The authentication of printed documents is a nowadays challenge. One of the promising solutions for document authentication is the use of copy sensitive graphical codes that can offer data storage and support authentication. Both data decoding and physical authentication are based on comparison between printed-and-scanned samples and original numerical codes. In this paper we want to evaluate different existing correlation measures (Kendall and Spearman) and to propose a new Kendall weighted correlation metric. We propose to evaluate this new method by considering its ability to decode stored messages and to evaluate document authenticity.