“…In [10,45], the authors consider the role of passive steganalysis is only to detect the presence or absence of a hidden message, whereas the active steganalysis objective is to extract, destroy or alter it. The same definition is also adopted by Wu in [46]. The estimation of the hidden message 'length' can be sometimes part of the passive approach like in [45], or part of the active approach as stated by Trivedi in [47].…”