In this letter, a scheme that achieves strong security using online fountain codes is proposed. By combining coset precoding, we minimize the number of leaked symbols of fountain codes to achieve strong security. We analyze the security performance of fountain codes using uni-partite graphs, and propose an optimal graph structure that achieves the smallest number of leaked symbols. Based on the optimal graph structure, we proposed a coding scheme called "Chain Buildup" and analyzed its expected number of leaked symbols. Simulation results show that our proposed scheme could achieve smaller number of leaked symbols compared to other security fountain codes. The proposed scheme also has smaller decoding overhead and lower complexity compared to conventional online fountain codes.