Franking method and mail transport system with central postage accounting

A method for operating a mailing system, comprising:

• initializing a franking device in a processor of the franking device by (a) generating a first franking image key IDAKeyi, for which i is a first key generation number, and (b) assigning an apparatus identifier g to the franking device;
• transmitting said first franking image key and said first key generation number and said apparatus identifier g from said franking device to a data center remote therefrom;
• franking a plurality of mail pieces successively with said franking device by respectively printing successively calculated franking images on said mail pieces, with one franking image on each mail piece;
• in said processor at said franking device, calculating a first of said successively calculated franking images using said first franking image key;
• in said processor of said franking device, for each franking image in a remainder of said successively calculated franking images, incrementing i by a predetermined amount h and deriving a new franking image key IDAKeyi+h therefor, according to a first crypto-algorithm, from an immediately preceding franking image key IDAKeyi−h and calculating each franking image in said remainder using the new franking image key IDAKeyi derived therefor;
• also in said processor of said franking device, calculating an integrity check code M for each of said successively calculated franking images, using a second crypto-algorithm, based on said franking image key IDAKeyi thereof, the value i thereof, and said apparatus identifier g;
• when printing the successively calculated franking images respectively on the plurality of mail pieces, also printing, on each mail piece, said integrity check code m and the value of i for the franking image on that mail piece, and printing the apparatus identifier on each mail piece;
• transporting the mail pieces with the respective franking images printed thereon to a mail sorting center in communication with said data center and, at said mail sorting center, scanning the franking image on each mail piece and, in a further processor in communication with said data center, determining a formulated franking image key IDAKeyi, using said first crypto-algorithm, from said first franking image key transmitted to said data center and incremented by the value of i scanned from said mail piece and, in said further processor, forming a comparison integrity check code, using said second crypto-algorithm, from said formulated franking image key, said apparatus identifier g, and said key generation number i scanned from said mail piece, and comparing the comparison integrity check code with the integrity check code M scanned from said mail piece;
• upon said comparison integrity check code matching said integrity check code M scanned from said mail piece, recording a fee for mailing the mail piece, and associating said fee at said data center with said apparatus identifier g;
• and
• at an end of an accounting period, invoicing a sender associated with the franking device identified by the apparatus identifier g for all fees recorded during said accounting period.