A robust digital fingerprint of a file ensures that one able to produce the robust digital fingerprint has possession of the file. A client obtains information that is unpredictable to the client and uses that information to modify the file and generate a robust digital fingerprint from the modified file. A server, with access to the same unpredictable information, verifies the generated robust digital fingerprint. An algorithm for generating the robust digital fingerprint has a property that different representations of the same content will produce matching digital fingerprints.