Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:A private key is required in the verifier side to generate N pseudo random locations to be used for retrieving the watermark; (2) a watermark is embedded into two or more halftoning images, where the retrieval approaches can be overlays of halftone images [15] or using a look up table (LUT) to decode the watermark [12].
Category: