Summary Secure and efficient enforcement of dynamic access control policies on shared data is a central problem in dynamic and scalable application scenarios, including the Internet of Things and smart cities. Key‐aggregate cryptosystem (KAC) is an efficient mechanism to delegate decryption rights of a subset of a large collection of data items to authorized users. While KAC provides an efficient solution to the problem of secure data sharing, we identify that using it to enforce continuously changing access policies is highly inefficient. In this article, we propose a novel dynamic KAC that, in addition to satisfying all key‐aggregate efficiency requirements, enforces dynamic updates to the aggregate sets with constant computation and constant communication cost. We propose a concrete construction for our dynamic KAC, prove its soundness and formal security, and analyze its theoretical and practical performance. We implement the procedures of a dynamic hierarchical key assignment scheme using the proposed as well as the existing dynamic KACs. We compare the costs of enforcing dynamic updates in large hierarchies. We conclude that the performance enhancement achieved by the proposed dynamic KAC compared to the existing KACs is even more significant in practical access control scenarios.

Hierarchical access control is for scenarios where some users have access to more organization data than others. In this paper, we propose an efficient key assignment scheme for dynamic hierarchies that features constant decryption cost and does not require any expensive operations like bilinear pairing. The proposed hierarchical key assignment scheme is secure against strong key distinguishability attacks. It also supports dynamic updates like addition and deletion of classes in the hierarchy with efficient procedures for preserving forward and backward secrecy. Another important highlight of the proposed scheme is that secret keys of none of the users have to be updated to preserve forward and backward secrecy in case of dynamic updates. Proposed is the first dynamic hierarchical key assignment scheme whose key derivation procedure has constant computation cost and is pairing-free with strong key indistinguishability.

Proxy re-encryption is a cryptographic primitive used to transform a ciphertext under one public key such that it becomes a ciphertext under another public key using a re-encryption key. Proxy invisibility is a desirable property of a proxy re-encryption scheme. This requires the ciphertexts directly intended for a recipient be indistinguishable from those re-encrypted for the same recipient. However, there may arise a situation where direct and re-encrypted ciphertexts need to be processed differently by a recipient. In such cases, it should be verifiable without decryption whether the ciphertext received is a direct or a re-encrypted ciphertext. Towards this, we design a proxy re-encryption scheme which relaxes the proxy invisibility property. With the proposed scheme, it is possible to publicly verify whether a given ciphertext is direct or re-encrypted for a particular user. This public verification does not fail even if anyone with malicious intent modifies the ciphertext.