Compressed accessibility map: Efficient access control for XML. Ting Yu, Divesh Srivastava, Laks V. S. Lakshmanan and H. V. Jagadish. XML is widely regarded as a promising means for data representation integration, and exchange. As companies transact business over the Internet, the sensitive nature of the information mandates that access must be provided selectively, using sophisticated access control specifications. Using the specification directly to determine if a user has access to a specific XML data item can hence be extremely inefficient. The alternative of fully materializing, for each data item, the users authorized to access it can be space-inefficient. In this paper, we propose a space- and time-efficient solution to the access control problem for XML data. Our solution is based on a novel notion of a compressed accessibility map (CAM), which compactly identifies the XML data items to which a user has access, by exploiting structural locality of accessibility in tree-structured data. We present a CAM lookup algorithm for determining if a user has access to a data item; it takes time proportional to the product of the depth of the item in the XML data and logarithm of the CAM size. We develop a linear-time algorithm for building an optimal size CAM. Finally, we experimentally demonstrate the effectiveness of the CAM for multiple users on both real and synthetic data sets.