DEPARTMENT OF MATHEMATICS AND COMPUTER SCIENCE                
                               ODENSE UNIVERSITY                               

                       Ripping Coins for a Fair Exchange                       

                                Markus Jakobsson                               
                 Department of Computer Science and Engineering                
                     University of California at San Diego                     

                       Tuesday, May 30, 1995, at 2:15 PM                       
                                The Seminar Room                               


  A fair exchange of payments for goods and  services is a barter where one of
  the parties cannot obtain the item desired  without handing over the item he
  offered. We introduce the concept of ripping digital coins to solve fairness
  problems in  payment transactions.  We  demonstrate  how to  implement  coin
  ripping for  a recently  proposed  payment scheme,  giving  a practical  and
  transparent coin ripping scheme. We then give a general solution that can be
  used in any payment scheme with a  challenge.  We also indicate how fairness
  can be obtained by building a contract into the coin.

  During the  first  part of  the  talk, basic  digital  cash issues  will  be
  presented along with  the cryptographic  primitives to  be used;  the second
  part will consist of the above mentioned coin ripping issues.


                                                                 Joan F. Boyar