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With the rapid development of cloud computing, an increasing number of individuals and organizations are sharing data in the public cloud. To protect the privacy of data stored in the cloud, a data owner usually encrypts his data in such a way that certain designated data users can decrypt the data. This raises a serious problem when the encrypted data needs to be shared to more people beyond those initially designated by the data owner. To address this problem, we introduce and formalize an identity-based encryption transformation (IBET) model by seamlessly integrating two well-established encryption mechanisms, namely identity-based encryption (IBE) and identity-based broadcast encryption (IBBE). In IBET, data users are identified and authorized for data access based on their recognizable identities, which avoids complicated certificate management in usual secure distributed systems. More importantly, IBET provides a transformation mechanism that converts an IBE ciphertext into an IBBE ciphertext so that a new group of users not specified during the IBE encryption can access the underlying data. We design a concrete IBET scheme based on bilinear groups and prove its security against powerful attacks. Thorough theoretical and experimental analyses demonstrate the high efficiency and practicability of the proposed scheme.

Many data driven networks have been developed so that data can be collected and shared for bettering our life. Various cryptosystems are presented to address security and privacy issues in such emerging networks. Observing that implementing various cryptographic schemes separately in one system leads to undesirable heavy development and management overheads, we create a versatile cryptographic toolkit from Ciphertext-Policy Attribute-Based Encryption (CP-ABE) systems. Our toolkit supports attributes, indices, and identities based access control. We achieve this goal by converting some CP-ABE schemes into conventional Broadcast Encryption (BE) and Identity-Based Broadcast Encryption (IBBE) schemes. By configuring the system parameters, one can further obtain conventional Public Key Encryption and Identity-Based Encryption systems.