To deal with the increasingly serious situation of document's security and better protect the controlled documents, in this paper, an identity-based On-The-Fly Encryption (OTFE) and decryption scheme was proposed for the controlled documents, which combined an Identity-Based Encryption (IBE) algorithm with an on-the-fly encryption technique. In the scheme, file system filter driver technology was used to monitor program's behaviors on the controlled documents; meanwhile, the IBE algorithm was used to encrypt and decrypt the controlled documents. Specifically, a new algorithm that associated the original ciphertext and divided the associated ciphertext into two parts stored in different locations was proposed. Therefore, it is impossible for an adversary to obtain the whole ciphertext and further recover the original plaintext. Finally, an elaborate description was made on the scheme from system level and algorithm level. The security analysis indicates that the proposed scheme is able to effectively protect the controlled documents.

Structured document plays a vital role in information carrier for information interaction and propagation in cloud computing. However, the existing research works lack structured document model that can describe this information carrier. In order to tackle this issue, the authors formalized the definitions of document element, atomic document element, connection, document fragment, and complex document element based on the in-depth analysis of the special characteristics of structured document in cloud computing. Furthermore, the authors proposed structured document model (referred to as the SDoc) satisfying the special characteristics in cloud computing. Finally, several open issues were indicated for structured document.