1. An encoder for encoding a covertext signal to generate a stegotext, the encoder comprising: first transformation means for carrying out a Fast Fourier Transform and rectangular polar conversion of the covertext signal so as to transform the covertext signal into a log power spectrogram; means for providing at least one key, the or each key being in the form of a two-dimensional pattern of predetermined size; a multiplier for adding or subtracting in the log power spectrogram domain multiples of the key or multiples of one or more of the keys if there is a plurality of keys, to blocks of the transformed covertext signal; means for controlling the addition or subtraction of the key or keys by the multiplier in accordance with data representing a desired code; and second transformation means for carrying out polar rectangular conversion and inverse Fast Fourier Transformation of the modulated covertext signal to generate a stegotext.