A watermark is said private if only authorized users can recover it. In other words, in private watermarking a mechanism is envisaged that makes it im possible for unauthorized people to extract the informatioii hidden within the host signal. Sometimes by private watermarking, non-blind algorithms are meant. Indeed, non-blind techniques are by themselves private, since only authorized users (e.g. the document owner) can access the original data needed to read the watermark. Here, we extend the concept of pri vateness to techniques using any mechanism to deny the extraction of the watermark to unauthorized personnel. For instance,