New Encryption Scheme Locks Cloud IoT Data by Time, Rank and Revocation

Billions of connected sensors are quietly rewriting how the world stores information. From hospital wards streaming patient health records to factories running industrial IoT platforms and vehicles exchanging telemetry, the number of IoT devices is projected to reach 30.9 billion by 2025, generating torrents of data that most organizations cannot realistically keep on their own machines. The obvious answer is to push everything into the cloud, where storage is cheap and access is ubiquitous. The obvious problem is that once data leaves the device that created it, the owner loses direct control over who reads it, when they read it, and whether it is still intact.

A research team led by Shitong Bai and Jun Wu of Fudan University, together with Jiawei Zhang of Xidian University, has now unveiled a cryptographic scheme designed to restore that control. Published in the open-access journal Cybersecurity, their work describes a revocable and time-based secure file hierarchy sharing scheme for cloud-assisted IoT, built on an advanced form of Ciphertext-Policy Attribute-Based Encryption, or CP-ABE. The scheme promises to cut encryption time by more than 30 percent and decryption time by more than 50 percent compared with leading prior designs, while saving over 50 percent of ciphertext storage on the cloud server.

CP-ABE has long been the workhorse of fine-grained access control for encrypted data. Instead of sharing a decryption key with a named individual, the data owner attaches an access policy to the ciphertext itself, written in terms of attributes such as doctor, pharmacist or researcher. A user whose attributes satisfy the policy can decrypt; everyone else sees only noise. The technique was first proposed by Bethencourt, Sahai and Waters in 2007 and has since spawned a rich family of extensions addressing user privacy, revocation and large attribute universes in decentralized settings.

Yet the Fudan and Xidian team identified three persistent gaps that keep existing CP-ABE schemes from working well in real cloud-assisted IoT deployments. First, most designs treat the cloud as an unlimited resource pool and ignore the long-term cost of storing ever-growing ciphertexts. File Hierarchy CP-ABE, introduced by Wang and colleagues in 2016, partially addressed this by exploiting the hierarchical structure of shared files, but early versions suffered from a collusion problem in which users at the same hierarchy level could decrypt each other’s files, and the fix that followed came at a steep computational price in both encryption and decryption.

Second, time matters. In healthcare IoT, patient health records must be read by doctors immediately for timely diagnosis, while pharmacists and researchers legitimately need access only later, for follow-up care and monitoring. Time-based CP-ABE schemes that fuse a time factor with attributes have been proposed, notably by Hong and colleagues in 2017, but they fail to handle dynamic users who join and leave the system. Conversely, schemes with efficient direct user revocation, such as that of Han and colleagues in 2020, impose heavy computation and offer no time-based access control at all. Third, many schemes simply trust the cloud. A malicious or negligent server might hide data breaches caused by mistakes or external attacks to protect its reputation, or quietly delete rarely accessed files to save cost, violating data integrity in ways that are unacceptable for quality of service.

The new scheme tackles all three challenges simultaneously through a construction the authors call a time-release hierarchical access policy tree. Like a traditional access tree, it contains leaf nodes representing attributes and non-leaf nodes representing AND, OR and threshold gates. But it adds two novel ingredients: level nodes that bundle multiple files with their own sub-policies into a single hierarchy, and trapdoor nodes, each bound to a stateful time trapdoor. When the ciphertext is created, every trapdoor is sealed. Only when a central authority publishes a time token at the designated moment can the cloud server unseal it, exposing the missing secret share that completes the access path. Before that instant, even users with perfectly matching attributes cannot decrypt the protected file block.

Dynamic users are handled by a binary tree technique borrowed from group key management. Each user occupies a leaf of a binary tree, and the intersection of a user’s path with the minimum cover set of the revocation list determines whether that user can still derive the necessary key components. Revoking a misbehaving user is direct: the authority updates the revocation list and hands the cloud an update key, and the server rewrites the relevant ciphertext components without touching the encrypted payloads themselves. The authors show that revocation costs grow only sublinearly with the size of the revocation list, a significant improvement over schemes whose costs scale linearly with both the user attribute set and the revocation list.

Integrity is enforced through an identity-based auditing mechanism. The scheme splits each ciphertext into two parts: the encrypted message components, protected by symmetric encryption and accompanied by signatures, and the policy-related components used for access control and verification. A third-party auditor challenges the cloud server with random queries and checks a aggregated proof against a compact verification equation, so a lazy or compromised server that returns incorrect decryption results or tampers with stored data will be caught. Formal security analysis proves the scheme selectively secure against chosen-plaintext attacks in the standard model under the decisional Bilinear Diffie-Hellman assumption, and its auditing security rests on the Computational Diffie-Hellman assumption together with an existentially unforgeable signing algorithm.

The performance story is where the design becomes genuinely striking. Implemented in Java with the JPBC pairing library on a 160-bit elliptic curve, the scheme keeps encryption and decryption time nearly constant as the number of shared file blocks grows, because the hierarchical structure lets the owner encrypt an entire file hierarchy in one pass and lets the cloud perform most of the heavy decryption on behalf of users. Experiments show encryption time roughly 8 percent lower than prior file-hierarchy schemes thanks to an online/offline technique in which the owner precomputes an intermediate ciphertext pool while the device is charging, and decryption time reduced by about 30 percent relative to comparable hierarchical designs. Ciphertext storage on the cloud drops by at least 50 percent, and the time trapdoors add only about 4 percent extra storage overhead. On the user’s side, the locally stored secret key is roughly 70 percent smaller than in competing schemes, because outsourced decryption means the bulky decryption key never needs to be kept on the device.

The team also validated the design in a real-world pilot deployment, using a server to simulate the cloud, auditor and authority, and four laptops to simulate data owners and users. Across 100 repeated runs of key generation, encryption, decryption and revocation, the proposed scheme outperformed the reference FH-CP-ABE and EFH-CP-ABE schemes in encryption and decryption time, key size and communication size, and it was the only one of the three to support user revocation and data auditing at all. For the exploding population of resource-poor IoT devices, that combination of fine-grained, time-aware, revocable and auditable sharing, delivered at a fraction of the usual cost, could mark a turning point in how sensitive data is entrusted to the cloud.

Subject of Research: Revocable and time-based secure file hierarchy sharing in cloud-assisted Internet of Things using ciphertext-policy attribute-based encryption

Article Title: Revocable and time-based secure file hierarchy sharing in cloud-assisted internet of things

Article References: Revocable and time-based secure file hierarchy sharing in cloud-assisted internet of things. (n.d.). https://doi.org/10.1186/s42400-026-00660-w

Image Credits: AI Generated

DOI: 10.1186/s42400-026-00660-w

Keywords: cloud-assisted IoT, ciphertext-policy attribute-based encryption, access control, time-release encryption, user revocation, data integrity auditing, file hierarchy, data privacy, cloud computing, cryptography, outsourced decryption, cybersecurity