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------------713/176 that are used for authentication by employing a short (even Single-bit) watermark vector to make the watermark and using the message needed for the authentication to deter mine where the watermark is located in the digital repre Sentation. Authentication of a digital representation is done by determining whether the watermark is present in the digital representation. In another technique, detection of the presence of a watermark is used to determine what areas of a digital representation have been Subject to alteration. Techniques for Synchronizing digital representations for watermark detection and other purposes include adding marks whose locations can be automatically detected only with the help of information that is external to the digital representation, Such as a key, and adding marks to a Sequence of digital representations and detecting the marks by Summing the Sequence.