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 office 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 aplying (asymmetric) degital 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, are hesitating to implement this concept.
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