Linking documents through digital watermarking

The present invention relates to linking documents through digital watermarking. In one implementation, an identification document is embedded with a first digital watermark. The first digital watermark includes a first identifier. Before issuing a second document, like a boarding pass or ticket, the first identifier is extracted from the identification document. The first identifier is manipulated or altered to produce a second identifier. The second identifier, however, remains related to the first identifier. The second document is embedded with a second digital watermark including the second identifier. Thus, the identification document and the second document are linked through the first and second watermark identifiers.