1. A method of classifying computer documents into confidential levels, the method comprising: clustering files of documents created by users into different groups of files based on characteristics of the files; generating a prediction confidential score for each group of files of each user; consulting a file transaction log to create connection graphs identifying users who sent or received files to or from other users; identifying a first unknown user having no associated confidential level downloading a first file; in response to identifying the first unknown user, determining a confidential level of the first file without analyzing a content of the first file by using a first prediction confidential score of a first group of files of a second user who uploaded the first file, the first group of files of the second user and the first file having a same file characteristic; identifying a third unknown user having no associated confidential level who uploaded a second file; and in response to identifying the third unknown user, determining a confidential level of the second file without analyzing a content of the second file by using a second prediction confidential score of a second group of files of a fourth user who downloaded the second file, the second group of files of the fourth user and the second file having same file characteristic.