Method to manage members of at least one group of decoders having access to broadcast data

A method to manage a population of decoders having access to broadcasted data, each decoder being temporarily assigned to a position in a group of decoders sharing a common broadcast encryption scheme, the method comprising the steps of:

• making a decoder a member of the group by performing the steps of:
• allocating an available position in the group to the decoder and marking the position as not available;
• sending, in a message to the decoder, keys pertaining to said position into the group according to the broadcast encryption scheme, as well as a current group access data comprising at least a current group access key common to said group, said message being encrypted with a key that is unique to the decoder;
• revoking at least one decoder and selectively renewing the current group access key for the group by sending a global group addressed message comprising at least a next group access data comprising at least a next group access key encrypted using the broadcast encryption scheme so that only the decoders corresponding to non-revoked positions can decrypt it, said group message being further encrypted by the current group access key and declaring the position of the revoked decoder as vacated;
• reusing a position vacated by a previously revoked decoder by performing the steps of
• making said next group access key the current group access key;
• marking the previously vacated position in the group as available;
• and
• repeating the allocating and sending steps using the position marked as available in the marking step.

A method for operating a decoder for accessing broadcasted data, the decoder being suitable for use in a group of decoders sharing a common broadcast encryption scheme, the method comprising the steps of:

• receiving keys pertaining to a position into the group according to the broadcast encryption scheme, as well as a current group access data comprising at least a current group access key common to said group, said message being encrypted with a key that is unique to the decoder;
• using the current group access data to directly or indirectly access the broadcasted data;
• receiving a global group addressed message comprising at least a next group access data comprising at least a next group access key encrypted using the broadcast encryption scheme so that only the decoders corresponding to non-revoked positions can decrypt it, said group message being further encrypted by the current group access key and declaring the position of the said revoked decoder as vacated;
• in a non-revoked decoder, decrypting the message using the current group access key, further decrypting the result using broadcast encryption keys pertaining to the position in the group, and storing the next group access data including updating the current group access key with the next group access key.