Patent ID: 7487349
Filing Date: 2009-02-03
Classification: H04L,H04N

Abstract:
1. A method for securing access to a content, encrypted by an encryption key, transmitted from a broadcaster to a plurality of multimedia units, each of which is associated with a security module comprising at least one unique authorisation key and at least two group authorisation keys, said method allowing the broadcaster to selectively revoke access rights from at least one of said security modules, this method involving the organisation of said security modules into a hierarchical tree of at least two levels of groups, the first of said levels comprising a plurality of security modules which share a common group authorisation key, the second of said levels comprising at least two groups of the first level groups of security modules each sharing a further common group key, said method comprising the following steps: generation of a temporary encryption key, generation of a value allowing the determination of the encryption key of the content, encryption under the temporary encryption key of said value, transmission of the encrypted value to the plurality of security modules, transmission of the content, encrypted under the encryption key, to the plurality of security modules, encryption and transmission of at least two cryptograms each comprising the temporary encryption key, the first cryptogram being encrypted under the unique authorisation key of a non-revoked security module and the second cryptogram being encrypted by a group authorisation key pertaining to a group of security modules to which the revoked security module does not belong, decryption, by at least one of the non-revoked security modules, of at least one of the plurality of cryptograms using either one of the group authorisation keys or the unique authorisation key to give the temporary encryption key, decryption, by at least one of the non-revoked security modules, of the encrypted value using the temporary encryption key to give the encryption key, decryption, by the multimedia unit associated with at least one non-revoked security module, of the encrypted content using the encryption key to get clear content.