Security method for preventing the unauthorized use of multimedia contents

A method of descrambling a first scrambled video stream to obtain a reconstructed video stream corresponding to an original video stream by a decoder comprising at least a security unit and a descrambling unit, comprising the steps of:

• receiving by the decoder the first scrambled video stream in which a plurality of original values of the original video stream at a plurality of locations have been modified by alternate values;
• receiving by the security unit a control object comprising a set of control data, each set of control data comprising data resulting in determination of at least the original value, the alternate value as well as a location where an alternate value has been introduced into the first scrambled stream;
• calculating for each set of control data a key parameter associated with a mathematical operation, wherein the operation and an associated key parameter are selected from among a plurality of different mathematical operations, said mathematical operations resulting in a reconstructed value from the alternate value by way of the key parameter;
• varying the selection of the mathematical operation based on a first internal parameter of the security unit for each set of control data;
• transmitting to the descrambler unit a set of correction data corresponding to a designation of the mathematical operation, the key parameter and the location of the alternate value;
• receiving the correction data by the descrambler unit;
• calculating the reconstructed value corresponding to the correction data and the alternate values retrieved from the first scrambled stream;
• and
• replacing the alternate value by the reconstructed value in the first scrambled stream to obtain the reconstructed video stream, wherein said reconstructed value is a personalized value determined according to the following steps:
• extracting the original value from the control object;
• calculating the personalized value based on the original value and a second internal parameter of the decoder;
• and
• applying the personalized value instead of the alternate value to calculate the key parameter.

A method of descrambling a first scrambled video stream to obtain a reconstructed video stream corresponding to an original video stream by a decoder comprising at least a security unit and a descrambling unit, comprising the steps of:

• receiving by the decoder the first scrambled video stream in which a plurality of original values of the original video stream at a plurality of locations have been modified by alternate values;
• receiving by the security unit a control object comprising a set of control data, each set of control data comprising data resulting in determination of at least the original value, the alternate value as well as a location where an alternate value has been introduced into the first scrambled stream;
• calculating for each set of control data a key parameter associated with a mathematical operation, wherein the operation and an associated key parameter are selected from among a plurality of different mathematical operations, said mathematical operations resulting in a reconstructed value from the alternate value by way of the key parameter;
• varying the selection of the mathematical operation based on a first internal parameter of the security unit for each set of control data;
• transmitting to the descrambler unit a set of correction data corresponding to a designation of the mathematical operation, the key parameter and the location of the alternate value;
• receiving the correction data by the descrambler unit;
• calculating the reconstructed value corresponding to the correction data and the alternate values retrieved from the first scrambled stream;
• and
• replacing the alternate value by the reconstructed value in the first scrambled stream to obtain the reconstructed video stream, wherein said control object further comprises a set of dedicated values associated with the original value in the modified data, said reconstructed value is a personalized value, said personalized value being determined according the following steps;
• extracting the original value and a set of dedicated values from the control object;
• selecting as personalized value among the original value and the set of the dedicated values based on a second internal parameter of the decoder;
• and
• applying the personalized value instead of the original value to calculate the key parameter.