Signature technique for electronically transmitted document

The system provides a simple method of validating a received document. The system provides a procedure for signature of an original document (Iorig) by imperceptible marking with the aid of a primary digital signature (Iinf0). The process includes a stage of pre-processing the primary digital signature (Iinf0) in order to obtain a secondary digital signature (Iinf1). The secondary digital signature (Iinf1) is inserted into the original document in order to produce a signed document (Is). The pre-processing is such that it is possible, on receiving a document (Im) which has been modified by a number of transformations, to extract the primary signature (Iinf0) from the document. By examination of this it is possible to determine whether the received document is derived from the original signed document.