With the rapid development of the Internet of things (IoT), more devices are connected in the same network and communicate frequently for sharing data and switching instructions. So that the traditional centralized security architecture of will be limited in terms of data storage space, data reliability, scalability and operating costs. In this paper, we propose a novel data management model based on blockchain technology. Specifically, all encrypted data files are uploaded to public cloud server for obtaining enough storage space, but the key information extracted from these data files (called communication logs) will be recorded in an IoT maintained by all devices. In fact, the ledger is a kind of blockchain structured distributed database, which can solve the problem of data reliability, scalability and would not involve high operating costs. Besides, in order to efficiently search logs and not reveal any sensitive information of data, we design a secure search scheme over such blockchain database, the asymmetric scalar-product preserving encryption (ASPE) approach is exploited to guarantee the data security, and the two layers index improves the search efficiency. Security analysis and experiments on synthetic dataset show that our scheme is secure and efficient.