The present invention allows the user (author or creator) of a document to specify that certain portions of a document be selected for encryption while other portions of the document remain displayed as created. In addition, each encrypted section could have multiple encryption keys such that some viewers...http://www.google.com.hk/patents/US20050235163?utm_source=gb-gplus-share專利 US20050235163 - Method for selective encryption within documents
