In many printer and publishing applications

it is desirable to embed data in halftone images for copyright and authentication purposes.In this paper

a novel data hiding algorithm for halftone images is proposed.The digital watermarking is embedded by DCT mid-frequency coefficients comparison method.Before embedding

the digital watermarking is transformed into binary data

and then they are embedded into DCT mid-frequency subbands by changing the order of these coefficients.Experiments show that this algorithm is robust for printing and scanning

and the original picture is not needed in the recover progress of the digital watermarking.This algorithm can also be used for forgery detection in certificates.