In this paper we propose some security mechanisms that can be used in a multimedia content distribution system with Digital Rights Management (DRM) to ensure that the software tools used in the client side are trusted not only in the moment when they are installed but during their whole life operation. For this purpose, certification, verification, reverification and recertification mechanisms are described, discussing the advantages as well as the drawbacks of using such techniques. A complex

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... se case is presented to complement the description of the proposed mechanisms. The presented architecture and mechanisms are being implemented in the AXMEDIS project, which aims to create an innovative technology framework for the automatic production, protection and distribution of digital cross media contents over a range of different media channels, including PC (on the Internet), PDA, kiosks, mobile phones and i-TV.