Sanitizable signatures allow a designated entity to modify some specific parts of a signed message and to produce a new signature of the resulting message without any interaction with the original signer. In this paper, we extend these sanitizable signatures to formally introduce trapdoor sanitizable signatures. In this concept, the power of saniti-zation is given to possibly several entities, for a given message/signature by using a trapdoor computed by the signer at any time. We also give a generic construction of such trapdoor sanitizable signatures. Eventually, we apply our new cryptographic tool to group content protection, permitting members of the group to distribute a protected content among themselves.
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