedicada al estudio de un criptosistema llamado recifrado delegado (en inglés, proxy re-encryption), que constituye una solución práctica a este problema, tanto desde el punto de vista funcional como de eficiencia. El recifrado delegado es un tipo de cifrado de clave pública que permite delegar en una entidad la capacidad de transformar textos cifrados de una clave pública a otra, sin que pueda obtener ninguna información sobre el mensaje subyacente. Desde un punto de vista funcional, el recifrado delegado puede verse como un medio de delegación segura de acceso a información cifrada, por lo que representa un candidato natural para construir mecanismos de control de acceso criptográficos. Aparte de esto, este tipo de cifrado es, en sí mismo, de gran interés teórico, ya que sus definiciones de seguridad deben balancear al mismo tiempo la seguridad de los textos cifrados con la posibilidad de transformarlos mediante el recifrado, lo que supone una estimulante dicotomía. Las contribuciones de esta tesis siguen un enfoque transversal, ya que van desde las propias definiciones de seguridad del recifrado delegado, hasta los detalles específicos de potenciales aplicaciones, pasando por construcciones concretas

Re-encryption, functional re-encryption, and multi-hop re-encryption: A framework for achieving obfuscation-based security and instantiations from lattices

In this work we define multiple relaxations to the definition of correctness in secure obfuscation. While still remaining meaningful, these relaxations provide ways to obfuscate many primitives in a more direct and efficient way. In particular, we first show how to construct a secure obfuscator for the re-encryption primitive from the Decisional Learning with Errors (DLWE) assumption, without going through fully homomorphic encryption. This can be viewed as a meaningful way to trade correctness for efficiency. Next, we show how our tools can be used to construct secure obfuscators for the functional re-encryption and multi-hop unidirectional re-encryption primitives. In the former case, we improve upon the efficiency of the only previously known construction that satisfies the stronger notion of collusion-resistant obfuscation (due to Chandran et al. - TCC 2012) and obtain a construction with input ciphertexts of constant length. In the latter case, we provide the first known obfuscation-based definition and construction; additionally, our scheme is the first scheme where the size of the ciphertexts does not grow with every hop

Fine-Grained Proxy Re-Encryption: Definitions & Constructions from LWE

Proxy re-encryption (PRE) allows a proxy with a re-encryption key to translate a ciphertext intended for Alice (delegator) to another ciphertext intended for Bob (delegatee) without revealing the underlying message. However, with PRE, Bob can obtain the whole message from the re-encrypted ciphertext, and Alice cannot take flexible control of the extent of the message transmitted to Bob. In this paper, we propose a new variant of PRE, called Fine-Grained PRE (FPRE), to support fine-grained re-encryptions. An FPRE is associated with a function family F, and each re-encryption key rk_{A→B}^f is associated with a function f ∈ F. With FPRE, Alice now can authorize re-encryption power to proxy by issuing rk_{A→B}^f to it, with f chosen by herself. Then the proxy can translate ciphertext encrypting m to Bob\u27s ciphertext encrypting f(m) with such a fine-grained re-encryption key, and Bob only obtains a function of message m. In this way, Alice can take flexible control of the message spread by specifying functions. For FPRE, we formally define its syntax and formalize security notions including CPA security, ciphertext pseudo-randomness, unidirectionality, non-transitivity, collusion-safety under adaptive corruptions in the multi-user setting. Moreover, we propose a new security notion named ciphertext unlinkability, which blurs the link between a ciphertext and its re-encrypted ciphertext to hide the proxy connections between users. We establish the relations between those security notions. As for constructions, we propose two FPRE schemes, one for bounded linear functions and the other for deletion functions, based on the learning-with-errors (LWE) assumption. Our FPRE schemes achieve all the aforementioned desirable securities under adaptive corruptions in the standard model. As far as we know, our schemes provide the first solution to PRE with security under adaptive corruptions in the standard model

Constant-Size Unbounded Multi-Hop Fully Homomorphic Proxy Re-Encryption from Lattices

Proxy re-encryption is a cryptosystem that achieves efficient encrypted data sharing by allowing a proxy to transform a ciphertext encrypted under one key into another ciphertext under a different key. Homomorphic proxy re-encryption (HPRE) extends this concept by integrating homomorphic encryption, allowing not only the sharing of encrypted data but also the homomorphic computations on such data. The existing HPRE schemes, however, are limited to a single or bounded number of hops of ciphertext re-encryptions. To address this limitation, this paper introduces a novel lattice-based, unbounded multi-hop fully homomorphic proxy re-encryption (FHPRE) scheme, with constant-size ciphertexts. Our FHPRE scheme supports an unbounded number of reencryption operations and enables arbitrary homomorphic computations over original, re-encrypted, and evaluated ciphertexts. Additionally, we propose a potential application of our FHPRE scheme in the form of a non-interactive, constant-size multi-user computation system for cloud computing environments

Non-Transferable Proxy Re-Encryption

Proxy re-encryption (PRE) allows a semi-trusted proxy to transform a ciphertext for Alice into a ciphertext of the same message for Bob. The traditional security notion of PRE focuses on preventing the proxy with the re-encryption key learning anything about the encrypted messages. However, such a basic security requirement is clearly not enough for many scenarios where the proxy can collude with Bob. A desirable security goal is therefore to prevent a malicious proxy colluding with Bob to re-delegate Alice’s decryption right. In 2005, Ateniese, Fu, Green and Hohenberger first proposed this intriguing problem called non-transferability, in the sense that the only way for Bob to transfer Alice’s decryption capability is to expose his own secret key. It captures the notion that Bob cannot collude with the proxy and transfer Alice’s decryption right without compromising his own decryption capability. However, over the last decade, no solutions have achieved this property. In this paper, we positively resolve this open problem. In particular, we give the first construction of nontransferable proxy re-encryption where the attacker is allowed to obtain one pair of keys consisting of Bob’s secret key and the corresponding re-encryption key. Using indistinguishability obfuscation and k-unforgeable authentication as main tools, our scheme is provably secure in the standard model. The essential idea behind our approach is to allow Bob’s secret key to be evoked in the process of decrypting Alice’s ciphertext while hiding the fact that only Bob could decrypt it by the obfuscated program. In addition, we also show a negative result: a CPA secure proxy re-encryption scheme with “error-freeness” property cannot be non-transferable

A Blockchain Framework for Patient-Centered Health Records and Exchange (HealthChain): Evaluation and Proof-of-Concept Study

Background: Blockchain has the potential to disrupt the current modes of patient data access, accumulation, contribution, exchange, and control. Using interoperability standards, smart contracts, and cryptographic identities, patients can securely exchange data with providers and regulate access. The resulting comprehensive, longitudinal medical records can significantly improve the cost and quality of patient care for individuals and populations alike. Objective: This work presents HealthChain, a novel patient-centered blockchain framework. The intent is to bolster patient engagement, data curation, and regulated dissemination of accumulated information in a secure, interoperable environment. A mixed-block blockchain is proposed to support immutable logging and redactable patient blocks. Patient data are generated and exchanged through Health Level-7 Fast Healthcare Interoperability Resources, allowing seamless transfer with compliant systems. In addition, patients receive cryptographic identities in the form of public and private key pairs. Public keys are stored in the blockchain and are suitable for securing and verifying transactions. Furthermore, the envisaged system uses proxy re-encryption (PRE) to share information through revocable, smart contracts, ensuring the preservation of privacy and confidentiality. Finally, several PRE improvements are offered to enhance performance and security. Methods: The framework was formulated to address key barriers to blockchain adoption in health care, namely, information security, interoperability, data integrity, identity validation, and scalability. It supports 16 configurations through the manipulation of 4 modes. An open-source, proof-of-concept tool was developed to evaluate the performance of the novel patient block components and system configurations. To demonstrate the utility of the proposed framework and evaluate resource consumption, extensive testing was performed on each of the 16 configurations over a variety of scenarios involving a variable number of existing and imported records. Results: The results indicate several clear high-performing, low-bandwidth configurations, although they are not the strongest cryptographically. Of the strongest models, one’s anticipated cumulative record size is shown to influence the selection. Although the most efficient algorithm is ultimately user specific, Advanced Encryption Standard–encrypted data with static keys, incremental server storage, and no additional server-side encryption are the fastest and least bandwidth intensive, whereas proxy re-encrypted data with dynamic keys, incremental server storage, and additional server-side encryption are the best performing of the strongest configurations. Conclusions: Blockchain is a potent and viable technology for patient-centered access to and exchange of health information. By integrating a structured, interoperable design with patient-accumulated and generated data shared through smart contracts into a universally accessible blockchain, HealthChain presents patients and providers with access to consistent and comprehensive medical records. Challenges addressed include data security, interoperability, block storage, and patient-administered data access, with several configurations emerging for further consideration regarding speed and security

Intelligent Computing for Big Data

Recent advances in artificial intelligence have the potential to further develop current big data research. The Special Issue on ‘Intelligent Computing for Big Data’ highlighted a number of recent studies related to the use of intelligent computing techniques in the processing of big data for text mining, autism diagnosis, behaviour recognition, and blockchain-based storage

A Parametric Family of Attack Models for Proxy Re-Encryption

Proxy Re-Encryption (PRE) is a type of Public-Key Encryption (PKE) that 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 fine-grained security notions for PRE, ranging from “plain” IND-CPA to “full” IND-CCA. We analyze some relations among these notions of security, and in particular, the separations, which further support the importance of the re-encryption oracle. The identified separations stem from the study of a new property of PRE, called privacy of re-encryption keys, which captures the requirement that re-encryption keys should not be leaked through the re-encryption function. Finally, we show that the scheme by Kirshanova (PKC 2014), which does not satisfy this property, cannot achieve a meaningful security notion for PRE since it is vulnerable to chosen-ciphertext attacks using the re-encryption oracle. This attack emphasizes the fact that PRE schemes that leak re-encryption keys cannot achieve strong security notions