A variety of \key recovery," \key escrow," and \trusted third-party" encryption requirements have been suggested in recent years by government agencies seeking to conduct covert surveillance within the changing environments brought about by new technologies. This report examines the fundamental properties of these requirements and attempts to outline the technical risks, costs, and implications of deploying systems that provide government access to encryption keys. MIT Laboratory for Computer Science/Hewlett-Packard, University of Cambridge, AT&T Laboratories { Research, Microsoft Research, AT&T Laboratories { Research, Sun Microsystems, SRI International, MIT Laboratory for Computer Science, MIT Information Systems, Counterpane Systems, The latest version of this document can be found on the world-wide-web at , in PostScript format at and in ASCII text format at .