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, this method comprising the steps of: • Receiving by the decoder the first scrambled 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 comprising data allowing to determine at least the original, the alternate value as well as the location where an alternate value have been introduced into the first scrambled stream, • For each set of control data, calculating a key parameter associated with a mathematical operation, this operation and the associated key parameter being selected among a plurality of different operations, said mathematical operations allowing to obtain a reconstructed value from the alternate value thanks to the key parameter, • Varying the selection process of the said mathematical operation based on an 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 the designation of the mathematical operation, the key parameter and the location of the alternate value, • Receiving the correction data by the descrambler unit and, • 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 so as to obtain the reconstructed video stream. • Receiving by the decoder the first scrambled 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 comprising data allowing to determine at least the original, the alternate value as well as the location where an alternate value have been introduced into the first scrambled stream, • For each set of control data, calculating a key parameter associated with a mathematical operation, this operation and the associated key parameter being selected among a plurality of different operations, said mathematical operations allowing to obtain a reconstructed value from the alternate value thanks to the key parameter, • Varying the selection process of the said mathematical operation based on an 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 the designation of the mathematical operation, the key parameter and the location of the alternate value, • Receiving the correction data by the descrambler unit and, • 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 so as to obtain the reconstructed video stream.