System mit bedingtem Zugang unter Verwendung von Nachrichten mit mehreren Verschlüsselungsschlüsseln

摘要

The invention relates to a conditional access system making it possible for a service provider to supply his services solely to users having acquired entitlements to these services. The services supplied by a service provider consist of an item scrambled by control words. To keep these control words secret, they are supplied in messages (MEC) after having been encrypted with an encryption algorithm with key K. According to the invention, one and the same message (MEC) contains the same control word (Cwi) encrypted several times, each encryption (E(CWi)Kj) of the control word depending on a different encryption key (Kj). The invention applies to any type of conditional access system, be this system either of "off-line" or "on-line" type.