A method for controlling access to a plurality of channels by a unit comprising a security module, each channel being encrypted by a specific control word and having a channel identifier, this method comprising the steps of: tuning to a first channel having first channel identifier and receiving first messages containing a first control word; decrypting the first messages and using the first control word; storing the first control word and the first channel identifier; tuning to a second channel having second channel identifier; calculating the second control word by: calculating a root control word with an inverse cryptographic function F−1 using the first control word and the first channel identifier; calculating the second control word with the cryptographic function F using the root control word and the second channel identifier; and using the second control word to access the second channel.