No matter what the format may be, it is very easy to modify the content of a digital file in your favour without leaving any trace and without showing who it was modified by. On the flip side, it is impossible to attach a reliable identity on an electronic document and do so easily without a specific tool. Therefore, cryptology needs to be used, which from its etymological root means the "science of secrecy". It encompasses cryptography - secret writing - and cryptanalysis - the analysis of the latter. The electronic signature, as well as all mechanisms managing electronic evidence rest entirely on this science.