Secret image sharing allows a secret image to be divided among multiple users, enabling reconstruction by collecting a predetermined number of images. One key challenge is minimising the size of these shadow images. This study introduces a new technique that employs downsampling and a public difference image to address this issue. By using average pooling for downsampling, we reduce the hidden image's size while preserving essential visual details. A public difference image is created by calculating the difference between the downsampled and permuted image, enabling recovery of the original image even if many shadow images are lost. Our method allows for adjustable pooling sizes, which control downsampling and maintain vital visual information. This reduction in size enhances the efficiency of the image-sharing system, making it more suitable for transmission and storage. Extensive evaluations across various scenarios confirm the approach's effectiveness, flexibility, and advantages over existing methods for secure image transmission and storage.

Summary A vast body of literature is filled with many key management schemes constructed using different cryptographic primitives. They aim toward either security goals or improvement in performance efficiency. However, the key management schemes based on proxy re‐encryption suffer from massive communication and computational costs. We propose an ElGamal proxy re‐encryption‐based construction for the key management scheme to resolve this. The proposed scheme involves constant computational and communication costs in rekeying operations and linear public bulletin size. We achieve essential security requirements of forward and backward secrecy in this scheme. The scheme is also secure against the collusion attack. In addition, we address the importance of adequately managing missed rekeying updates for offline users. The existing schemes trivially keep track of each rekeying message on the public bulletin board, which increases the size of the public bulletin with every join/leave operation. However, the proposed scheme uses the public bulletin board so that the handling of offline users is built into the scheme itself, which manages it efficiently. Also, the public bulletin size in the proposed scheme is not dependent on the number of rekeying operations but linear in the number of users.

In the literature, secure group key management schemes have focused on either rekeying cost or security requirements, i.e., forward and backward secrecy. There is little or no work that adds new features to the secure group key management scheme. In the existing key management schemes, any user can join or leave the group any number of times during the group's lifetime. There is a need to restrict the number of times a user joins the group during the group's lifetime. We propose a user-driven general framework wherein a cap is enforced on the number of times a user can join the group. Any existing key management scheme can use this framework. In the proposed scheme, the user is prevented from joining a group, say, more than t > 0 times. We analyse the scheme and show that the proposed scheme indeed caps the number of times a user can join the group.

Proxy re-encryption is used to delegate the task of providing access control to the outsourced data on a cloud storage server. However, the straightforward application of proxy re-encryption requires the cloud storage server to be trusted. The cloud storage servers are however, often, honest-but-curious or untrusted. Towards solving the problem of access control enforcement delegation of outsourced data, we design a quorum controlled proxy re-encryption scheme. We show that, using the proposed scheme, task of enforcing access control is delegated to a set of proxies, such that a quorum of proxies should come together to enforce access control. By distributing trust among multiple proxies, single point of trust is eliminated, and the system is made fault tolerant. We prove the IND-CPA security of the proposed scheme under the DBDHI assumption and show that it satisfies most of the desirable properties of a proxy re-encryption scheme thus outperforming the existing schemes.

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.

Abstract In group‐oriented encryption, a sender encrypts a message and sends it to a set of users, which form a group. Encryption is carried out using the group's public key. Only the legitimate group users are capable of decrypting the ciphertext using their individual private keys. Existing literature in group‐oriented encryption schemes considers only static groups in secure group communication. Extension of the existing schemes to support dynamic groups results in the one‐affects‐all problem. We propose a group‐oriented encryption scheme which is capable of handling dynamic groups in secure group communication. In the proposed scheme, we consider groups which are dynamic in nature and involve joining and leaving of members thereby giving rise to the problem of forward and backward secrecy for which group public key needs to be changed. In the proposed scheme, updating the group public key does not affect the group users, and they are not required to update any of their secret key components. The group members can continue their operations with the same secret keys which they are possessing since the time they joined the group. Also, size of the secret key at users, the public key and the ciphertext, remains constant. Copyright © 2016 John Wiley & Sons, Ltd.

ABSTRACT Secure group communication model poses the challenge of key management to enforce the access control among the users of the group. Secure group key management schemes should be designed such that there is less rekeying and storage cost involved to manage the group keys. We propose a centralized, scalable, share‐based secure group key management scheme employing a new approach on the basis of the polynomial interpolation technique. We prove the security of the scheme. We design a multilayer share‐based key management scheme to manage the keys in the multilayer communication model. A new aggregate tree structure is proposed to manage the keys in the multilayer communication model. We analyze and evaluate the performance of the designed schemes in detail for storage and rekeying cost. We show that the proposed share‐based group key management scheme and the multilayer key management schemes are efficient in terms of storage and rekeying cost in comparison with two schemes based on threshold secret‐sharing method available in the literature. Copyright © 2012 John Wiley & Sons, Ltd.