report on privacy protecting off line cash available i recently published a new privacy protecting off line electronic cash system as a technical report at cwi being a phd student at david chaum s cryptography group our group has a long history in research in the field of privacy protecting cash systems the report is called cs r ps z contains pages and can be retrieved from ftp cwi nl from the directory pub cwireports aa the postscript file is suitable for dpi laserprinters abstract from coverpage we present a new off line electronic cash system based on a problem called the representation problem of which little use has been made in literature thus far our system is the first to be based entirely on discrete logarithms using the representation problem as a basic concept some techniques are introduced that enable us to construct protocols for withdrawal and payment that do not use the cut and choose methodology of earlier systems as a consequence our cash system is much more efficient in both computation and communication complexity than any such system proposed previously another important aspect of our system concerns its provability contrary to previously proposed systems its correctness can be mathematically proven to a very great extent specifically if we make one plausible assumption concerning a single hash function the ability to break the system seems to imply that one can break the diffie hellman problem our system offers a number of extensions that are hard to achieve in previously known systems in our opinion the most interesting of these is that the entire cash system including all the extensions can be incorporated in a setting based on wallets with observers which has the important advantage that double spending can be prevented in the first place rather than detecting the identity of a double spender after the fact in particular it can be incorporated even under the most stringent requirements conceivable about the privacy of the user which seems to be impossible to do with previously proposed systems another benefit of our system is that framing attempts by a bank have negligible probability of success independent of computing power by a simple mechanism from within the system which is something that previous solutions lack entirely furthermore the basic cash system can be extended to checks multi show cash and divisibility while retaining its computational efficiency cryptographers are challenged to try to break this system i made a particular effort to keep the report as self contained as possible nevertheless if you have any questions please e mail to me and i will try to reply as good as i can any comments are also welcome stefan brands cwi kruislaan sj amsterdam the netherlands tel e mail brands cwi nl
