Backscatter networks are widely applied in real world such as inventory management, RFID-enabled passport, and access control. The security issues rapidly grow in its importance with the widespread uses. Especially, if the access control application identifies a user using a message from a backscatter device, the message can be used for tracking or replay attacks. However, most of the existing security protocols depending on cryptosystems cannot be applied into the backscatter networks due to the minimalist design of backscatter devices. In order to alleviate the security menaces, we propose a novel concept of authentication scheme for backscatter networks. Our scheme uses an unpredictable collision by applying our own data structure called Shifted Counting Bloom Filter (SCBF). Based on the SCBF, the scheme can reduce the overhead of backscatter devices required for security. Our performance evaluation shows the information-theoretic secrecy and the reliability of our scheme.