It is commonsense that the data should be encrypted by searchable encryption before being outsourced to the cloud for its confidentiality and availability. Most of existing studies focus on forward privacy, we initiate the study of forward unforgeability, which defines the time periods when files can be uploaded while the forward privacy defines the time periods when files can be queried. For example in log-files management, we should block the attackers from revising the logs of previous time periods. We find that forward unforgeability is helpful in dealing with the problem. However, it is still challenging to achieve forward unforgeability in case of searchable encryption. In this paper, we formalize the key exposure problem in keyword-field-free conjunctive keyword search (KFF-CKS) and define the forward unforgeability of KFF-CKS formally. Then we design a general framework where any forward unforgeable KFF-CKS scheme can be applied there for practical use. After that, we construct a novel KFF-CKS scheme with forward unforgeability (FU-KFF-CKS) under this framework. Furthermore, we formally prove the security properties of our FU-KFF-CKS under random oracle model and compare our construction with previous ones in a theoretical view. Finally, we conduct extensive experiments and the results validate the effectiveness and efficiency of the proposed FU-KFF-CKS.

众所周知，在将数据外包给云之前，应该通过可搜索的加密方式对其进行加密，以确保数据的机密性和可用性。现有的大多数研究都集中在前向隐私上，我们开始进行前向不可伪造性研究，前向不可伪造性定义了可以上传文件的时间段，而前向隐私性定义了可以查询文件的时间段。例如，在日志文件管理中，我们应阻止攻击者修改以前时间段的日志。我们发现，向前不可伪造有助于解决问题。但是，在可搜索加密的情况下，实现前不可伪性仍然是一个挑战。在本文中，我们将无关键字字段的联合关键字搜索（KFF-CKS）中的关键暴露问题形式化，并正式定义KFF-CKS的正向不可伪造性。然后，我们设计了一个通用框架，在该框架中任何前向不可伪造的KFF-CKS方案都可以在那里实际应用。之后，在此框架下，我们构建了具有前不可伪性的新型KFF-CKS方案（FU-KFF-CKS）。此外，我们在随机预言机模型下正式证明了FU-KFF-CKS的安全性，并在理论上将我们的构造与以前的构造进行了比较。最后，我们进行了广泛的实验，结果验证了所提出的FU-KFF-CKS的有效性和效率。我们在随机预言机模型下正式证明了FU-KFF-CKS的安全性，并从理论角度将其构造与以前的构造进行了比较。最后，我们进行了广泛的实验，结果验证了所提出的FU-KFF-CKS的有效性和效率。我们在随机预言机模型下正式证明了FU-KFF-CKS的安全性，并从理论角度将其构造与以前的构造进行了比较。最后，我们进行了广泛的实验，结果验证了所提出的FU-KFF-CKS的有效性和效率。