1. A method for signing a primary information file, comprising the steps of: defining a secret datum which is specific to an identity of at least one signer, and a portable object having memory and processing circuits, said portable object being dedicated to and at the disposal of said at least of signer wherein said memory has a secret zone used for storing said secret datum; selecting, in said primary information file, an arbitrary file portion on which the at least one signer intends to calculate a signature; calculating the signature by using an information processing device incorporating a signature algorithm and carrying out a calculation with said algorithm based on said secret datum from said portable object and said arbitrary file portion, wherein a result of said calculation defines said signature; using a secondary file for storing file identification information adapted for identifying, in said primary information file, said arbitrary file portion used to calculate said signature; and linking said primary information file, said signature and said secondary file.