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Data protection is an important issue for computer forensics in a digitalized world. We construct secure data protection methods by adopting cryptographic building blocks. In our proposed scheme, an investigating authority delegates a group composed of n judicial policemen to collect digital evidence. Any t or more of them can cooperatively generate valid authenticated evidence for collected ordinary evidence on behalf of the investigating authority. To ensure confidentiality, the authenticated evidence can only be decrypted and verified by a designated investigator of Investigation of Bureau, Ministry of Justice (MJIB). For the litigation process, the designated investigator is capable of further converting the authenticated evidence into an ordinary one and giving it to a judge or prosecutor without leaking the information of his private key. We also present a variant with message linkages for benefiting the encryption of a large message. To guarantee the feasibility of practical implementation, we show that our construction achieves the IND-CCA2 and the EF-CMA security in the random oracle model.

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