1. An authentication method for verifying authenticity of a target authentication device, wherein the target authentication device comprises a reference pattern and a security pattern, and each one of the reference pattern and the security pattern comprises pattern defining elements which are distributed according to a characteristic frequency, and the security pattern comprises an embedded security device that is coded with predetermined relative phase modulation information of the characteristic frequency with respect to phase of the reference pattern; and wherein the method comprises: obtaining frequency domain data from spatial domain pixel data of an image of the target authentication device; digitally filtering the characteristic frequency; and extracting said predetermined relative phase modulation information comprising information on relative phase shift between the security pattern and the reference pattern to construct characteristic features of the security device for verification of authenticity of the target authentication device.