A method of generating a digital signature at a postage meter, in which method a postal imprint is printed on a mail item once said mail item has been weighed and once its postage amount has been determined, said postal imprint being associated with a digital signature generated by means of a secret key on the basis of constant data and variable data, said variable data comprising only a fingerprint code extracted upstream of the weighing from a three-dimensional structure of the fibers of the mail item.