A method to identify the origin of a security module in pay-tv decoder system this method comprising the steps of: - receiving by the pay-tv decoder system a service comprising at least a first stream, a second stream and a control word stream, the second stream being a duplicate of the first stream, said first stream being encrypted by a first control word and said second stream being encrypted by a second control word, - extracting from the control word stream, entitlement messages containing at least one main control word allowing to retrieve the first and the second control words, and access conditions, - transferring the control word stream to the security module, - extracting the entitlement messages from the control word stream and checking the access conditions, - if the access conditions are met, determining the first and second control words from the main control word in the entitlement message, - determining an internal parameter of the security module, this internal parameter being unique per security module, - selecting a current control word from the first or the second control word based on part of the internal parameter, corresponding to the first or the second stream, - transmitting the current control word to the pay-tv decoder, - selecting a current stream from the first or the second stream in accordance with the selection of the first or the second control word, - decrypting the current stream with the current control word, characterized in that, the internal parameter is a unique address of the security module and having a plurality of bits, and the method comprises the steps of: a. initializing a sweep register (CNTR) pointing to the first bit of the unique address, b. using the sweep register to point to a bit of the unique address, c. using the state of said bit to select the current control word, d. incrementing the sweep register to point to the next bit of the unique address each time the selection of a current control word from the first or the second control word is performed, e. repeating the steps b to d until all of the bits of the unique address have been used, and reinitializing the sweep register. - receiving by the pay-tv decoder system a service comprising at least a first stream, a second stream and a control word stream, the second stream being a duplicate of the first stream, said first stream being encrypted by a first control word and said second stream being encrypted by a second control word, - extracting from the control word stream, entitlement messages containing at least one main control word allowing to retrieve the first and the second control words, and access conditions, - transferring the control word stream to the security module, - extracting the entitlement messages from the control word stream and checking the access conditions, - if the access conditions are met, determining the first and second control words from the main control word in the entitlement message, - determining an internal parameter of the security module, this internal parameter being unique per security module, - selecting a current control word from the first or the second control word based on part of the internal parameter, corresponding to the first or the second stream, - transmitting the current control word to the pay-tv decoder, - selecting a current stream from the first or the second stream in accordance with the selection of the first or the second control word, - decrypting the current stream with the current control word, a. initializing a sweep register (CNTR) pointing to the first bit of the unique address, b. using the sweep register to point to a bit of the unique address, c. using the state of said bit to select the current control word, d. incrementing the sweep register to point to the next bit of the unique address each time the selection of a current control word from the first or the second control word is performed, e. repeating the steps b to d until all of the bits of the unique address have been used, and reinitializing the sweep register.