一些協助遠距合作的編輯器 (Web-Based Collaborative Editors) 提供良好的環境讓參與任務的使用者可以緊密地合作 (Tightly-Coupled Collaboration)。此外這些編輯器的使用者還可以參與多個任務。也由於這些編輯器被廣泛的使用，對於想要竊取別人合作的內容的攻擊者而言，這些協助遠距合作的編輯器就是明顯的攻擊目標。而且一旦攻擊者在這些編輯器上找到別人合作的內容後，他們就有機會能夠繼續取得之後的合作內容。
為了增加合作內容的隱密性 (Confidentiality)，本論文提出了一個具備租約機制的超儲存空間 (Lease-Based Meta-Storage)。這個具備租約機制的超儲存空間從兩個方面來增加合作內容的隱密性。一方面，由於合作的內容會伴隨著租約 (Lease) 一起被送出，當合作內容的租約期限到了以後，具備租約的超儲存空間會把這些合作的內容移除以降低這些合作內容被攻擊者發現的風險。另一方面，合作內容的相關的資訊 (Context Information) 被隱藏在租約中要租借超儲存空間的虛擬地址上，而不同合作內容的虛擬地址是不一樣的。如果不知道這些虛擬地址以及它們的順序，攻擊者就無法將合作的內容還原。此外由於具備租約機制的超儲存空間還允許空間的擁有者出租他們的空間且合作內容的實際存放位置 (Physical Location) 會隨著空間的被出租情形而有所差異，即使是超儲存空間的管理者 (Administrator) 也很難追蹤合作內容的實際位置，更遑論攻擊者要從已發現的合作內容中推論出其它的合作內容的實際位置了。

Although Web-based collaborative editors are suitable for tightly-coupled collaboration and can support users with multiple tasks, data stored in servers of the editors can be disclosed because the editors are famous and unhidden targets. Lease-Based Meta-Storage is proposed to enhance confidentiality by exploiting the notion of lease. Lease on collaborative data contains the term that collaborative data can exist in Lease-Based Meta-Storage and the meta-block number which is the virtual address of storage in Lease-Based Meta-Storage. Lease-Based Meta-Storage also allows storage owners to lease their storages. The probability that attackers can discover collaborative data is reduced because attackers cannot discover the collaborative data once its term expires. The damage that attackers discover part of collaborative is reduced for two reasons. One is the fact that the context information is encapsulated in meta-block number. Only with the correct sequence of meta-blocks the collaborative data can be recovered. The other is that the physical location of collaborative data is not static and depends on the storages leased to Lease-Based Meta-Storage. Finally, prototype of Collaborative System with Lease-Based Meta-Storage (Calm) is introduced after considering the future changes.

[1] D. E. Comer, Computer Networks and Internets. Prentice-Hall, Inc., 1997.
[2] K. Kourai and S. Chiba, "A Secure Access Control Mechanism against Internet Crackers," in Proceedings of the 21st IEEE International Conference on Distributed Computing Systems, Apr. 2001.
[3] OneNote, http://office.microsoft.com/en-us/onenote/FX100487701033.aspx, 2008.
[4] Google Docs, http://docs.google.com, 2008.
[5] Zoho Writer, http://writer.zoho.com, 2008.
[6] UCLA alerts 800,000 to data breach, http://www.securityfocus.com/news/11429, 2008.
[7] TJX Settles Visa Suit over Data Breach, http://www.consumeraffairs.com/news04/2007/11/tjx_visa.html, 2008.
[8] R. Anderson, R. Needham, and A. Shamir, "The Steganographic File System," Proc. Information Hiding, Second Int'l Workshop, Apr. 1998.
[9] A. McDonald and M. Kuhn, "Stegfs: A Steganographic File System for Linux," Proc. Information Hiding, Third Int'l Workshop, Sept. 1999.
[10] H. Pang, K.-L. Tan, and X. Zhou, "Steganographic Schemes for File System and B-Tree," IEEE Transactions on Knowledge and Data Engineering, vol. 16, no. 6, pp. 701-713, June 2004.
[11] C. Gray and D. Cheriton, Leases: An Efficient Fault-Tolerant Mechanism for Distributed File Cache Consistency," Proceedings of the 12th ACM Symposium on Operating Systems Principles, pp. 202-210, 1989.
[12] Jini Network Technology, http://www.sun.com/software/jini, 2008.
[13] The Free Online Dictionary, http://www.thefreedictionary.com, 2008.
[14] Gmail, http://mail.google.com, 2008.
[15] Advanced Encryption Standard. National Institute of Science and Technology, FIPS 197, 2001.
[16] PyCrypto, http://sourceforge.net/projects/pycrypto, 2008.
[17] libgmail X Python binding for Google's Gmail service, http://libgmail.sourceforge.net, 2008.