Watermarking digital content is a very common approach leveraged by creators of copyrighted digital data to embed fingerprints into their data. The rationale of such operation is to mark each copy of the data in order to uniquely identify it. These watermarks are embedded in a suitable way to prevent their stripping or modification by users for illegal distribution of the copy. If a copy is illegally distributed by a pirate user (or a set of users referred as coalition) it can be identified by the distributor that can analyze the fingerprint and accuse as traitor the person in charge of that copy. Actions can then be taken against this user, to prevent further illegal distribution. Many approaches have been defined to obtain optimal fingerprinting code based on the well-known Tardos encoding. However, such approaches suffer a great limitation, i.e., when the fingerprinting code embedded in a document is too short, it is not useful for accusing a traitor in a trial, as the probability that s/he might be innocent is too high, nevertheless, the fingerprint code can be used to drive further investigation. To overcome this limitation, in this work we provide a simple yet powerful accusation scheme that can be applied for a widely used approach such phrase substitution that prove itself to be effective in many real world applications when the length of the fingerprinting codes is too short to be used for accusing a traitor in a trial with the classical approaches.
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