With the rapid development of cloud computing services, it is common to have a large server shared by many different users. As the shared server is involved in a criminal case, it is hard to clone a copy of data in forensic investigation due to the huge volume of data. Besides, those users irrelevant to the crime are not willing to disclose their private data for investigation. To solve these problems, Hou et al. presented a solution to let the server administrator (without knowing the investigation subject) cooperate with the investigator in performing forensic investigation. By using encrypted keyword(s) to search over encrypted data, they realized that the investigator can collect the necessary evidence while the private data of irrelevant users can be protected from disclosing. However, the authenticity and integrity of the collected evidence are not considered there. The authenticity and integrity are two fundamental requirements for the evidence admitted in court. So in this paper, we aim to prove the authenticity and integrity of the evidence collected by the existing work. Based on commutative encryption, we construct a blind signature and propose a “encryption-then-blind signature with designated verifier” scheme to tackle the problem.