1. A watermark coding method for hiding a digital watermark into a digital image to generate a robust protection, the method comprising the steps of: determining a first pseudo random code PN 1 and using this PN code to perform a watermark series, Wj, conversion to generate a permuted watermark series Wi′; converting an original image Vm to generate a frequency domain of a modified original image Vm″; using the frequency domain of the modified original image Vm″ and the permuted watermark series Wi′ to generate an encrypted watermark series Wi″; modulating the encrypted watermark series Wi″ to generate a modulated watermark series Wi′″; hiding the modulated watermark series Wi′″ in the frequency of the modified original image Vm″ by a frequency hopping rule; and converting the frequency domain of the modified original image back to its spatial domain to obtain the watermarked image Vm′″.