Method and arrangement for detecting a watermark

Abstract

Recently developed methods for copy protection rely on a watermark detector to judge whether multimedia content can be copied or not. In such copy protection schemes, a watermark detector examines the multimedia content and outputs a signal (D) indicating whether a watermark is present or not. Known watermark detectors determine a decision variable (y) indicating to which extent the watermark is present, for example, the amount of correlation between the input signal and a reference copy of the watermark to be detected. The watermark is detected if the decision variable exceeds a predetermined threshold (y 2 ). Such a detector is vulnerable to an attack which is described in this patent application. Disclosed is a watermark detector which increases the work load for an attacker by several orders of magnitude. To this end, the detector generates a random output signal for a predetermined range of decision values (y) between the threshold (y 2 ) and a further threshold (y 1 ).

1 . A method of detecting a watermark (w) embedded in an information signal (q), comprising the steps of: 
 determining a decision variable (y) indicating to which extent said watermark is present in said signal; and    generating an output signal (D) indicating detection of the watermark if the decision variable exceeds a predetermined threshold (y 2 ); characterized by the step of randomly generating said output signal for decision values below said threshold (y 2 ).    
     
     
         2 . A method as claimed in  claim 1 , characterized by the step of randomly generating said output signal for a range of decision values between said threshold (y 2 ) and a further predetermined threshold (y 1 ).  
     
     
         3 . A method as claimed in  claim 1  or  2 , wherein said random output signal is generated with an increasing probability as the decision variable becomes closer to the predetermined threshold (y 2 ).  
     
     
         4 . A method as claimed in  claim 3 , wherein said probability is a linear function of the decision variable within said range between the threshold (y 2 ) and the further threshold (y 1 ).  
     
     
         5 . A method as claimed in  claim 3 , wherein said probability is a raised cosine function of the decision variable within said range between the threshold (y 2 ) and the further threshold (y 1 ).  
     
     
         6 . A method as claimed in  claim 1  or  2 , wherein the step of randomly generating the output signal includes generating the same output signal whenever the same information signal is received.  
     
     
         7 . An arrangement for detecting a watermark (w) embedded in an information signal (q), comprising: 
 means ( 21 , 22 ) for determining a decision variable (y) indicating to which extent said watermark is present in said signal; and    means for generating an output signal (D) indicating detection of the watermark if the decision variable exceeds a predetermined threshold (y 2 );    characterized in that the arrangement comprises means for randomly generating said output signal for decision values below said threshold (y 2 ).    
     
     
         8 . An arrangement as claimed in  claim 7 , characterized in that the arrangement comprises means for randomly generating said output signal for a range of decision values between said threshold (y 2 ) and a further predetermined threshold (y 1 ).  
     
     
         9 . An arrangement as claimed in  claim 7  or  8 , comprising means for generating said random output signal with an increasing probability as the decision variable becomes closer to the predetermined threshold (y 2 ).  
     
     
         10 . An arrangement as claimed  claim 9 , wherein said means for randomly generating the output signal includes a random number generator with a seed input, and means for deriving the seed input from the information signal in accordance with a predetermined function.  
     
     
         11 . An arrangement as claimed in  claim 9 , wherein the means ( 21 , 22 ) for determining the decision variable constitute said means for deriving the seed input.  
     
     
         12 . A method of removing a watermark from an information signal having an embedded watermark, comprising the steps of: 
 applying the information signal to a watermark detector generating an indication on whether the information signal has an embedded watermark;    modifying selected portions of the information signal until the watermark detector reverses said indication, to obtain respective portions of an estimated watermark causing said reversal; and    subtracting the estimated watermark from the information signal.    
     
     
         13 . A multimedia playing and/or recording apparatus comprising an arrangement as claimed in any one of claims  7 - 11 .