CloudKeyBank: Privacy and Owner Authorization Enforced Key Management Framework

In this paper, an identity-based conditional proxy re-encryption (PRE) scheme is proposed, where a delegator provides a re-encryption key satisfying one condition to a semi-trusted proxy who can convert a ciphertext encrypted under the delegator’s public key into one that can be decrypted using the delegatee’s private key. We address the identity-based proxy re-encryption scheme, where the delegator and the delegatee request keys from a trusted party known as a key generator center (KGC), who generates private keys for delegator and delegatee based on their identities. Meanwhile, the identity-based conditional proxy re-encryption scheme satisfies the properties of PRE including unidirectionality, non-interactivity and multi-hop. Additionally, the identity-based conditional proxy re-encryption scheme is efficient in terms of both the communication cost and the computing cost, and can realize security secret sharing in cloud computing environments.

Proxy re-encryption (PRE) allows a proxy to transform one ciphertext to another under different encryption keys while keeping the underlying plaintext secret. Because of the ciphertext transformability of PRE, there are many potential private communicating applications of this feature. However, existing PRE schemes are not as full-fledged as expected. The lack of necessary features makes them hard to apply in real-world scenarios. So far, there does not exist a unidirectional multihop PRE scheme with constant decryption efficiency and constant ciphertext size without extensions. Impractical performance and weak scalability also hinder PRE from most real-world applications. In this work, we present a new PRE scheme with secure hardware enclave named <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hPRESS</monospace> (hardware-enhanced PRE scheme using secure enclave). To the best of our knowledge, <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hPRESS</monospace> is the first unidirectional multihop PRE scheme which achieves both constant decryption efficiency and constant ciphertext size without extensions. A detailed security analysis demonstrates that our proposal is CCA secure based on the security of the underlying encryption schemes and the secure enclave. We also implement a prototype based on Intel SGX, one of the most popular secure enclave techniques in recent years, and evaluate its performance. The experimental results show that, compared with previous PRE schemes, our <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hPRESS</monospace> is almost one order of magnitude faster in terms of the decryption and transformation.

In a proxy re-encryption (PRE) scheme, a semi-trusted proxy is given special information that allows it to transform cipher texts encrypted under Alice's public key into the different cipher texts that can be decrypted by Bob's secret key. In 2006, Green and Ateniese proposed identity-based proxy re-encryption schemes, in which cipher texts are transformed from one identity to another. Based on this, we propose a new construction for certificate less proxy re-encryption scheme and the corresponding security model in this paper. The scheme is proved secure against chosen-plaintext attack in the random oracle model assuming that an underlying problem related to the Decisional Bilinear Diffie-Hellman problem is computationally hard.

Outsourcing heavy storage and computation to the cloud servers now becomes more and more popular. How to secure share the cloud storage is an important problem for many mobile users. Proxy re-encryption is such a cryptographic primitive which can be used to secure share cloud data. Until now there are many kinds of proxy re-encryption schemes with various properties, such as conditional proxy re-encryption, proxy re-encryption with keyword search etc. However until now there exists no work focus on proxy re-encryption for mobile cloud storage sharing. In mobile cloud storage, almost all the users are mobile ones, they only have resource-restricted equipments. In this paper we try to initialize this research, we give a very basic outsourced online/offline proxy re-encryption scheme for mobile cloud storage sharing and leave many interesting open problems as the future work.

Outsourcing private data and heavy computation tasks to the cloud may lead to privacy breach as attackers (e.g., malicious outsiders or cloud administrators) may correlate any relevant information to penetrate information of their interests. Therefore, how to preserve cloud users' privacy has been a top concern when adopting cloud solutions. In this paper, we investigate the identity privacy problem for the proxy re-encryption, which allows any third party (e.g., cloud) to re-encrypt ciphertexts in order to delegate the decryption right from one to another user. The relevant identity information, e.g., whose ciphertext was re-encrypted to the ciphertext under whose public key, may leak because re-encryption keys and ciphertexts (before and after re-encryption) are known to the third party. We review prior anonymity (identity privacy) notions, and find that these notions are either impractical or too weak. To address this problem thoroughly, we rigorously define the anonymity notion that not only embraces the prior anonymity notions but also captures the necessary anonymity requirement for practical applications. In addition, we propose a new and efficient proxy re-encryption scheme. The scheme satisfies the proposed anonymity notion under the Squared Decisional Bilinear Diffie-Hellman assumption and achieves security against chosen ciphertext attack under the Decisional Bilinear Diffie-Hellman assumption in the random oracle model. To the best of our knowledge, it is the first proxy re-encryption scheme attaining both chosen-ciphertext security and anonymity simultaneously.We implement a prototype based on the proposed proxy re-encryption scheme and the performance study shows that it is efficient.

Multi-use and unidirectional identity-based proxy re-encryption schemes

Storage is a promising application for permission-less blockchains. Before blockchain, cloud storage was hosted by a trusted service provider. The centralized system controls the permission of the data access. In web3, users own their data. Data must be encrypted in a permission-less decentralized storage network, and the permission control should be pure cryptographic. Proxy re-encryption (PRE) is ideal for cryptographic access control, which allows a proxy to transfer Alice’s ciphertext to Bob with Alice’s authorization. The encrypted data are stored in several copies for redundancy in a permission-less decentralized storage network. The redundancy suffers from the outsourcing attack. The malicious resource provider may fetch the content from others and respond to the verifiers. This harms data integrity security. Thus, proof-of-replication (PoRep) must be applied to convince the user that the storage provider is using dedicated storage. PoRep is an expensive operation that encodes the original content into a replication. Existing PRE schemes cannot satisfy PoRep, as the cryptographic permission granting generates an extra ciphertext. A new ciphertext would result in several expensive replication operations. We searched most of the PRE schemes for the combination of the cryptographic methods to avoid transforming the ciphertext. Therefore, we propose a new PRE scheme. The proposed scheme does not require the proxy to transfer the ciphertext into a new one. It reduces the computation and operation time when allowing a new user to access a file. Furthermore, the PRE scheme is CCA (chosen-ciphertext attack) security and only needs one key pair.

Users share a group key to decrypt encryptions for the group using a group key management scheme. In this paper, we propose a re-encryption-based group key management scheme, which uses a unidirectional proxy re-encryption scheme with special properties to enable group members share the updated group key with minimum storage and computation overhead. In particular, we propose a proxy re-encryption scheme that supports direct re-encryption key derivation using intermediate re-encryption keys. Unlike multi-hop re-encryption, the proposed proxy re-encryption scheme does not involve repeated re-encryption of the message. All the computations are done on the re-encryption key level and only one re-encryption is sufficient for making the group key available to the users. The proposed scheme is the first for group key management based on proxy re-encryption that is secure against collusion. The individual users store just one individual secret key with group key derivation requiring O\((\log N)\) computation steps for a group of N users. Size of the public bulletin maintained to facilitate access to the most recent group key for off-line members is O(N) and remains constant with respect to the number of group updates. The proposed group key management scheme confronts attacks by a non-member and even a collusion attack under standard cryptographic assumptions.

Proxy re-encryption is an efficient solution to ciphertext delegation and distribution, which also enables the sender to carry out fine-grained control on his ciphertext. This paper summarizes the progress on the proxy re-encryption schemes and their practical applications. Universal models for proxy re-encryption and its security are also induced for detailed investigation on different types of PRE schemes.

Recently, blockchain technology has attracted much attention of the research community in several domains requiring transparency of data accountability, due to the removal of intermediate trust assumptions from third parties. One such application is enabling file sharing in blockchain enabled distributed cloud storage. Proxy re-encryption is a cryptographic primitive that allows such file sharing by re-encrypting ciphertexts towards legitimate users via semi-trusted proxies, without them learning any information about the underlying message. To facilitate secure data sharing in the distributed cloud, it is essential to construct efficient proxy re-encryption protocols. In this paper, we introduce the notion of proxy self re-encryption (SE-PRE) that is highly efficient, as compared to the existing PRE schemes in the literature. We show that our self encryption scheme is provably CCA secure based on the DLP assumption and our proxy re-encryption scheme with self encryption is CCA secure under the hardness of the Computational Diffie Hellman (CDH) and Discrete Logarithm (DLP) assumption. Our novel encryption scheme, called self encryption, has no exponentiation or costly pairing operation. Even the re-encryption in SE-PRE does not have such operations and this facilitates the service provider with efficiency gain.

Proxy re-encryption is a cryptographic primitive proposed by Blaze, Bleumer and Strauss in 1998. It allows a user, Alice, to decide that in case of unavailability, one (or several) particular user, the delegatee, Bob, will be able to read her confidential messages. This is made possible thanks to a semi-trusted third party, the proxy, which is given by Alice a re-encryption key, computed with Alice’s secret key and Bob’s public key. This information allows the proxy to transform a ciphertext intended to Alice into a ciphertext intended to Bob. Very few constructions of proxy re-encryption scheme actually handle the concern that the original sender may not want his message to be read by Bob instead of Alice. In this article, we adapt the primitive of proxy re-encryption to allow a sender to choose who among the potential delegatees will be able to decrypt his messages, and propose a simple and efficient scheme which is secure under chosen plaintext attack under standard algorithmic assumptions in a bilinear setting. We also add to our scheme a traceability of the proxy so that Alice can detect if it has leaked some re-encryption keys.

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 Stehle 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.

A proxy re-encryption (PRE) scheme involves three parties: Alice, Bob, and a proxy. PRE allows the proxy to translate a ciphertext encrypted under Alice's public key into one that can be decrypted by Bob's secret key. We present a general method to construct an identity-based proxy re-encryption scheme from an existing identity-based encryption scheme. The transformed scheme satisfies the properties of PRE, such as unidirectionality, non-interactivity and multi-use. Moreover, the proposed scheme has master key security, allows the encryptor to decide whether the ciphertext can be re-encrypted.

Multi-authority proxy re-encryption based on CPABE for cloud storage systems

Due to the limited computational capability of mobile devices, the research organization and academia are working on computationally secure schemes that have capability for offloading the computational intensive data access operations on the cloud/trusted entity for execution. Most of the existing security schemes, such as proxy re-encryption, manager-based re-encryption, and cloud-based re-encryption, are based on El-Gamal cryptosystem for offloading the computational intensive data access operation on the cloud/trusted entity. However, the resource hungry pairing-based cryptographic operations, such as encryption and decryption, are executed using the limited computational power of mobile device. Similarly, if the data owner wants to modify the encrypted file uploaded on the cloud storage, after modification the data owner must encrypt and upload the entire file on the cloud storage without considering the altered portion(s) of the file. In this paper, we have proposed an incremental version of proxy re-encryption scheme for improving the file modification operation and compared with the original version of the proxy re-encryption scheme on the basis of turnaround time, energy consumption, CPU utilization, and memory consumption while executing the security operations on mobile device. The incremental version of proxy re-encryption scheme shows significant improvement in results while performing file modification operations using limited processing capability of mobile devices.