Patent ID: 8571213

Claim:
A method for 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 extracted from the original stream and 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 extracted from the original stream and, 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; replacing the alternate value by the reconstructed value in the first scrambled stream to obtain the reconstructed video stream; and 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.