Attribute-based signature (ABS) enables users to sign messages over attributes without revealing any information other than the fact that they have attested to the messages. However, heavy computational cost is required during signing in existing work of ABS, which grows linearly with the size of the predicate formula. As a result, this presents a significant challenge for resource-constrained devices (such as mobile devices or RFID tags) to perform such heavy computations independently. Aiming at tackling the challenge above, we first propose and formalize a new paradigm called Outsourced ABS, i.e., OABS, in which the computational overhead at user side is greatly reduced through outsourcing intensive computations to an untrusted signing-cloud service provider (S-CSP). Furthermore, we apply this novel paradigm to existing ABS schemes to reduce the complexity. As a result, we present two concrete OABS schemes: i) in the first OABS scheme, the number of exponentiations involving in signing is reduced from $O(d)$ to $O(1)$ (nearly three), where $d$ is the upper bound of threshold value defined in the predicate; ii) our second scheme is built on Herranz et al.’s construction with constant-size signatures. The number of exponentiations in signing is reduced from $O(d^2)$ to $O(d)$ and the communication overhead is $O(1)$ . Security analysis demonstrates that both OABS schemes are secure in terms of the unforgeability and attribute-signer privacy definitions specified in the proposed security model. Finally, to allow for high efficiency and flexibility, we discuss extensions of OABS and show how to achieve accountability as well.

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Secure Outsourced Attribute-Based Signatures

The concept of functional encryption (FE) has been introduced to address the shortcomings of public-key encryption (PKE) in many emerging applications which require both data storage and data sharing (e.g., cloud storage service). One of the major issues existing in most FE schemes is the efficiency, as they are built from bilinear pairings of which the computation is very expensive. A widely accepted solution to this problem is outsourcing the heavy workloads to a powerful third party and leaving the user with the light computation. Nevertheless, it is impractical to assume that the third party (e.g., the cloud) will provide free services. To our knowledge, no attention has been paid to the payment procedure between the user and the third party in an FE with outsourced decryption (FEOD) scheme under the assumption that neither of them should be trusted. Leveraging the transactions on cryptocurrencies supported by the blockchain technology, in this paper, we aim to design FE with payable outsourced decryption (FEPOD) schemes. The payment in an FEPOD scheme is achieved through a blockchain-based cryptocurrency, which enables the user to pay a third party when it correctly completes the outsourced decryption. We define the adversarial model for FEPOD schemes, and then present a generic construction of FEPOD schemes. Also, we evaluate the performance of the proposed generic construction by implementing a concrete FEPOD scheme over a blockchain platform.

Pay as You Decrypt: Decryption Outsourcing for Functional Encryption Using Blockchain

Cyber-physical systems CPS are a key component in industrial control systems ICS, which are widely used in the critical infrastructure sectors. The increasing reliance on CPS, however, affords exploitative opportunities for malicious actors targeting our critical infrastructure. The real-time requirement of control systems, coupled with the deployment of resource-constrained field devices, complicate efforts to secure our critical infrastructure. A key technical limitation for security solutions is that they should be lightweight. While lightweight cryptography is useful to some extent, enforcement of asymmetric key cryptographic primitives in control systems is known to be problematic. In this paper, we suggest investigating the enforcement of lightweight security solutions in ICS from a different perspective. Rather than focusing on designing lightweight individual cryptographic primitives, we propose taking a whole-of-system approach to 1 achieve system/collective lightweightness, 2 outsource expensive computations from resource-constrained field devices to neighboring devices and equipments that have more computational capacity, and 3 selectively protect critical data partial/selective protection of Data of Interest.

On Lightweight Security Enforcement in Cyber-Physical Systems

A broadcast encryption scheme enables a broadcaster to send an encrypted message to the multiple privileged users. In identity-based broadcast encryption schemes, a broadcaster typically encrypts a message by combining public identities of receivers and system parameters, which therefore reveals identities of receivers to the public and raised a concern on user privacy. On the other hand, identity-based systems generally require pairing computations which are computationally costly. It would be desirable if pairing computations could be avoided in resource constrained devices. The aim of this paper is to provide the solutions to these problems by introducing a new concept called server-aided identity-based anonymous broadcast encryption. We formalise the notion of server-aided identity-based anonymous broadcast encryption and present a construction in the random oracle model based on pairings that achieve anonymity under adaptive corruptions in the chosen-plaintext setting. Comparing with other broadcast encryption schemes, our scheme requires less computation cost for encryption and decryption while preserving receiver's privacy.

Server-aided identity-based anonymous broadcast encryption

For practical data sharing applications, many attributebased encryption (ABE) schemes were proposed with different kinds of properties, such as supporting large universe, revocation, verification and so on. However, existing schemes seldom support these three important properties simultaneously. In this paper, we present a directly revocable and verifiable key-policy ABE scheme for large universe (DRV-KP-ABE). The new scheme supports large universe, and attributes do not need to be enumerated at stage of setup. Meanwhile, our scheme allows the trusted authority to revoke users by only updating the revocation list without interaction with non-revoked users. We use the subset difference method for revocation which greatly improves the broadcast efficiency compared with the complete subtree scheme. In addition, the proposed scheme enables the third party to update ciphertexts with public information, and the auditor assures the third party updated ciphertexts correctly. The DRV-KP-ABE scheme is selectively secure under q-type assumption in the standard model.

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Directly Revocable and Verifiable Key-Policy Attribute-based Encryption for Large Universe.

Identity-Based Cryptosystem plays an important role in the modern cryptography world, due to the elimination of the costly certificate. However, all practical identity-based encryption schemes require pairing operation in the decryption stage. Pairing is a heavy mathematical algorithm, especially for resource-constrained devices such as smart cards or wireless sensors. In other words, decryption can hardly be done in these devices if identity-based cryptosystem is employed.We solve this problem by proposing a new notion called Identity-Based Server-Aided Decryption. It is similar to normal identity-based encryption scheme, but it further enables the receiver to decrypt the ciphertext without needing to compute pairing with the assistance of an external server. Secure mechanisms are provided to detect whether the server has computed correctly and prevent the server from getting any information about the plaintext or the user secret key. We give two concrete instantiations of this notion.

Cheng Kang Chu

Papers

Identity-based server-aided decryption