Secure and Cost Eecient Electronic Stamps

Even small companies do not use physical stamps to prepay postal services, but have a franking machine which can be logically loaded with a certain amount of stamps which are printed on letters for example. Special purpose franking machines, which are still widely used in practice, however have security and handling problems. It was relatively easy to forge stamps and in some cases one needed to bring the franking machine to the post o ce to get it loaded. Therefore the USPS (US Postal Service) initiated an information based indicia program (IBIP) [1] to provide "electronic stamps". While the two problems mentioned above were solved by applying (asymmetric) digital signatures and a special purpose hardware device on the client side, it seems that this approach introduces an unreasonable overhead. In fact the problem of verifying the huge number of stamps / signatures in reasonable time is not adressed in [1] at all. Therefore many international postal service providers, like the German Post AG for example, will most likely not implement this concept. In this short note we will introduce an alternative approach using symmetric algorithms and general purpose smart cards to provide electronic stamps. Thus it will be less expensive to implement this concept. Furthermore our approach will allow an e cient veri cation of all stamps in a very short time because one does not need to contact a certi cate directory. Note that in this special scenario it is no problem at all that only the postal service provider is able to verify the "symmetric signatures" and hence the authenticity of the stamps.